Adds support for type parameter defaults

src/compiler/declarationEmitter.ts

@@ -1000,6 +1000,23 @@ namespace ts {
                         emitTypeWithNewGetSymbolAccessibilityDiagnostic(node.constraint, getTypeParameterConstraintVisibilityError);
                     }
                 }
+                if (node.default && !isPrivateMethodTypeParameter(node)) {
+                    write(" = ");
+                    if (node.parent.kind === SyntaxKind.FunctionType ||
+                        node.parent.kind === SyntaxKind.ConstructorType ||
+                        (node.parent.parent && node.parent.parent.kind === SyntaxKind.TypeLiteral)) {
+                        Debug.assert(node.parent.kind === SyntaxKind.MethodDeclaration ||
+                            node.parent.kind === SyntaxKind.MethodSignature ||
+                            node.parent.kind === SyntaxKind.FunctionType ||
+                            node.parent.kind === SyntaxKind.ConstructorType ||
+                            node.parent.kind === SyntaxKind.CallSignature ||
+                            node.parent.kind === SyntaxKind.ConstructSignature);
+                        emitType(node.default);
+                    }
+                    else {
+                        emitTypeWithNewGetSymbolAccessibilityDiagnostic(node.default, getTypeParameterConstraintVisibilityError);
+                    }
+                }
 
                 function getTypeParameterConstraintVisibilityError(): SymbolAccessibilityDiagnostic {
                     // Type parameter constraints are named by user so we should always be able to name it

src/compiler/diagnosticMessages.json

@@ -2035,6 +2035,18 @@
         "category": "Error",
         "code": 2704
     },
+    "Required type parameters may not follow optional type parameters.": {
+        "category": "Error",
+        "code": 2705
+    },
+     "Type parameter '{0}' has a circular default.": {
+        "category": "Error",
+        "code": 2706
+    },
+     "Generic type '{0}' requires between {1} and {2} type arguments.": {
+        "category": "Error",
+        "code": 2707
+    },
 
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",

src/compiler/parser.ts

@@ -66,6 +66,7 @@ namespace ts {
             case SyntaxKind.TypeParameter:
                 return visitNode(cbNode, (<TypeParameterDeclaration>node).name) ||
                     visitNode(cbNode, (<TypeParameterDeclaration>node).constraint) ||
+                    visitNode(cbNode, (<TypeParameterDeclaration>node).default) ||
                     visitNode(cbNode, (<TypeParameterDeclaration>node).expression);
             case SyntaxKind.ShorthandPropertyAssignment:
                 return visitNodes(cbNodes, node.decorators) ||
@@ -2098,6 +2099,10 @@ namespace ts {
                 }
             }
 
+            if (parseOptional(SyntaxKind.EqualsToken)) {
+                node.default = parseType();
+            }
+
             return finishNode(node);
         }
 

src/compiler/types.ts

@@ -604,6 +604,7 @@ namespace ts {
         kind: SyntaxKind.TypeParameter;
         name: Identifier;
         constraint?: TypeNode;
+        default?: TypeNode;
 
         // For error recovery purposes.
         expression?: Expression;
@@ -2679,6 +2680,7 @@ namespace ts {
         type?: Type;                        // Type of value symbol
         declaredType?: Type;                // Type of class, interface, enum, type alias, or type parameter
         typeParameters?: TypeParameter[];   // Type parameters of type alias (undefined if non-generic)
+        minTypeArgumentCount?: number;
         inferredClassType?: Type;           // Type of an inferred ES5 class
         instantiations?: Map<Type>;         // Instantiations of generic type alias (undefined if non-generic)
         mapper?: TypeMapper;                // Type mapper for instantiation alias
@@ -2880,6 +2882,8 @@ namespace ts {
         typeParameters: TypeParameter[];           // Type parameters (undefined if non-generic)
         outerTypeParameters: TypeParameter[];      // Outer type parameters (undefined if none)
         localTypeParameters: TypeParameter[];      // Local type parameters (undefined if none)
+        /* @internal */
+        minTypeArgumentCount: number;
         thisType: TypeParameter;                   // The "this" type (undefined if none)
         /* @internal */
         resolvedBaseConstructorType?: Type;        // Resolved base constructor type of class
@@ -2986,18 +2990,23 @@ namespace ts {
         /* @internal */
         resolvedBaseConstraint: Type;
         /* @internal */
+        resolvedDefault: Type;
+        /* @internal */
         resolvedIndexType: IndexType;
     }
 
     // Type parameters (TypeFlags.TypeParameter)
     export interface TypeParameter extends TypeVariable {
         constraint: Type;        // Constraint
+        default?: Type;
         /* @internal */
         target?: TypeParameter;  // Instantiation target
         /* @internal */
         mapper?: TypeMapper;     // Instantiation mapper
         /* @internal */
         isThisType?: boolean;
+        /* @internal */
+        resolvedDefaultType?: Type;
     }
 
     // Indexed access types (TypeFlags.IndexedAccess)
@@ -3021,6 +3030,8 @@ namespace ts {
     export interface Signature {
         declaration: SignatureDeclaration;  // Originating declaration
         typeParameters: TypeParameter[];    // Type parameters (undefined if non-generic)
+        /* @internal */
+        minTypeArgumentCount: number;       // Number of non-optional type parameters
         parameters: Symbol[];               // Parameters
         /* @internal */
         thisParameter?: Symbol;             // symbol of this-type parameter
@@ -3084,6 +3095,7 @@ namespace ts {
         inferUnionTypes: boolean;           // Infer union types for disjoint candidates (otherwise undefinedType)
         inferences: TypeInferences[];       // Inferences made for each type parameter
         inferredTypes: Type[];              // Inferred type for each type parameter
+        suppliedTypes?: Type[];             // Supplied types for non-default type parameters
         mapper?: TypeMapper;                // Type mapper for this inference context
         failedTypeParameterIndex?: number;  // Index of type parameter for which inference failed
         // It is optional because in contextual signature instantiation, nothing fails

src/compiler/utilities.ts

@@ -24,6 +24,20 @@ namespace ts {
         return undefined;
     }
 
+    export function findDeclaration<T extends Declaration>(symbol: Symbol, predicate: (node: Declaration) => node is T): T | undefined;
+    export function findDeclaration(symbol: Symbol, predicate: (node: Declaration) => boolean): Declaration | undefined;
+    export function findDeclaration(symbol: Symbol, predicate: (node: Declaration) => boolean): Declaration | undefined {
+        const declarations = symbol.declarations;
+        if (declarations) {
+            for (const declaration of declarations) {
+                if (predicate(declaration)) {
+                    return declaration;
+                }
+            }
+        }
+        return undefined;
+    }
+
     export interface StringSymbolWriter extends SymbolWriter {
         string(): string;
     }

src/lib/es2015.promise.d.ts

@@ -8,45 +8,14 @@ interface Promise<T> {
      * @param onrejected The callback to execute when the Promise is rejected.
      * @returns A Promise for the completion of which ever callback is executed.
      */
-    then(onfulfilled?: ((value: T) => T | PromiseLike<T>) | undefined | null, onrejected?: ((reason: any) => T | PromiseLike<T>) | undefined | null): Promise<T>;
-
-    /**
-     * Attaches callbacks for the resolution and/or rejection of the Promise.
-     * @param onfulfilled The callback to execute when the Promise is resolved.
-     * @param onrejected The callback to execute when the Promise is rejected.
-     * @returns A Promise for the completion of which ever callback is executed.
-     */
-    then<TResult>(onfulfilled: ((value: T) => T | PromiseLike<T>) | undefined | null, onrejected: (reason: any) => TResult | PromiseLike<TResult>): Promise<T | TResult>;
-
-    /**
-     * Attaches callbacks for the resolution and/or rejection of the Promise.
-     * @param onfulfilled The callback to execute when the Promise is resolved.
-     * @param onrejected The callback to execute when the Promise is rejected.
-     * @returns A Promise for the completion of which ever callback is executed.
-     */
-    then<TResult>(onfulfilled: (value: T) => TResult | PromiseLike<TResult>, onrejected?: ((reason: any) => TResult | PromiseLike<TResult>) | undefined | null): Promise<TResult>;
-
-    /**
-     * Attaches callbacks for the resolution and/or rejection of the Promise.
-     * @param onfulfilled The callback to execute when the Promise is resolved.
-     * @param onrejected The callback to execute when the Promise is rejected.
-     * @returns A Promise for the completion of which ever callback is executed.
-     */
-    then<TResult1, TResult2>(onfulfilled: (value: T) => TResult1 | PromiseLike<TResult1>, onrejected: (reason: any) => TResult2 | PromiseLike<TResult2>): Promise<TResult1 | TResult2>;
-
-    /**
-     * Attaches a callback for only the rejection of the Promise.
-     * @param onrejected The callback to execute when the Promise is rejected.
-     * @returns A Promise for the completion of the callback.
-     */
-    catch(onrejected?: ((reason: any) => T | PromiseLike<T>) | undefined | null): Promise<T>;
+    then<TResult1 = T, TResult2 = never>(onfulfilled?: ((value: T) => TResult1 | PromiseLike<TResult1>) | undefined | null, onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | undefined | null): Promise<TResult1 | TResult2>;
 
     /**
      * Attaches a callback for only the rejection of the Promise.
      * @param onrejected The callback to execute when the Promise is rejected.
      * @returns A Promise for the completion of the callback.
      */
-    catch<TResult>(onrejected: (reason: any) => TResult | PromiseLike<TResult>): Promise<T | TResult>;
+    catch<TResult = never>(onrejected?: ((reason: any) => TResult | PromiseLike<TResult>) | undefined | null): Promise<T | TResult>;
 }
 
 interface PromiseConstructor {

src/lib/es5.d.ts

@@ -1316,39 +1316,7 @@ interface PromiseLike<T> {
      * @param onrejected The callback to execute when the Promise is rejected.
      * @returns A Promise for the completion of which ever callback is executed.
      */
-    then(
-        onfulfilled?: ((value: T) => T | PromiseLike<T>) | undefined | null,
-        onrejected?: ((reason: any) => T | PromiseLike<T>) | undefined | null): PromiseLike<T>;
-
-    /**
-     * Attaches callbacks for the resolution and/or rejection of the Promise.
-     * @param onfulfilled The callback to execute when the Promise is resolved.
-     * @param onrejected The callback to execute when the Promise is rejected.
-     * @returns A Promise for the completion of which ever callback is executed.
-     */
-    then<TResult>(
-        onfulfilled: ((value: T) => T | PromiseLike<T>) | undefined | null,
-        onrejected: (reason: any) => TResult | PromiseLike<TResult>): PromiseLike<T | TResult>;
-
-    /**
-     * Attaches callbacks for the resolution and/or rejection of the Promise.
-     * @param onfulfilled The callback to execute when the Promise is resolved.
-     * @param onrejected The callback to execute when the Promise is rejected.
-     * @returns A Promise for the completion of which ever callback is executed.
-     */
-    then<TResult>(
-        onfulfilled: (value: T) => TResult | PromiseLike<TResult>,
-        onrejected?: ((reason: any) => TResult | PromiseLike<TResult>) | undefined | null): PromiseLike<TResult>;
-
-    /**
-     * Attaches callbacks for the resolution and/or rejection of the Promise.
-     * @param onfulfilled The callback to execute when the Promise is resolved.
-     * @param onrejected The callback to execute when the Promise is rejected.
-     * @returns A Promise for the completion of which ever callback is executed.
-     */
-    then<TResult1, TResult2>(
-        onfulfilled: (value: T) => TResult1 | PromiseLike<TResult1>,
-        onrejected: (reason: any) => TResult2 | PromiseLike<TResult2>): PromiseLike<TResult1 | TResult2>;
+    then<TResult1 = T, TResult2 = never>(onfulfilled?: ((value: T) => TResult1 | PromiseLike<TResult1>) | undefined | null, onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | undefined | null): PromiseLike<TResult1 | TResult2>;
 }
 
 interface ArrayLike<T> {

src/services/services.ts

@@ -396,6 +396,7 @@ namespace ts {
         parameters: Symbol[];
         thisParameter: Symbol;
         resolvedReturnType: Type;
+        minTypeArgumentCount: number;
         minArgumentCount: number;
         hasRestParameter: boolean;
         hasLiteralTypes: boolean;

tests/baselines/reference/asyncFunctionDeclaration15_es5.errors.txt

@@ -6,7 +6,7 @@ tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration1
 tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration15_es5.ts(10,23): error TS1055: Type 'typeof Thenable' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.
   Type 'Thenable' is not assignable to type 'PromiseLike<any>'.
     Types of property 'then' are incompatible.
-      Type '() => void' is not assignable to type '{ (onfulfilled?: (value: any) => any, onrejected?: (reason: any) => any): PromiseLike<any>; <TResult>(onfulfilled: (value: any) => any, onrejected: (reason: any) => TResult | PromiseLike<TResult>): PromiseLike<any>; <TResult>(onfulfilled: (value: any) => TResult | PromiseLike<TResult>, onrejected?: (reason: any) => TResult | PromiseLike<TResult>): PromiseLike<TResult>; <TResult1, TResult2>(onfulfilled: (value: any) => TResult1 | PromiseLike<TResult1>, onrejected: (reason: any) => TResult2 | PromiseLike<TResult2>): PromiseLike<TResult1 | TResult2>; }'.
+      Type '() => void' is not assignable to type '<TResult1 = any, TResult2 = never>(onfulfilled?: (value: any) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => PromiseLike<TResult1 | TResult2>'.
         Type 'void' is not assignable to type 'PromiseLike<any>'.
 tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration15_es5.ts(17,16): error TS1059: Return expression in async function does not have a valid callable 'then' member.
 tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration15_es5.ts(23,25): error TS1058: Operand for 'await' does not have a valid callable 'then' member.
@@ -37,7 +37,7 @@ tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration1
 !!! error TS1055: Type 'typeof Thenable' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.
 !!! error TS1055:   Type 'Thenable' is not assignable to type 'PromiseLike<any>'.
 !!! error TS1055:     Types of property 'then' are incompatible.
-!!! error TS1055:       Type '() => void' is not assignable to type '{ (onfulfilled?: (value: any) => any, onrejected?: (reason: any) => any): PromiseLike<any>; <TResult>(onfulfilled: (value: any) => any, onrejected: (reason: any) => TResult | PromiseLike<TResult>): PromiseLike<any>; <TResult>(onfulfilled: (value: any) => TResult | PromiseLike<TResult>, onrejected?: (reason: any) => TResult | PromiseLike<TResult>): PromiseLike<TResult>; <TResult1, TResult2>(onfulfilled: (value: any) => TResult1 | PromiseLike<TResult1>, onrejected: (reason: any) => TResult2 | PromiseLike<TResult2>): PromiseLike<TResult1 | TResult2>; }'.
+!!! error TS1055:       Type '() => void' is not assignable to type '<TResult1 = any, TResult2 = never>(onfulfilled?: (value: any) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => PromiseLike<TResult1 | TResult2>'.
 !!! error TS1055:         Type 'void' is not assignable to type 'PromiseLike<any>'.
     async function fn7() { return; } // valid: Promise<void>
     async function fn8() { return 1; } // valid: Promise<number>