diff --git a/tests/baselines/reference/narrowingAssignmentReadonlyRespectsAssertion.js b/tests/baselines/reference/narrowingAssignmentReadonlyRespectsAssertion.js
new file mode 100644
index 0000000000000..5db0ab74ca7ae
--- /dev/null
+++ b/tests/baselines/reference/narrowingAssignmentReadonlyRespectsAssertion.js
@@ -0,0 +1,71 @@
+//// [narrowingAssignmentReadonlyRespectsAssertion.ts]
+// https://github.com/microsoft/TypeScript/issues/41984
+
+interface TestCase<T extends string | number> {
+  readonly val1: T | ReadonlyArray<T>;
+  readonly val2: ReadonlyArray<T>;
+}
+
+interface MultiCaseFixture<T> {
+  cases: T[];
+}
+
+function subDataFunc(): TestCase<string | number>[] {
+  return [
+      { val1: "a", val2: ["a", "b", "c"] },
+      { val1: 2, val2: [1, 2, 3] },
+      { val1: ["a", "z"], val2: ["x", "y", "z"] },
+      { val1: [5, 10], val2: [10, 100, 1000] },
+  ];
+}
+
+function dataFunc<T>(subFunc: () => T[]): MultiCaseFixture<T> {
+  return { cases: subFunc() };
+}
+
+function testFunc() {
+  const fixture = dataFunc<TestCase<string | number>>(subDataFunc);
+  fixture.cases.forEach(({ val1, val2 }) => {
+      if (Array.isArray(val1)) {
+          // This should retain val1 as being an array
+          const reversedVal1 = val1.slice().reverse();
+          console.log(reversedVal1);
+      } else {
+          console.log(val1);
+      }
+      console.log(val2);
+  });
+}
+
+testFunc();
+
+
+//// [narrowingAssignmentReadonlyRespectsAssertion.js]
+// https://github.com/microsoft/TypeScript/issues/41984
+function subDataFunc() {
+    return [
+        { val1: "a", val2: ["a", "b", "c"] },
+        { val1: 2, val2: [1, 2, 3] },
+        { val1: ["a", "z"], val2: ["x", "y", "z"] },
+        { val1: [5, 10], val2: [10, 100, 1000] },
+    ];
+}
+function dataFunc(subFunc) {
+    return { cases: subFunc() };
+}
+function testFunc() {
+    var fixture = dataFunc(subDataFunc);
+    fixture.cases.forEach(function (_a) {
+        var val1 = _a.val1, val2 = _a.val2;
+        if (Array.isArray(val1)) {
+            // This should retain val1 as being an array
+            var reversedVal1 = val1.slice().reverse();
+            console.log(reversedVal1);
+        }
+        else {
+            console.log(val1);
+        }
+        console.log(val2);
+    });
+}
+testFunc();
diff --git a/tests/baselines/reference/narrowingAssignmentReadonlyRespectsAssertion.symbols b/tests/baselines/reference/narrowingAssignmentReadonlyRespectsAssertion.symbols
new file mode 100644
index 0000000000000..8b7fcfebae44f
--- /dev/null
+++ b/tests/baselines/reference/narrowingAssignmentReadonlyRespectsAssertion.symbols
@@ -0,0 +1,123 @@
+=== tests/cases/compiler/narrowingAssignmentReadonlyRespectsAssertion.ts ===
+// https://github.com/microsoft/TypeScript/issues/41984
+
+interface TestCase<T extends string | number> {
+>TestCase : Symbol(TestCase, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 0, 0))
+>T : Symbol(T, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 2, 19))
+
+  readonly val1: T | ReadonlyArray<T>;
+>val1 : Symbol(TestCase.val1, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 2, 47))
+>T : Symbol(T, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 2, 19))
+>ReadonlyArray : Symbol(ReadonlyArray, Decl(lib.es5.d.ts, --, --))
+>T : Symbol(T, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 2, 19))
+
+  readonly val2: ReadonlyArray<T>;
+>val2 : Symbol(TestCase.val2, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 3, 38))
+>ReadonlyArray : Symbol(ReadonlyArray, Decl(lib.es5.d.ts, --, --))
+>T : Symbol(T, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 2, 19))
+}
+
+interface MultiCaseFixture<T> {
+>MultiCaseFixture : Symbol(MultiCaseFixture, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 5, 1))
+>T : Symbol(T, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 7, 27))
+
+  cases: T[];
+>cases : Symbol(MultiCaseFixture.cases, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 7, 31))
+>T : Symbol(T, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 7, 27))
+}
+
+function subDataFunc(): TestCase<string | number>[] {
+>subDataFunc : Symbol(subDataFunc, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 9, 1))
+>TestCase : Symbol(TestCase, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 0, 0))
+
+  return [
+      { val1: "a", val2: ["a", "b", "c"] },
+>val1 : Symbol(val1, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 13, 7))
+>val2 : Symbol(val2, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 13, 18))
+
+      { val1: 2, val2: [1, 2, 3] },
+>val1 : Symbol(val1, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 14, 7))
+>val2 : Symbol(val2, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 14, 16))
+
+      { val1: ["a", "z"], val2: ["x", "y", "z"] },
+>val1 : Symbol(val1, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 15, 7))
+>val2 : Symbol(val2, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 15, 25))
+
+      { val1: [5, 10], val2: [10, 100, 1000] },
+>val1 : Symbol(val1, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 16, 7))
+>val2 : Symbol(val2, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 16, 22))
+
+  ];
+}
+
+function dataFunc<T>(subFunc: () => T[]): MultiCaseFixture<T> {
+>dataFunc : Symbol(dataFunc, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 18, 1))
+>T : Symbol(T, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 20, 18))
+>subFunc : Symbol(subFunc, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 20, 21))
+>T : Symbol(T, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 20, 18))
+>MultiCaseFixture : Symbol(MultiCaseFixture, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 5, 1))
+>T : Symbol(T, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 20, 18))
+
+  return { cases: subFunc() };
+>cases : Symbol(cases, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 21, 10))
+>subFunc : Symbol(subFunc, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 20, 21))
+}
+
+function testFunc() {
+>testFunc : Symbol(testFunc, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 22, 1))
+
+  const fixture = dataFunc<TestCase<string | number>>(subDataFunc);
+>fixture : Symbol(fixture, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 25, 7))
+>dataFunc : Symbol(dataFunc, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 18, 1))
+>TestCase : Symbol(TestCase, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 0, 0))
+>subDataFunc : Symbol(subDataFunc, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 9, 1))
+
+  fixture.cases.forEach(({ val1, val2 }) => {
+>fixture.cases.forEach : Symbol(Array.forEach, Decl(lib.es5.d.ts, --, --))
+>fixture.cases : Symbol(MultiCaseFixture.cases, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 7, 31))
+>fixture : Symbol(fixture, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 25, 7))
+>cases : Symbol(MultiCaseFixture.cases, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 7, 31))
+>forEach : Symbol(Array.forEach, Decl(lib.es5.d.ts, --, --))
+>val1 : Symbol(val1, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 26, 26))
+>val2 : Symbol(val2, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 26, 32))
+
+      if (Array.isArray(val1)) {
+>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, --, --))
+>val1 : Symbol(val1, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 26, 26))
+
+          // This should retain val1 as being an array
+          const reversedVal1 = val1.slice().reverse();
+>reversedVal1 : Symbol(reversedVal1, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 29, 15))
+>val1.slice().reverse : Symbol(Array.reverse, Decl(lib.es5.d.ts, --, --))
+>val1.slice : Symbol(Array.slice, Decl(lib.es5.d.ts, --, --))
+>val1 : Symbol(val1, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 26, 26))
+>slice : Symbol(Array.slice, Decl(lib.es5.d.ts, --, --))
+>reverse : Symbol(Array.reverse, Decl(lib.es5.d.ts, --, --))
+
+          console.log(reversedVal1);
+>console.log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>console : Symbol(console, Decl(lib.dom.d.ts, --, --))
+>log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>reversedVal1 : Symbol(reversedVal1, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 29, 15))
+
+      } else {
+          console.log(val1);
+>console.log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>console : Symbol(console, Decl(lib.dom.d.ts, --, --))
+>log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>val1 : Symbol(val1, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 26, 26))
+      }
+      console.log(val2);
+>console.log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>console : Symbol(console, Decl(lib.dom.d.ts, --, --))
+>log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>val2 : Symbol(val2, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 26, 32))
+
+  });
+}
+
+testFunc();
+>testFunc : Symbol(testFunc, Decl(narrowingAssignmentReadonlyRespectsAssertion.ts, 22, 1))
+
diff --git a/tests/baselines/reference/narrowingAssignmentReadonlyRespectsAssertion.types b/tests/baselines/reference/narrowingAssignmentReadonlyRespectsAssertion.types
new file mode 100644
index 0000000000000..9fd13d661027a
--- /dev/null
+++ b/tests/baselines/reference/narrowingAssignmentReadonlyRespectsAssertion.types
@@ -0,0 +1,147 @@
+=== tests/cases/compiler/narrowingAssignmentReadonlyRespectsAssertion.ts ===
+// https://github.com/microsoft/TypeScript/issues/41984
+
+interface TestCase<T extends string | number> {
+  readonly val1: T | ReadonlyArray<T>;
+>val1 : T | readonly T[]
+
+  readonly val2: ReadonlyArray<T>;
+>val2 : readonly T[]
+}
+
+interface MultiCaseFixture<T> {
+  cases: T[];
+>cases : T[]
+}
+
+function subDataFunc(): TestCase<string | number>[] {
+>subDataFunc : () => TestCase<string | number>[]
+
+  return [
+>[      { val1: "a", val2: ["a", "b", "c"] },      { val1: 2, val2: [1, 2, 3] },      { val1: ["a", "z"], val2: ["x", "y", "z"] },      { val1: [5, 10], val2: [10, 100, 1000] },  ] : ({ val1: string; val2: string[]; } | { val1: number; val2: number[]; } | { val1: string[]; val2: string[]; } | { val1: number[]; val2: number[]; })[]
+
+      { val1: "a", val2: ["a", "b", "c"] },
+>{ val1: "a", val2: ["a", "b", "c"] } : { val1: string; val2: string[]; }
+>val1 : string
+>"a" : "a"
+>val2 : string[]
+>["a", "b", "c"] : string[]
+>"a" : "a"
+>"b" : "b"
+>"c" : "c"
+
+      { val1: 2, val2: [1, 2, 3] },
+>{ val1: 2, val2: [1, 2, 3] } : { val1: number; val2: number[]; }
+>val1 : number
+>2 : 2
+>val2 : number[]
+>[1, 2, 3] : number[]
+>1 : 1
+>2 : 2
+>3 : 3
+
+      { val1: ["a", "z"], val2: ["x", "y", "z"] },
+>{ val1: ["a", "z"], val2: ["x", "y", "z"] } : { val1: string[]; val2: string[]; }
+>val1 : string[]
+>["a", "z"] : string[]
+>"a" : "a"
+>"z" : "z"
+>val2 : string[]
+>["x", "y", "z"] : string[]
+>"x" : "x"
+>"y" : "y"
+>"z" : "z"
+
+      { val1: [5, 10], val2: [10, 100, 1000] },
+>{ val1: [5, 10], val2: [10, 100, 1000] } : { val1: number[]; val2: number[]; }
+>val1 : number[]
+>[5, 10] : number[]
+>5 : 5
+>10 : 10
+>val2 : number[]
+>[10, 100, 1000] : number[]
+>10 : 10
+>100 : 100
+>1000 : 1000
+
+  ];
+}
+
+function dataFunc<T>(subFunc: () => T[]): MultiCaseFixture<T> {
+>dataFunc : <T>(subFunc: () => T[]) => MultiCaseFixture<T>
+>subFunc : () => T[]
+
+  return { cases: subFunc() };
+>{ cases: subFunc() } : { cases: T[]; }
+>cases : T[]
+>subFunc() : T[]
+>subFunc : () => T[]
+}
+
+function testFunc() {
+>testFunc : () => void
+
+  const fixture = dataFunc<TestCase<string | number>>(subDataFunc);
+>fixture : MultiCaseFixture<TestCase<string | number>>
+>dataFunc<TestCase<string | number>>(subDataFunc) : MultiCaseFixture<TestCase<string | number>>
+>dataFunc : <T>(subFunc: () => T[]) => MultiCaseFixture<T>
+>subDataFunc : () => TestCase<string | number>[]
+
+  fixture.cases.forEach(({ val1, val2 }) => {
+>fixture.cases.forEach(({ val1, val2 }) => {      if (Array.isArray(val1)) {          // This should retain val1 as being an array          const reversedVal1 = val1.slice().reverse();          console.log(reversedVal1);      } else {          console.log(val1);      }      console.log(val2);  }) : void
+>fixture.cases.forEach : (callbackfn: (value: TestCase<string | number>, index: number, array: TestCase<string | number>[]) => void, thisArg?: any) => void
+>fixture.cases : TestCase<string | number>[]
+>fixture : MultiCaseFixture<TestCase<string | number>>
+>cases : TestCase<string | number>[]
+>forEach : (callbackfn: (value: TestCase<string | number>, index: number, array: TestCase<string | number>[]) => void, thisArg?: any) => void
+>({ val1, val2 }) => {      if (Array.isArray(val1)) {          // This should retain val1 as being an array          const reversedVal1 = val1.slice().reverse();          console.log(reversedVal1);      } else {          console.log(val1);      }      console.log(val2);  } : ({ val1, val2 }: TestCase<string | number>) => void
+>val1 : string | number | readonly (string | number)[]
+>val2 : readonly (string | number)[]
+
+      if (Array.isArray(val1)) {
+>Array.isArray(val1) : boolean
+>Array.isArray : (arg: any) => arg is any[]
+>Array : ArrayConstructor
+>isArray : (arg: any) => arg is any[]
+>val1 : string | number | readonly (string | number)[]
+
+          // This should retain val1 as being an array
+          const reversedVal1 = val1.slice().reverse();
+>reversedVal1 : any[]
+>val1.slice().reverse() : any[]
+>val1.slice().reverse : () => any[]
+>val1.slice() : any[]
+>val1.slice : (start?: number, end?: number) => any[]
+>val1 : any[]
+>slice : (start?: number, end?: number) => any[]
+>reverse : () => any[]
+
+          console.log(reversedVal1);
+>console.log(reversedVal1) : void
+>console.log : (...data: any[]) => void
+>console : Console
+>log : (...data: any[]) => void
+>reversedVal1 : any[]
+
+      } else {
+          console.log(val1);
+>console.log(val1) : void
+>console.log : (...data: any[]) => void
+>console : Console
+>log : (...data: any[]) => void
+>val1 : string | number | readonly (string | number)[]
+      }
+      console.log(val2);
+>console.log(val2) : void
+>console.log : (...data: any[]) => void
+>console : Console
+>log : (...data: any[]) => void
+>val2 : readonly (string | number)[]
+
+  });
+}
+
+testFunc();
+>testFunc() : void
+>testFunc : () => void
+
diff --git a/tests/cases/compiler/narrowingAssignmentReadonlyRespectsAssertion.ts b/tests/cases/compiler/narrowingAssignmentReadonlyRespectsAssertion.ts
new file mode 100644
index 0000000000000..adc138d60d466
--- /dev/null
+++ b/tests/cases/compiler/narrowingAssignmentReadonlyRespectsAssertion.ts
@@ -0,0 +1,39 @@
+// https://github.com/microsoft/TypeScript/issues/41984
+
+interface TestCase<T extends string | number> {
+  readonly val1: T | ReadonlyArray<T>;
+  readonly val2: ReadonlyArray<T>;
+}
+
+interface MultiCaseFixture<T> {
+  cases: T[];
+}
+
+function subDataFunc(): TestCase<string | number>[] {
+  return [
+      { val1: "a", val2: ["a", "b", "c"] },
+      { val1: 2, val2: [1, 2, 3] },
+      { val1: ["a", "z"], val2: ["x", "y", "z"] },
+      { val1: [5, 10], val2: [10, 100, 1000] },
+  ];
+}
+
+function dataFunc<T>(subFunc: () => T[]): MultiCaseFixture<T> {
+  return { cases: subFunc() };
+}
+
+function testFunc() {
+  const fixture = dataFunc<TestCase<string | number>>(subDataFunc);
+  fixture.cases.forEach(({ val1, val2 }) => {
+      if (Array.isArray(val1)) {
+          // This should retain val1 as being an array
+          const reversedVal1 = val1.slice().reverse();
+          console.log(reversedVal1);
+      } else {
+          console.log(val1);
+      }
+      console.log(val2);
+  });
+}
+
+testFunc();