diff --git a/src/lib/es5.d.ts b/src/lib/es5.d.ts
index 0d1481dd7d26b..d39978f5c1a0b 100644
--- a/src/lib/es5.d.ts
+++ b/src/lib/es5.d.ts
@@ -1341,7 +1341,7 @@ interface ArrayConstructor {
     (arrayLength?: number): any[];
     <T>(arrayLength: number): T[];
     <T>(...items: T[]): T[];
-    isArray(arg: any): arg is Array<any>;
+    isArray<T>(arg: T): arg is T extends ReadonlyArray<any> ? T : Array<any> extends T ? (T & any[]) : never;
     readonly prototype: Array<any>;
 }
 
diff --git a/tests/baselines/reference/declarationsWithRecursiveInternalTypesProduceUniqueTypeParams.types b/tests/baselines/reference/declarationsWithRecursiveInternalTypesProduceUniqueTypeParams.types
index 68e2e12c65dbb..8fce630fecb93 100644
--- a/tests/baselines/reference/declarationsWithRecursiveInternalTypesProduceUniqueTypeParams.types
+++ b/tests/baselines/reference/declarationsWithRecursiveInternalTypesProduceUniqueTypeParams.types
@@ -67,9 +67,9 @@ export const updateIfChanged = <T>(t: T) => {
 >assign : { <T, U>(target: T, source: U): T & U; <T, U, V>(target: T, source1: U, source2: V): T & U & V; <T, U, V, W>(target: T, source1: U, source2: V, source3: W): T & U & V & W; (target: object, ...sources: any[]): any; }
 >Array.isArray(u) ? [] : {} : undefined[] | {}
 >Array.isArray(u) : boolean
->Array.isArray : (arg: any) => arg is any[]
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
 >Array : ArrayConstructor
->isArray : (arg: any) => arg is any[]
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
 >u : U
 >[] : undefined[]
 >{} : {}
diff --git a/tests/baselines/reference/fixSignatureCaching.types b/tests/baselines/reference/fixSignatureCaching.types
index 4fda03be7a901..914e651864670 100644
--- a/tests/baselines/reference/fixSignatureCaching.types
+++ b/tests/baselines/reference/fixSignatureCaching.types
@@ -1109,9 +1109,9 @@ define(function () {
 >Array : ArrayConstructor
 
         Array.isArray : function (value) { return Object.prototype.toString.call(value) === '[object Array]'; };
->Array.isArray : (arg: any) => arg is any[]
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
 >Array : ArrayConstructor
->isArray : (arg: any) => arg is any[]
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
 >function (value) { return Object.prototype.toString.call(value) === '[object Array]'; } : (value: any) => boolean
 >value : any
 >Object.prototype.toString.call(value) === '[object Array]' : boolean
diff --git a/tests/baselines/reference/isArray.errors.txt b/tests/baselines/reference/isArray.errors.txt
new file mode 100644
index 0000000000000..558c6b042be0c
--- /dev/null
+++ b/tests/baselines/reference/isArray.errors.txt
@@ -0,0 +1,128 @@
+tests/cases/compiler/isArray.ts(21,33): error TS2339: Property 'push' does not exist on type 'readonly string[]'.
+tests/cases/compiler/isArray.ts(22,33): error TS2339: Property 'push' does not exist on type 'readonly string[]'.
+tests/cases/compiler/isArray.ts(28,38): error TS2345: Argument of type '10' is not assignable to parameter of type 'string'.
+tests/cases/compiler/isArray.ts(36,33): error TS2339: Property 'push' does not exist on type 'readonly string[] | number[]'.
+  Property 'push' does not exist on type 'readonly string[]'.
+tests/cases/compiler/isArray.ts(40,33): error TS2339: Property 'push' does not exist on type 'readonly string[] | number[]'.
+  Property 'push' does not exist on type 'readonly string[]'.
+tests/cases/compiler/isArray.ts(44,38): error TS2345: Argument of type '10' is not assignable to parameter of type 'string'.
+tests/cases/compiler/isArray.ts(50,33): error TS2339: Property 'push' does not exist on type 'MyReadOnlyArray<string>'.
+tests/cases/compiler/isArray.ts(51,41): error TS2345: Argument of type '10' is not assignable to parameter of type 'string'.
+tests/cases/compiler/isArray.ts(61,33): error TS2339: Property 'push' does not exist on type 'readonly T[]'.
+tests/cases/compiler/isArray.ts(70,33): error TS2339: Property 'push' does not exist on type 'readonly [T]'.
+tests/cases/compiler/isArray.ts(76,38): error TS2345: Argument of type '10' is not assignable to parameter of type 'string'.
+tests/cases/compiler/isArray.ts(83,15): error TS2345: Argument of type '"str"' is not assignable to parameter of type 'string[]'.
+
+
+==== tests/cases/compiler/isArray.ts (12 errors) ====
+    interface MyArray<T> extends Array<T> { manifest: any; }
+    interface MyReadOnlyArray<T> extends ReadonlyArray<T> { manifest: any; }
+    
+    function fn1(arg: string | string[]) {
+        if (Array.isArray(arg)) arg.push(""); // Should OK
+    }
+    
+    function fn2(arg: unknown) {
+        if (Array.isArray(arg)) arg.push(""); // Should OK
+    }
+    
+    function fn3(arg: object) {
+        if (Array.isArray(arg)) arg.push(""); // Should OK
+    }
+    
+    function fn4(arg: {}) {
+        if (Array.isArray(arg)) arg.push(""); // Should OK
+    }
+    
+    function fn5(arg: string | ReadonlyArray<string>) {
+        if (Array.isArray(arg)) arg.push(10); // Should FAIL
+                                    ~~~~
+!!! error TS2339: Property 'push' does not exist on type 'readonly string[]'.
+        if (Array.isArray(arg)) arg.push(""); // Should FAIL
+                                    ~~~~
+!!! error TS2339: Property 'push' does not exist on type 'readonly string[]'.
+        if (Array.isArray(arg)) arg.indexOf(""); // Should OK
+        if (!Array.isArray(arg)) arg.toUpperCase(); // Should OK
+    }
+    
+    function fn6(arg: string | string[]) {
+        if (Array.isArray(arg)) arg.push(10); // Should FAIL
+                                         ~~
+!!! error TS2345: Argument of type '10' is not assignable to parameter of type 'string'.
+    }
+    
+    function fn7(arg: boolean | number[] | string[], stringAndNumber: string & number) {
+        if (Array.isArray(arg)) arg.push(stringAndNumber); // Should OK
+    }
+    
+    function fn8(arg: string | number[] | readonly string[]) {
+        if (Array.isArray(arg)) arg.push(10); // Should FAIL
+                                    ~~~~
+!!! error TS2339: Property 'push' does not exist on type 'readonly string[] | number[]'.
+!!! error TS2339:   Property 'push' does not exist on type 'readonly string[]'.
+    }
+    
+    function fn9(arg: string | number[] | readonly string[]) {
+        if (Array.isArray(arg)) arg.push(10); // Should FAIL
+                                    ~~~~
+!!! error TS2339: Property 'push' does not exist on type 'readonly string[] | number[]'.
+!!! error TS2339:   Property 'push' does not exist on type 'readonly string[]'.
+    }
+    
+    function fn10(arg: string | MyArray<string>) {
+        if (Array.isArray(arg)) arg.push(10); // Should FAIL
+                                         ~~
+!!! error TS2345: Argument of type '10' is not assignable to parameter of type 'string'.
+        if (Array.isArray(arg)) arg.push(""); // Should OK
+        if (Array.isArray(arg)) arg.manifest; // Should OK
+    }
+    
+    function fn11(arg: string | MyReadOnlyArray<string>) {
+        if (Array.isArray(arg)) arg.push(""); // Should FAIL
+                                    ~~~~
+!!! error TS2339: Property 'push' does not exist on type 'MyReadOnlyArray<string>'.
+        if (Array.isArray(arg)) arg.indexOf(10); // Should FAIL
+                                            ~~
+!!! error TS2345: Argument of type '10' is not assignable to parameter of type 'string'.
+        if (Array.isArray(arg)) arg.indexOf(""); // Should OK
+        if (Array.isArray(arg)) arg.manifest; // Should OK
+    }
+    
+    function fn12<T>(arg: T | T[], t: T) {
+        if (Array.isArray(arg)) arg.push(t); // Should OK
+    }
+    
+    function fn13<T>(arg: T | ReadonlyArray<T>, t: T) {
+        if (Array.isArray(arg)) arg.push(t); // Should fail
+                                    ~~~~
+!!! error TS2339: Property 'push' does not exist on type 'readonly T[]'.
+        if (Array.isArray(arg)) arg.indexOf(t); // OK
+    }
+    
+    function fn14<T>(arg: T | [T], t: T) {
+        if (Array.isArray(arg)) arg.push(t); // Should OK
+    }
+    
+    function fn15<T>(arg: T | readonly [T], t: T) {
+        if (Array.isArray(arg)) arg.push(t); // Should fail
+                                    ~~~~
+!!! error TS2339: Property 'push' does not exist on type 'readonly [T]'.
+        if (Array.isArray(arg)) arg.indexOf(t); // Should OK
+    }
+    
+    function fn16<T extends string | string[]>(arg: T) {
+        if (Array.isArray(arg)) arg.push("10"); // Should OK
+        if (Array.isArray(arg)) arg.push(10); // Should fail
+                                         ~~
+!!! error TS2345: Argument of type '10' is not assignable to parameter of type 'string'.
+    }
+    
+    function fn17() {
+        const s: Array<string | string[]> = [];
+        const arrs = s.filter(Array.isArray);
+        arrs.push(["one"]); // Should OK
+        arrs.push("str");  // Should fail
+                  ~~~~~
+!!! error TS2345: Argument of type '"str"' is not assignable to parameter of type 'string[]'.
+    }
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/isArray.js b/tests/baselines/reference/isArray.js
index a90898fd65b26..e52ad526d7508 100644
--- a/tests/baselines/reference/isArray.js
+++ b/tests/baselines/reference/isArray.js
@@ -1,19 +1,180 @@
 //// [isArray.ts]
-var maybeArray: number | number[];
+interface MyArray<T> extends Array<T> { manifest: any; }
+interface MyReadOnlyArray<T> extends ReadonlyArray<T> { manifest: any; }
 
+function fn1(arg: string | string[]) {
+    if (Array.isArray(arg)) arg.push(""); // Should OK
+}
 
-if (Array.isArray(maybeArray)) {
-    maybeArray.length; // OK
+function fn2(arg: unknown) {
+    if (Array.isArray(arg)) arg.push(""); // Should OK
 }
-else {
-    maybeArray.toFixed(); // OK
-}
+
+function fn3(arg: object) {
+    if (Array.isArray(arg)) arg.push(""); // Should OK
+}
+
+function fn4(arg: {}) {
+    if (Array.isArray(arg)) arg.push(""); // Should OK
+}
+
+function fn5(arg: string | ReadonlyArray<string>) {
+    if (Array.isArray(arg)) arg.push(10); // Should FAIL
+    if (Array.isArray(arg)) arg.push(""); // Should FAIL
+    if (Array.isArray(arg)) arg.indexOf(""); // Should OK
+    if (!Array.isArray(arg)) arg.toUpperCase(); // Should OK
+}
+
+function fn6(arg: string | string[]) {
+    if (Array.isArray(arg)) arg.push(10); // Should FAIL
+}
+
+function fn7(arg: boolean | number[] | string[], stringAndNumber: string & number) {
+    if (Array.isArray(arg)) arg.push(stringAndNumber); // Should OK
+}
+
+function fn8(arg: string | number[] | readonly string[]) {
+    if (Array.isArray(arg)) arg.push(10); // Should FAIL
+}
+
+function fn9(arg: string | number[] | readonly string[]) {
+    if (Array.isArray(arg)) arg.push(10); // Should FAIL
+}
+
+function fn10(arg: string | MyArray<string>) {
+    if (Array.isArray(arg)) arg.push(10); // Should FAIL
+    if (Array.isArray(arg)) arg.push(""); // Should OK
+    if (Array.isArray(arg)) arg.manifest; // Should OK
+}
+
+function fn11(arg: string | MyReadOnlyArray<string>) {
+    if (Array.isArray(arg)) arg.push(""); // Should FAIL
+    if (Array.isArray(arg)) arg.indexOf(10); // Should FAIL
+    if (Array.isArray(arg)) arg.indexOf(""); // Should OK
+    if (Array.isArray(arg)) arg.manifest; // Should OK
+}
+
+function fn12<T>(arg: T | T[], t: T) {
+    if (Array.isArray(arg)) arg.push(t); // Should OK
+}
+
+function fn13<T>(arg: T | ReadonlyArray<T>, t: T) {
+    if (Array.isArray(arg)) arg.push(t); // Should fail
+    if (Array.isArray(arg)) arg.indexOf(t); // OK
+}
+
+function fn14<T>(arg: T | [T], t: T) {
+    if (Array.isArray(arg)) arg.push(t); // Should OK
+}
+
+function fn15<T>(arg: T | readonly [T], t: T) {
+    if (Array.isArray(arg)) arg.push(t); // Should fail
+    if (Array.isArray(arg)) arg.indexOf(t); // Should OK
+}
+
+function fn16<T extends string | string[]>(arg: T) {
+    if (Array.isArray(arg)) arg.push("10"); // Should OK
+    if (Array.isArray(arg)) arg.push(10); // Should fail
+}
+
+function fn17() {
+    const s: Array<string | string[]> = [];
+    const arrs = s.filter(Array.isArray);
+    arrs.push(["one"]); // Should OK
+    arrs.push("str");  // Should fail
+}
+
 
 //// [isArray.js]
-var maybeArray;
-if (Array.isArray(maybeArray)) {
-    maybeArray.length; // OK
+function fn1(arg) {
+    if (Array.isArray(arg))
+        arg.push(""); // Should OK
+}
+function fn2(arg) {
+    if (Array.isArray(arg))
+        arg.push(""); // Should OK
+}
+function fn3(arg) {
+    if (Array.isArray(arg))
+        arg.push(""); // Should OK
+}
+function fn4(arg) {
+    if (Array.isArray(arg))
+        arg.push(""); // Should OK
+}
+function fn5(arg) {
+    if (Array.isArray(arg))
+        arg.push(10); // Should FAIL
+    if (Array.isArray(arg))
+        arg.push(""); // Should FAIL
+    if (Array.isArray(arg))
+        arg.indexOf(""); // Should OK
+    if (!Array.isArray(arg))
+        arg.toUpperCase(); // Should OK
+}
+function fn6(arg) {
+    if (Array.isArray(arg))
+        arg.push(10); // Should FAIL
+}
+function fn7(arg, stringAndNumber) {
+    if (Array.isArray(arg))
+        arg.push(stringAndNumber); // Should OK
+}
+function fn8(arg) {
+    if (Array.isArray(arg))
+        arg.push(10); // Should FAIL
+}
+function fn9(arg) {
+    if (Array.isArray(arg))
+        arg.push(10); // Should FAIL
+}
+function fn10(arg) {
+    if (Array.isArray(arg))
+        arg.push(10); // Should FAIL
+    if (Array.isArray(arg))
+        arg.push(""); // Should OK
+    if (Array.isArray(arg))
+        arg.manifest; // Should OK
+}
+function fn11(arg) {
+    if (Array.isArray(arg))
+        arg.push(""); // Should FAIL
+    if (Array.isArray(arg))
+        arg.indexOf(10); // Should FAIL
+    if (Array.isArray(arg))
+        arg.indexOf(""); // Should OK
+    if (Array.isArray(arg))
+        arg.manifest; // Should OK
+}
+function fn12(arg, t) {
+    if (Array.isArray(arg))
+        arg.push(t); // Should OK
+}
+function fn13(arg, t) {
+    if (Array.isArray(arg))
+        arg.push(t); // Should fail
+    if (Array.isArray(arg))
+        arg.indexOf(t); // OK
+}
+function fn14(arg, t) {
+    if (Array.isArray(arg))
+        arg.push(t); // Should OK
+}
+function fn15(arg, t) {
+    if (Array.isArray(arg))
+        arg.push(t); // Should fail
+    if (Array.isArray(arg))
+        arg.indexOf(t); // Should OK
+}
+function fn16(arg) {
+    if (Array.isArray(arg))
+        arg.push("10"); // Should OK
+    if (Array.isArray(arg))
+        arg.push(10); // Should fail
 }
-else {
-    maybeArray.toFixed(); // OK
+function fn17() {
+    var s = [];
+    var arrs = s.filter(Array.isArray);
+    arrs.push(["one"]); // Should OK
+    arrs.push("str"); // Should fail
 }
diff --git a/tests/baselines/reference/isArray.symbols b/tests/baselines/reference/isArray.symbols
index 411cb42918daa..04a2fc1258f9b 100644
--- a/tests/baselines/reference/isArray.symbols
+++ b/tests/baselines/reference/isArray.symbols
@@ -1,22 +1,385 @@
 === tests/cases/compiler/isArray.ts ===
-var maybeArray: number | number[];
->maybeArray : Symbol(maybeArray, Decl(isArray.ts, 0, 3))
+interface MyArray<T> extends Array<T> { manifest: any; }
+>MyArray : Symbol(MyArray, Decl(isArray.ts, 0, 0))
+>T : Symbol(T, Decl(isArray.ts, 0, 18))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>T : Symbol(T, Decl(isArray.ts, 0, 18))
+>manifest : Symbol(MyArray.manifest, Decl(isArray.ts, 0, 39))
+
+interface MyReadOnlyArray<T> extends ReadonlyArray<T> { manifest: any; }
+>MyReadOnlyArray : Symbol(MyReadOnlyArray, Decl(isArray.ts, 0, 56))
+>T : Symbol(T, Decl(isArray.ts, 1, 26))
+>ReadonlyArray : Symbol(ReadonlyArray, Decl(lib.es5.d.ts, --, --))
+>T : Symbol(T, Decl(isArray.ts, 1, 26))
+>manifest : Symbol(MyReadOnlyArray.manifest, Decl(isArray.ts, 1, 55))
 
+function fn1(arg: string | string[]) {
+>fn1 : Symbol(fn1, Decl(isArray.ts, 1, 72))
+>arg : Symbol(arg, Decl(isArray.ts, 3, 13))
 
-if (Array.isArray(maybeArray)) {
+    if (Array.isArray(arg)) arg.push(""); // Should OK
 >Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
 >Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
 >isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
->maybeArray : Symbol(maybeArray, Decl(isArray.ts, 0, 3))
+>arg : Symbol(arg, Decl(isArray.ts, 3, 13))
+>arg.push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 3, 13))
+>push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+}
 
-    maybeArray.length; // OK
->maybeArray.length : Symbol(Array.length, Decl(lib.es5.d.ts, --, --))
->maybeArray : Symbol(maybeArray, Decl(isArray.ts, 0, 3))
->length : Symbol(Array.length, Decl(lib.es5.d.ts, --, --))
+function fn2(arg: unknown) {
+>fn2 : Symbol(fn2, Decl(isArray.ts, 5, 1))
+>arg : Symbol(arg, Decl(isArray.ts, 7, 13))
+
+    if (Array.isArray(arg)) arg.push(""); // Should OK
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 7, 13))
+>arg.push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 7, 13))
+>push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
 }
-else {
-    maybeArray.toFixed(); // OK
->maybeArray.toFixed : Symbol(Number.toFixed, Decl(lib.es5.d.ts, --, --))
->maybeArray : Symbol(maybeArray, Decl(isArray.ts, 0, 3))
->toFixed : Symbol(Number.toFixed, Decl(lib.es5.d.ts, --, --))
+
+function fn3(arg: object) {
+>fn3 : Symbol(fn3, Decl(isArray.ts, 9, 1))
+>arg : Symbol(arg, Decl(isArray.ts, 11, 13))
+
+    if (Array.isArray(arg)) arg.push(""); // Should OK
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 11, 13))
+>arg.push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 11, 13))
+>push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
 }
+
+function fn4(arg: {}) {
+>fn4 : Symbol(fn4, Decl(isArray.ts, 13, 1))
+>arg : Symbol(arg, Decl(isArray.ts, 15, 13))
+
+    if (Array.isArray(arg)) arg.push(""); // Should OK
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 15, 13))
+>arg.push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 15, 13))
+>push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+}
+
+function fn5(arg: string | ReadonlyArray<string>) {
+>fn5 : Symbol(fn5, Decl(isArray.ts, 17, 1))
+>arg : Symbol(arg, Decl(isArray.ts, 19, 13))
+>ReadonlyArray : Symbol(ReadonlyArray, Decl(lib.es5.d.ts, --, --))
+
+    if (Array.isArray(arg)) arg.push(10); // Should FAIL
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 19, 13))
+>arg : Symbol(arg, Decl(isArray.ts, 19, 13))
+
+    if (Array.isArray(arg)) arg.push(""); // Should FAIL
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 19, 13))
+>arg : Symbol(arg, Decl(isArray.ts, 19, 13))
+
+    if (Array.isArray(arg)) arg.indexOf(""); // Should OK
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 19, 13))
+>arg.indexOf : Symbol(ReadonlyArray.indexOf, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 19, 13))
+>indexOf : Symbol(ReadonlyArray.indexOf, Decl(lib.es5.d.ts, --, --))
+
+    if (!Array.isArray(arg)) arg.toUpperCase(); // Should OK
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 19, 13))
+>arg.toUpperCase : Symbol(String.toUpperCase, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 19, 13))
+>toUpperCase : Symbol(String.toUpperCase, Decl(lib.es5.d.ts, --, --))
+}
+
+function fn6(arg: string | string[]) {
+>fn6 : Symbol(fn6, Decl(isArray.ts, 24, 1))
+>arg : Symbol(arg, Decl(isArray.ts, 26, 13))
+
+    if (Array.isArray(arg)) arg.push(10); // Should FAIL
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 26, 13))
+>arg.push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 26, 13))
+>push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+}
+
+function fn7(arg: boolean | number[] | string[], stringAndNumber: string & number) {
+>fn7 : Symbol(fn7, Decl(isArray.ts, 28, 1))
+>arg : Symbol(arg, Decl(isArray.ts, 30, 13))
+>stringAndNumber : Symbol(stringAndNumber, Decl(isArray.ts, 30, 48))
+
+    if (Array.isArray(arg)) arg.push(stringAndNumber); // Should OK
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 30, 13))
+>arg.push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 30, 13))
+>push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>stringAndNumber : Symbol(stringAndNumber, Decl(isArray.ts, 30, 48))
+}
+
+function fn8(arg: string | number[] | readonly string[]) {
+>fn8 : Symbol(fn8, Decl(isArray.ts, 32, 1))
+>arg : Symbol(arg, Decl(isArray.ts, 34, 13))
+
+    if (Array.isArray(arg)) arg.push(10); // Should FAIL
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 34, 13))
+>arg : Symbol(arg, Decl(isArray.ts, 34, 13))
+}
+
+function fn9(arg: string | number[] | readonly string[]) {
+>fn9 : Symbol(fn9, Decl(isArray.ts, 36, 1))
+>arg : Symbol(arg, Decl(isArray.ts, 38, 13))
+
+    if (Array.isArray(arg)) arg.push(10); // Should FAIL
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 38, 13))
+>arg : Symbol(arg, Decl(isArray.ts, 38, 13))
+}
+
+function fn10(arg: string | MyArray<string>) {
+>fn10 : Symbol(fn10, Decl(isArray.ts, 40, 1))
+>arg : Symbol(arg, Decl(isArray.ts, 42, 14))
+>MyArray : Symbol(MyArray, Decl(isArray.ts, 0, 0))
+
+    if (Array.isArray(arg)) arg.push(10); // Should FAIL
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 42, 14))
+>arg.push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 42, 14))
+>push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+
+    if (Array.isArray(arg)) arg.push(""); // Should OK
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 42, 14))
+>arg.push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 42, 14))
+>push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+
+    if (Array.isArray(arg)) arg.manifest; // Should OK
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 42, 14))
+>arg.manifest : Symbol(MyArray.manifest, Decl(isArray.ts, 0, 39))
+>arg : Symbol(arg, Decl(isArray.ts, 42, 14))
+>manifest : Symbol(MyArray.manifest, Decl(isArray.ts, 0, 39))
+}
+
+function fn11(arg: string | MyReadOnlyArray<string>) {
+>fn11 : Symbol(fn11, Decl(isArray.ts, 46, 1))
+>arg : Symbol(arg, Decl(isArray.ts, 48, 14))
+>MyReadOnlyArray : Symbol(MyReadOnlyArray, Decl(isArray.ts, 0, 56))
+
+    if (Array.isArray(arg)) arg.push(""); // Should FAIL
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 48, 14))
+>arg : Symbol(arg, Decl(isArray.ts, 48, 14))
+
+    if (Array.isArray(arg)) arg.indexOf(10); // Should FAIL
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 48, 14))
+>arg.indexOf : Symbol(ReadonlyArray.indexOf, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 48, 14))
+>indexOf : Symbol(ReadonlyArray.indexOf, Decl(lib.es5.d.ts, --, --))
+
+    if (Array.isArray(arg)) arg.indexOf(""); // Should OK
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 48, 14))
+>arg.indexOf : Symbol(ReadonlyArray.indexOf, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 48, 14))
+>indexOf : Symbol(ReadonlyArray.indexOf, Decl(lib.es5.d.ts, --, --))
+
+    if (Array.isArray(arg)) arg.manifest; // Should OK
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 48, 14))
+>arg.manifest : Symbol(MyReadOnlyArray.manifest, Decl(isArray.ts, 1, 55))
+>arg : Symbol(arg, Decl(isArray.ts, 48, 14))
+>manifest : Symbol(MyReadOnlyArray.manifest, Decl(isArray.ts, 1, 55))
+}
+
+function fn12<T>(arg: T | T[], t: T) {
+>fn12 : Symbol(fn12, Decl(isArray.ts, 53, 1))
+>T : Symbol(T, Decl(isArray.ts, 55, 14))
+>arg : Symbol(arg, Decl(isArray.ts, 55, 17))
+>T : Symbol(T, Decl(isArray.ts, 55, 14))
+>T : Symbol(T, Decl(isArray.ts, 55, 14))
+>t : Symbol(t, Decl(isArray.ts, 55, 30))
+>T : Symbol(T, Decl(isArray.ts, 55, 14))
+
+    if (Array.isArray(arg)) arg.push(t); // Should OK
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 55, 17))
+>arg.push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 55, 17))
+>push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+>t : Symbol(t, Decl(isArray.ts, 55, 30))
+}
+
+function fn13<T>(arg: T | ReadonlyArray<T>, t: T) {
+>fn13 : Symbol(fn13, Decl(isArray.ts, 57, 1))
+>T : Symbol(T, Decl(isArray.ts, 59, 14))
+>arg : Symbol(arg, Decl(isArray.ts, 59, 17))
+>T : Symbol(T, Decl(isArray.ts, 59, 14))
+>ReadonlyArray : Symbol(ReadonlyArray, Decl(lib.es5.d.ts, --, --))
+>T : Symbol(T, Decl(isArray.ts, 59, 14))
+>t : Symbol(t, Decl(isArray.ts, 59, 43))
+>T : Symbol(T, Decl(isArray.ts, 59, 14))
+
+    if (Array.isArray(arg)) arg.push(t); // Should fail
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 59, 17))
+>arg : Symbol(arg, Decl(isArray.ts, 59, 17))
+>t : Symbol(t, Decl(isArray.ts, 59, 43))
+
+    if (Array.isArray(arg)) arg.indexOf(t); // OK
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 59, 17))
+>arg.indexOf : Symbol(ReadonlyArray.indexOf, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 59, 17))
+>indexOf : Symbol(ReadonlyArray.indexOf, Decl(lib.es5.d.ts, --, --))
+>t : Symbol(t, Decl(isArray.ts, 59, 43))
+}
+
+function fn14<T>(arg: T | [T], t: T) {
+>fn14 : Symbol(fn14, Decl(isArray.ts, 62, 1))
+>T : Symbol(T, Decl(isArray.ts, 64, 14))
+>arg : Symbol(arg, Decl(isArray.ts, 64, 17))
+>T : Symbol(T, Decl(isArray.ts, 64, 14))
+>T : Symbol(T, Decl(isArray.ts, 64, 14))
+>t : Symbol(t, Decl(isArray.ts, 64, 30))
+>T : Symbol(T, Decl(isArray.ts, 64, 14))
+
+    if (Array.isArray(arg)) arg.push(t); // Should OK
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 64, 17))
+>arg.push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 64, 17))
+>push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+>t : Symbol(t, Decl(isArray.ts, 64, 30))
+}
+
+function fn15<T>(arg: T | readonly [T], t: T) {
+>fn15 : Symbol(fn15, Decl(isArray.ts, 66, 1))
+>T : Symbol(T, Decl(isArray.ts, 68, 14))
+>arg : Symbol(arg, Decl(isArray.ts, 68, 17))
+>T : Symbol(T, Decl(isArray.ts, 68, 14))
+>T : Symbol(T, Decl(isArray.ts, 68, 14))
+>t : Symbol(t, Decl(isArray.ts, 68, 39))
+>T : Symbol(T, Decl(isArray.ts, 68, 14))
+
+    if (Array.isArray(arg)) arg.push(t); // Should fail
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 68, 17))
+>arg : Symbol(arg, Decl(isArray.ts, 68, 17))
+>t : Symbol(t, Decl(isArray.ts, 68, 39))
+
+    if (Array.isArray(arg)) arg.indexOf(t); // Should OK
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 68, 17))
+>arg.indexOf : Symbol(ReadonlyArray.indexOf, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 68, 17))
+>indexOf : Symbol(ReadonlyArray.indexOf, Decl(lib.es5.d.ts, --, --))
+>t : Symbol(t, Decl(isArray.ts, 68, 39))
+}
+
+function fn16<T extends string | string[]>(arg: T) {
+>fn16 : Symbol(fn16, Decl(isArray.ts, 71, 1))
+>T : Symbol(T, Decl(isArray.ts, 73, 14))
+>arg : Symbol(arg, Decl(isArray.ts, 73, 43))
+>T : Symbol(T, Decl(isArray.ts, 73, 14))
+
+    if (Array.isArray(arg)) arg.push("10"); // Should OK
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 73, 43))
+>arg.push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 73, 43))
+>push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+
+    if (Array.isArray(arg)) arg.push(10); // Should fail
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 73, 43))
+>arg.push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+>arg : Symbol(arg, Decl(isArray.ts, 73, 43))
+>push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+}
+
+function fn17() {
+>fn17 : Symbol(fn17, Decl(isArray.ts, 76, 1))
+
+    const s: Array<string | string[]> = [];
+>s : Symbol(s, Decl(isArray.ts, 79, 9))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+
+    const arrs = s.filter(Array.isArray);
+>arrs : Symbol(arrs, Decl(isArray.ts, 80, 9))
+>s.filter : Symbol(Array.filter, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>s : Symbol(s, Decl(isArray.ts, 79, 9))
+>filter : Symbol(Array.filter, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+
+    arrs.push(["one"]); // Should OK
+>arrs.push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+>arrs : Symbol(arrs, Decl(isArray.ts, 80, 9))
+>push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+
+    arrs.push("str");  // Should fail
+>arrs.push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+>arrs : Symbol(arrs, Decl(isArray.ts, 80, 9))
+>push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
+}
+
diff --git a/tests/baselines/reference/isArray.types b/tests/baselines/reference/isArray.types
index bc452b12bef01..71a9af0a866ff 100644
--- a/tests/baselines/reference/isArray.types
+++ b/tests/baselines/reference/isArray.types
@@ -1,24 +1,445 @@
 === tests/cases/compiler/isArray.ts ===
-var maybeArray: number | number[];
->maybeArray : number | number[]
+interface MyArray<T> extends Array<T> { manifest: any; }
+>manifest : any
 
+interface MyReadOnlyArray<T> extends ReadonlyArray<T> { manifest: any; }
+>manifest : any
 
-if (Array.isArray(maybeArray)) {
->Array.isArray(maybeArray) : boolean
->Array.isArray : (arg: any) => arg is any[]
+function fn1(arg: string | string[]) {
+>fn1 : (arg: string | string[]) => void
+>arg : string | string[]
+
+    if (Array.isArray(arg)) arg.push(""); // Should OK
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : string | string[]
+>arg.push("") : number
+>arg.push : (...items: string[]) => number
+>arg : string[]
+>push : (...items: string[]) => number
+>"" : ""
+}
+
+function fn2(arg: unknown) {
+>fn2 : (arg: unknown) => void
+>arg : unknown
+
+    if (Array.isArray(arg)) arg.push(""); // Should OK
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : unknown
+>arg.push("") : number
+>arg.push : (...items: any[]) => number
+>arg : any[]
+>push : (...items: any[]) => number
+>"" : ""
+}
+
+function fn3(arg: object) {
+>fn3 : (arg: object) => void
+>arg : object
+
+    if (Array.isArray(arg)) arg.push(""); // Should OK
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
 >Array : ArrayConstructor
->isArray : (arg: any) => arg is any[]
->maybeArray : number | number[]
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : object
+>arg.push("") : number
+>arg.push : (...items: any[]) => number
+>arg : object & any[]
+>push : (...items: any[]) => number
+>"" : ""
+}
+
+function fn4(arg: {}) {
+>fn4 : (arg: {}) => void
+>arg : {}
 
-    maybeArray.length; // OK
->maybeArray.length : number
->maybeArray : number[]
->length : number
+    if (Array.isArray(arg)) arg.push(""); // Should OK
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : {}
+>arg.push("") : number
+>arg.push : (...items: any[]) => number
+>arg : any[]
+>push : (...items: any[]) => number
+>"" : ""
 }
-else {
-    maybeArray.toFixed(); // OK
->maybeArray.toFixed() : string
->maybeArray.toFixed : (fractionDigits?: number) => string
->maybeArray : number
->toFixed : (fractionDigits?: number) => string
+
+function fn5(arg: string | ReadonlyArray<string>) {
+>fn5 : (arg: string | readonly string[]) => void
+>arg : string | readonly string[]
+
+    if (Array.isArray(arg)) arg.push(10); // Should FAIL
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : string | readonly string[]
+>arg.push(10) : any
+>arg.push : any
+>arg : readonly string[]
+>push : any
+>10 : 10
+
+    if (Array.isArray(arg)) arg.push(""); // Should FAIL
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : string | readonly string[]
+>arg.push("") : any
+>arg.push : any
+>arg : readonly string[]
+>push : any
+>"" : ""
+
+    if (Array.isArray(arg)) arg.indexOf(""); // Should OK
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : string | readonly string[]
+>arg.indexOf("") : number
+>arg.indexOf : (searchElement: string, fromIndex?: number | undefined) => number
+>arg : readonly string[]
+>indexOf : (searchElement: string, fromIndex?: number | undefined) => number
+>"" : ""
+
+    if (!Array.isArray(arg)) arg.toUpperCase(); // Should OK
+>!Array.isArray(arg) : boolean
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : string | readonly string[]
+>arg.toUpperCase() : string
+>arg.toUpperCase : () => string
+>arg : string
+>toUpperCase : () => string
+}
+
+function fn6(arg: string | string[]) {
+>fn6 : (arg: string | string[]) => void
+>arg : string | string[]
+
+    if (Array.isArray(arg)) arg.push(10); // Should FAIL
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : string | string[]
+>arg.push(10) : number
+>arg.push : (...items: string[]) => number
+>arg : string[]
+>push : (...items: string[]) => number
+>10 : 10
+}
+
+function fn7(arg: boolean | number[] | string[], stringAndNumber: string & number) {
+>fn7 : (arg: boolean | string[] | number[], stringAndNumber: never) => void
+>arg : boolean | string[] | number[]
+>stringAndNumber : never
+
+    if (Array.isArray(arg)) arg.push(stringAndNumber); // Should OK
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : boolean | string[] | number[]
+>arg.push(stringAndNumber) : number
+>arg.push : ((...items: string[]) => number) | ((...items: number[]) => number)
+>arg : string[] | number[]
+>push : ((...items: string[]) => number) | ((...items: number[]) => number)
+>stringAndNumber : never
+}
+
+function fn8(arg: string | number[] | readonly string[]) {
+>fn8 : (arg: string | readonly string[] | number[]) => void
+>arg : string | readonly string[] | number[]
+
+    if (Array.isArray(arg)) arg.push(10); // Should FAIL
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : string | readonly string[] | number[]
+>arg.push(10) : any
+>arg.push : any
+>arg : readonly string[] | number[]
+>push : any
+>10 : 10
+}
+
+function fn9(arg: string | number[] | readonly string[]) {
+>fn9 : (arg: string | readonly string[] | number[]) => void
+>arg : string | readonly string[] | number[]
+
+    if (Array.isArray(arg)) arg.push(10); // Should FAIL
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : string | readonly string[] | number[]
+>arg.push(10) : any
+>arg.push : any
+>arg : readonly string[] | number[]
+>push : any
+>10 : 10
+}
+
+function fn10(arg: string | MyArray<string>) {
+>fn10 : (arg: string | MyArray<string>) => void
+>arg : string | MyArray<string>
+
+    if (Array.isArray(arg)) arg.push(10); // Should FAIL
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : string | MyArray<string>
+>arg.push(10) : number
+>arg.push : (...items: string[]) => number
+>arg : MyArray<string>
+>push : (...items: string[]) => number
+>10 : 10
+
+    if (Array.isArray(arg)) arg.push(""); // Should OK
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : string | MyArray<string>
+>arg.push("") : number
+>arg.push : (...items: string[]) => number
+>arg : MyArray<string>
+>push : (...items: string[]) => number
+>"" : ""
+
+    if (Array.isArray(arg)) arg.manifest; // Should OK
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : string | MyArray<string>
+>arg.manifest : any
+>arg : MyArray<string>
+>manifest : any
 }
+
+function fn11(arg: string | MyReadOnlyArray<string>) {
+>fn11 : (arg: string | MyReadOnlyArray<string>) => void
+>arg : string | MyReadOnlyArray<string>
+
+    if (Array.isArray(arg)) arg.push(""); // Should FAIL
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : string | MyReadOnlyArray<string>
+>arg.push("") : any
+>arg.push : any
+>arg : MyReadOnlyArray<string>
+>push : any
+>"" : ""
+
+    if (Array.isArray(arg)) arg.indexOf(10); // Should FAIL
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : string | MyReadOnlyArray<string>
+>arg.indexOf(10) : number
+>arg.indexOf : (searchElement: string, fromIndex?: number | undefined) => number
+>arg : MyReadOnlyArray<string>
+>indexOf : (searchElement: string, fromIndex?: number | undefined) => number
+>10 : 10
+
+    if (Array.isArray(arg)) arg.indexOf(""); // Should OK
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : string | MyReadOnlyArray<string>
+>arg.indexOf("") : number
+>arg.indexOf : (searchElement: string, fromIndex?: number | undefined) => number
+>arg : MyReadOnlyArray<string>
+>indexOf : (searchElement: string, fromIndex?: number | undefined) => number
+>"" : ""
+
+    if (Array.isArray(arg)) arg.manifest; // Should OK
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : string | MyReadOnlyArray<string>
+>arg.manifest : any
+>arg : MyReadOnlyArray<string>
+>manifest : any
+}
+
+function fn12<T>(arg: T | T[], t: T) {
+>fn12 : <T>(arg: T | T[], t: T) => void
+>arg : T | T[]
+>t : T
+
+    if (Array.isArray(arg)) arg.push(t); // Should OK
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : T | T[]
+>arg.push(t) : number
+>arg.push : (...items: T[]) => number
+>arg : T[]
+>push : (...items: T[]) => number
+>t : T
+}
+
+function fn13<T>(arg: T | ReadonlyArray<T>, t: T) {
+>fn13 : <T>(arg: T | readonly T[], t: T) => void
+>arg : T | readonly T[]
+>t : T
+
+    if (Array.isArray(arg)) arg.push(t); // Should fail
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : T | readonly T[]
+>arg.push(t) : any
+>arg.push : any
+>arg : readonly T[]
+>push : any
+>t : T
+
+    if (Array.isArray(arg)) arg.indexOf(t); // OK
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : T | readonly T[]
+>arg.indexOf(t) : number
+>arg.indexOf : (searchElement: T, fromIndex?: number | undefined) => number
+>arg : readonly T[]
+>indexOf : (searchElement: T, fromIndex?: number | undefined) => number
+>t : T
+}
+
+function fn14<T>(arg: T | [T], t: T) {
+>fn14 : <T>(arg: T | [T], t: T) => void
+>arg : T | [T]
+>t : T
+
+    if (Array.isArray(arg)) arg.push(t); // Should OK
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : T | [T]
+>arg.push(t) : number
+>arg.push : (...items: T[]) => number
+>arg : [T]
+>push : (...items: T[]) => number
+>t : T
+}
+
+function fn15<T>(arg: T | readonly [T], t: T) {
+>fn15 : <T>(arg: T | readonly [T], t: T) => void
+>arg : T | readonly [T]
+>t : T
+
+    if (Array.isArray(arg)) arg.push(t); // Should fail
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : T | readonly [T]
+>arg.push(t) : any
+>arg.push : any
+>arg : readonly [T]
+>push : any
+>t : T
+
+    if (Array.isArray(arg)) arg.indexOf(t); // Should OK
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : T | readonly [T]
+>arg.indexOf(t) : number
+>arg.indexOf : (searchElement: T, fromIndex?: number | undefined) => number
+>arg : readonly [T]
+>indexOf : (searchElement: T, fromIndex?: number | undefined) => number
+>t : T
+}
+
+function fn16<T extends string | string[]>(arg: T) {
+>fn16 : <T extends string | string[]>(arg: T) => void
+>arg : T
+
+    if (Array.isArray(arg)) arg.push("10"); // Should OK
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : T
+>arg.push("10") : number
+>arg.push : (...items: string[]) => number
+>arg : T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>push : (...items: string[]) => number
+>"10" : "10"
+
+    if (Array.isArray(arg)) arg.push(10); // Should fail
+>Array.isArray(arg) : boolean
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>arg : T
+>arg.push(10) : number
+>arg.push : (...items: string[]) => number
+>arg : T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>push : (...items: string[]) => number
+>10 : 10
+}
+
+function fn17() {
+>fn17 : () => void
+
+    const s: Array<string | string[]> = [];
+>s : (string | string[])[]
+>[] : never[]
+
+    const arrs = s.filter(Array.isArray);
+>arrs : string[][]
+>s.filter(Array.isArray) : string[][]
+>s.filter : { <S extends string | string[]>(callbackfn: (value: string | string[], index: number, array: (string | string[])[]) => value is S, thisArg?: any): S[]; (callbackfn: (value: string | string[], index: number, array: (string | string[])[]) => unknown, thisArg?: any): (string | string[])[]; }
+>s : (string | string[])[]
+>filter : { <S extends string | string[]>(callbackfn: (value: string | string[], index: number, array: (string | string[])[]) => value is S, thisArg?: any): S[]; (callbackfn: (value: string | string[], index: number, array: (string | string[])[]) => unknown, thisArg?: any): (string | string[])[]; }
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+>Array : ArrayConstructor
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
+
+    arrs.push(["one"]); // Should OK
+>arrs.push(["one"]) : number
+>arrs.push : (...items: string[][]) => number
+>arrs : string[][]
+>push : (...items: string[][]) => number
+>["one"] : string[]
+>"one" : "one"
+
+    arrs.push("str");  // Should fail
+>arrs.push("str") : number
+>arrs.push : (...items: string[][]) => number
+>arrs : string[][]
+>push : (...items: string[][]) => number
+>"str" : "str"
+}
+
diff --git a/tests/baselines/reference/literalFreshnessPropagationOnNarrowing.types b/tests/baselines/reference/literalFreshnessPropagationOnNarrowing.types
index 8fb03bc413f30..5d6f68578070d 100644
--- a/tests/baselines/reference/literalFreshnessPropagationOnNarrowing.types
+++ b/tests/baselines/reference/literalFreshnessPropagationOnNarrowing.types
@@ -66,9 +66,9 @@ function f2() {
 >a4 : string | false | (string | false)[]
 >Array.isArray(elOrA) ? elOrA : [elOrA] : (string | false)[]
 >Array.isArray(elOrA) : boolean
->Array.isArray : (arg: any) => arg is any[]
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
 >Array : ArrayConstructor
->isArray : (arg: any) => arg is any[]
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
 >elOrA : string | false | (string | false)[]
 >elOrA : (string | false)[]
 >[elOrA] : (string | false)[]
@@ -83,9 +83,9 @@ function f2() {
 >...Array.isArray(elOrA) ? elOrA : [elOrA] : string | false
 >Array.isArray(elOrA) ? elOrA : [elOrA] : (string | false)[]
 >Array.isArray(elOrA) : boolean
->Array.isArray : (arg: any) => arg is any[]
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
 >Array : ArrayConstructor
->isArray : (arg: any) => arg is any[]
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
 >elOrA : string | false | (string | false)[]
 >elOrA : (string | false)[]
 >[elOrA] : (string | false)[]
diff --git a/tests/baselines/reference/malformedTags.types b/tests/baselines/reference/malformedTags.types
index c3af959586790..1d3e2582bafa1 100644
--- a/tests/baselines/reference/malformedTags.types
+++ b/tests/baselines/reference/malformedTags.types
@@ -6,7 +6,7 @@
  */
 var isArray = Array.isArray;
 >isArray : Function
->Array.isArray : (arg: any) => arg is any[]
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
 >Array : ArrayConstructor
->isArray : (arg: any) => arg is any[]
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
 
diff --git a/tests/baselines/reference/parserharness.types b/tests/baselines/reference/parserharness.types
index 5fde386f90acf..55e4c1cbf1e8e 100644
--- a/tests/baselines/reference/parserharness.types
+++ b/tests/baselines/reference/parserharness.types
@@ -3061,27 +3061,27 @@ module Harness {
 >identifier : any
 
             public normalizeToArray(arg: any) {
->normalizeToArray : (arg: any) => any[]
+>normalizeToArray : (arg: any) => any
 >arg : any
 
                 if ((Array.isArray && Array.isArray(arg)) || arg instanceof Array)
 >(Array.isArray && Array.isArray(arg)) || arg instanceof Array : boolean
 >(Array.isArray && Array.isArray(arg)) : boolean
 >Array.isArray && Array.isArray(arg) : boolean
->Array.isArray : (arg: any) => arg is any[]
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
 >Array : ArrayConstructor
->isArray : (arg: any) => arg is any[]
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
 >Array.isArray(arg) : boolean
->Array.isArray : (arg: any) => arg is any[]
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
 >Array : ArrayConstructor
->isArray : (arg: any) => arg is any[]
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
 >arg : any
 >arg instanceof Array : boolean
 >arg : any
 >Array : ArrayConstructor
 
                     return arg;
->arg : any[]
+>arg : any
 
                 return [arg];
 >[arg] : any[]
@@ -3240,12 +3240,12 @@ module Harness {
 >others : any
 
                 others = this.normalizeToArray(others);
->others = this.normalizeToArray(others) : any[]
+>others = this.normalizeToArray(others) : any
 >others : any
->this.normalizeToArray(others) : any[]
->this.normalizeToArray : (arg: any) => any[]
+>this.normalizeToArray(others) : any
+>this.normalizeToArray : (arg: any) => any
 >this : this
->normalizeToArray : (arg: any) => any[]
+>normalizeToArray : (arg: any) => any
 >others : any
 
                 for (var i = 0; i < others.length; i++) {
@@ -3294,12 +3294,12 @@ module Harness {
 >others : any
 
                 others = this.normalizeToArray(others);
->others = this.normalizeToArray(others) : any[]
+>others = this.normalizeToArray(others) : any
 >others : any
->this.normalizeToArray(others) : any[]
->this.normalizeToArray : (arg: any) => any[]
+>this.normalizeToArray(others) : any
+>this.normalizeToArray : (arg: any) => any
 >this : this
->normalizeToArray : (arg: any) => any[]
+>normalizeToArray : (arg: any) => any
 >others : any
 
                 for (var i = 0; i < others.length; i++) {
@@ -3449,12 +3449,12 @@ module Harness {
 >others : any
 
                 others = this.normalizeToArray(others);
->others = this.normalizeToArray(others) : any[]
+>others = this.normalizeToArray(others) : any
 >others : any
->this.normalizeToArray(others) : any[]
->this.normalizeToArray : (arg: any) => any[]
+>this.normalizeToArray(others) : any
+>this.normalizeToArray : (arg: any) => any
 >this : this
->normalizeToArray : (arg: any) => any[]
+>normalizeToArray : (arg: any) => any
 >others : any
 
                 for (var i = 0; i < others.length; i++) {
@@ -3505,12 +3505,12 @@ module Harness {
 >others : any
 
                 others = this.normalizeToArray(others);
->others = this.normalizeToArray(others) : any[]
+>others = this.normalizeToArray(others) : any
 >others : any
->this.normalizeToArray(others) : any[]
->this.normalizeToArray : (arg: any) => any[]
+>this.normalizeToArray(others) : any
+>this.normalizeToArray : (arg: any) => any
 >this : this
->normalizeToArray : (arg: any) => any[]
+>normalizeToArray : (arg: any) => any
 >others : any
 
                 for (var i = 0; i < others.length; i++) {
diff --git a/tests/baselines/reference/partiallyDiscriminantedUnions.types b/tests/baselines/reference/partiallyDiscriminantedUnions.types
index f1d44249c0eb2..0530f42889ade 100644
--- a/tests/baselines/reference/partiallyDiscriminantedUnions.types
+++ b/tests/baselines/reference/partiallyDiscriminantedUnions.types
@@ -77,9 +77,9 @@ function isShape(s : Shapes): s is Shape {
     return !Array.isArray(s);
 >!Array.isArray(s) : boolean
 >Array.isArray(s) : boolean
->Array.isArray : (arg: any) => arg is any[]
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
 >Array : ArrayConstructor
->isArray : (arg: any) => arg is any[]
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
 >s : Shapes
 }
 
diff --git a/tests/baselines/reference/spreadBooleanRespectsFreshness.types b/tests/baselines/reference/spreadBooleanRespectsFreshness.types
index c607422b0629a..f3392084a6cab 100644
--- a/tests/baselines/reference/spreadBooleanRespectsFreshness.types
+++ b/tests/baselines/reference/spreadBooleanRespectsFreshness.types
@@ -22,9 +22,9 @@ foo1 = [...Array.isArray(foo2) ? foo2 : [foo2]];
 >...Array.isArray(foo2) ? foo2 : [foo2] : FooBase
 >Array.isArray(foo2) ? foo2 : [foo2] : FooBase[]
 >Array.isArray(foo2) : boolean
->Array.isArray : (arg: any) => arg is any[]
+>Array.isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
 >Array : ArrayConstructor
->isArray : (arg: any) => arg is any[]
+>isArray : <T>(arg: T) => arg is T extends readonly any[] ? T : any[] extends T ? T & any[] : never
 >foo2 : Foo
 >foo2 : FooBase[]
 >[foo2] : FooBase[]
diff --git a/tests/cases/compiler/isArray.ts b/tests/cases/compiler/isArray.ts
index dbdd875e21ecc..756634e12f80f 100644
--- a/tests/cases/compiler/isArray.ts
+++ b/tests/cases/compiler/isArray.ts
@@ -1,9 +1,86 @@
-var maybeArray: number | number[];
+// @strictNullChecks: true
 
+interface MyArray<T> extends Array<T> { manifest: any; }
+interface MyReadOnlyArray<T> extends ReadonlyArray<T> { manifest: any; }
 
-if (Array.isArray(maybeArray)) {
-    maybeArray.length; // OK
+function fn1(arg: string | string[]) {
+    if (Array.isArray(arg)) arg.push(""); // Should OK
+}
+
+function fn2(arg: unknown) {
+    if (Array.isArray(arg)) arg.push(""); // Should OK
+}
+
+function fn3(arg: object) {
+    if (Array.isArray(arg)) arg.push(""); // Should OK
+}
+
+function fn4(arg: {}) {
+    if (Array.isArray(arg)) arg.push(""); // Should OK
+}
+
+function fn5(arg: string | ReadonlyArray<string>) {
+    if (Array.isArray(arg)) arg.push(10); // Should FAIL
+    if (Array.isArray(arg)) arg.push(""); // Should FAIL
+    if (Array.isArray(arg)) arg.indexOf(""); // Should OK
+    if (!Array.isArray(arg)) arg.toUpperCase(); // Should OK
+}
+
+function fn6(arg: string | string[]) {
+    if (Array.isArray(arg)) arg.push(10); // Should FAIL
+}
+
+function fn7(arg: boolean | number[] | string[], stringAndNumber: string & number) {
+    if (Array.isArray(arg)) arg.push(stringAndNumber); // Should OK
+}
+
+function fn8(arg: string | number[] | readonly string[]) {
+    if (Array.isArray(arg)) arg.push(10); // Should FAIL
+}
+
+function fn9(arg: string | number[] | readonly string[]) {
+    if (Array.isArray(arg)) arg.push(10); // Should FAIL
+}
+
+function fn10(arg: string | MyArray<string>) {
+    if (Array.isArray(arg)) arg.push(10); // Should FAIL
+    if (Array.isArray(arg)) arg.push(""); // Should OK
+    if (Array.isArray(arg)) arg.manifest; // Should OK
+}
+
+function fn11(arg: string | MyReadOnlyArray<string>) {
+    if (Array.isArray(arg)) arg.push(""); // Should FAIL
+    if (Array.isArray(arg)) arg.indexOf(10); // Should FAIL
+    if (Array.isArray(arg)) arg.indexOf(""); // Should OK
+    if (Array.isArray(arg)) arg.manifest; // Should OK
+}
+
+function fn12<T>(arg: T | T[], t: T) {
+    if (Array.isArray(arg)) arg.push(t); // Should OK
+}
+
+function fn13<T>(arg: T | ReadonlyArray<T>, t: T) {
+    if (Array.isArray(arg)) arg.push(t); // Should fail
+    if (Array.isArray(arg)) arg.indexOf(t); // OK
+}
+
+function fn14<T>(arg: T | [T], t: T) {
+    if (Array.isArray(arg)) arg.push(t); // Should OK
+}
+
+function fn15<T>(arg: T | readonly [T], t: T) {
+    if (Array.isArray(arg)) arg.push(t); // Should fail
+    if (Array.isArray(arg)) arg.indexOf(t); // Should OK
+}
+
+function fn16<T extends string | string[]>(arg: T) {
+    if (Array.isArray(arg)) arg.push("10"); // Should OK
+    if (Array.isArray(arg)) arg.push(10); // Should fail
+}
+
+function fn17() {
+    const s: Array<string | string[]> = [];
+    const arrs = s.filter(Array.isArray);
+    arrs.push(["one"]); // Should OK
+    arrs.push("str");  // Should fail
 }
-else {
-    maybeArray.toFixed(); // OK
-}
\ No newline at end of file