Merge pull request #16305 from Microsoft/contextualGenericTypes

Contextual generic function types

Author: ahejlsberg

src/compiler/checker.ts

@@ -10261,7 +10261,7 @@ namespace ts {
             const objectFlags = getObjectFlags(type);
             return !!(type.flags & TypeFlags.TypeVariable ||
                 objectFlags & ObjectFlags.Reference && forEach((<TypeReference>type).typeArguments, couldContainTypeVariables) ||
-                objectFlags & ObjectFlags.Anonymous && type.symbol && type.symbol.flags & (SymbolFlags.Method | SymbolFlags.TypeLiteral | SymbolFlags.Class) ||
+                objectFlags & ObjectFlags.Anonymous && type.symbol && type.symbol.flags & (SymbolFlags.Function | SymbolFlags.Method | SymbolFlags.TypeLiteral | SymbolFlags.Class) ||
                 objectFlags & ObjectFlags.Mapped ||
                 type.flags & TypeFlags.UnionOrIntersection && couldUnionOrIntersectionContainTypeVariables(<UnionOrIntersectionType>type));
         }
@@ -13066,13 +13066,13 @@ namespace ts {
             return node ? node.contextualMapper : identityMapper;
         }
 
-        // If the given type is an object or union type, if that type has a single signature, and if
-        // that signature is non-generic, return the signature. Otherwise return undefined.
-        function getNonGenericSignature(type: Type, node: FunctionExpression | ArrowFunction | MethodDeclaration): Signature {
+        // If the given type is an object or union type with a single signature, and if that signature has at
+        // least as many parameters as the given function, return the signature. Otherwise return undefined.
+        function getContextualCallSignature(type: Type, node: FunctionExpression | ArrowFunction | MethodDeclaration): Signature {
             const signatures = getSignaturesOfStructuredType(type, SignatureKind.Call);
             if (signatures.length === 1) {
                 const signature = signatures[0];
-                if (!signature.typeParameters && !isAritySmaller(signature, node)) {
+                if (!isAritySmaller(signature, node)) {
                     return signature;
                 }
             }
@@ -13123,12 +13123,12 @@ namespace ts {
                 return undefined;
             }
             if (!(type.flags & TypeFlags.Union)) {
-                return getNonGenericSignature(type, node);
+                return getContextualCallSignature(type, node);
             }
             let signatureList: Signature[];
             const types = (<UnionType>type).types;
             for (const current of types) {
-                const signature = getNonGenericSignature(current, node);
+                const signature = getContextualCallSignature(current, node);
                 if (signature) {
                     if (!signatureList) {
                         // This signature will contribute to contextual union signature
@@ -14988,11 +14988,21 @@ namespace ts {
                     // We clone the contextual mapper to avoid disturbing a resolution in progress for an
                     // outer call expression. Effectively we just want a snapshot of whatever has been
                     // inferred for any outer call expression so far.
-                    const mapper = cloneTypeMapper(getContextualMapper(node));
-                    const instantiatedType = instantiateType(contextualType, mapper);
-                    const returnType = getReturnTypeOfSignature(signature);
-                    // Inferences made from return types have lower priority than all other inferences.
-                    inferTypes(context.inferences, instantiatedType, returnType, InferencePriority.ReturnType);
+                    const instantiatedType = instantiateType(contextualType, cloneTypeMapper(getContextualMapper(node)));
+                    // If the contextual type is a generic pure function type, we instantiate the type with
+                    // its own type parameters and type arguments. This ensures that the type parameters are
+                    // not erased to type any during type inference such that they can be inferred as actual
+                    // types from the contextual type. For example:
+                    //   declare function arrayMap<T, U>(f: (x: T) => U): (a: T[]) => U[];
+                    //   const boxElements: <A>(a: A[]) => { value: A }[] = arrayMap(value => ({ value }));
+                    // Above, the type of the 'value' parameter is inferred to be 'A'.
+                    const contextualSignature = getSingleCallSignature(instantiatedType);
+                    const inferenceSourceType = contextualSignature && contextualSignature.typeParameters ?
+                        getOrCreateTypeFromSignature(getSignatureInstantiation(contextualSignature, contextualSignature.typeParameters)) :
+                        instantiatedType;
+                    const inferenceTargetType = getReturnTypeOfSignature(signature);
+                     // Inferences made from return types have lower priority than all other inferences.
+                    inferTypes(context.inferences, inferenceSourceType, inferenceTargetType, InferencePriority.ReturnType);
                 }
             }
 

tests/baselines/reference/contextualTypingWithGenericSignature.types

@@ -16,10 +16,10 @@ var f2: {
 };
 
 f2 = (x, y) => { return x }
->f2 = (x, y) => { return x } : (x: any, y: any) => any
+>f2 = (x, y) => { return x } : (x: T, y: U) => T
 >f2 : <T, U>(x: T, y: U) => T
->(x, y) => { return x } : (x: any, y: any) => any
->x : any
->y : any
->x : any
+>(x, y) => { return x } : (x: T, y: U) => T
+>x : T
+>y : U
+>x : T
 

tests/baselines/reference/genericContextualTypes1.js

@@ -0,0 +1,107 @@
+//// [genericContextualTypes1.ts]
+type Box<T> = { value: T };
+
+declare function wrap<A, B>(f: (a: A) => B): (a: A) => B;
+
+declare function compose<A, B, C>(f: (a: A) => B, g: (b: B) => C): (a: A) => C;
+
+declare function list<T>(a: T): T[];
+
+declare function unlist<T>(a: T[]): T;
+
+declare function box<V>(x: V): Box<V>;
+
+declare function unbox<W>(x: Box<W>): W;
+
+declare function map<T, U>(a: T[], f: (x: T) => U): U[];
+
+declare function identity<T>(x: T): T;
+
+declare function zip<A, B>(a: A, b: B): [A, B];
+
+declare function flip<X, Y, Z>(f: (x: X, y: Y) => Z): (y: Y, x: X) => Z;
+
+const f00: <A>(x: A) => A[] = list;
+const f01: <A>(x: A) => A[] = x => [x];
+const f02: <A>(x: A) => A[] = wrap(list);
+const f03: <A>(x: A) => A[] = wrap(x => [x]);
+
+const f10: <T>(x: T) => Box<T[]> = compose(a => list(a), b => box(b));
+const f11: <T>(x: T) => Box<T[]> = compose(list, box);
+const f12: <T>(x: Box<T[]>) => T = compose(a => unbox(a), b => unlist(b));
+const f13: <T>(x: Box<T[]>) => T = compose(unbox, unlist);
+
+const arrayMap = <T, U>(f: (x: T) => U) => (a: T[]) => a.map(f);
+const arrayFilter = <T>(f: (x: T) => boolean) => (a: T[]) => a.filter(f);
+
+const f20: (a: string[]) => number[] = arrayMap(x => x.length);
+const f21: <A>(a: A[]) => A[][] = arrayMap(x => [x]);
+const f22: <A>(a: A[]) => A[] = arrayMap(identity);
+const f23: <A>(a: A[]) => Box<A>[] = arrayMap(value => ({ value }));
+
+const f30: (a: string[]) => string[] = arrayFilter(x => x.length > 10);
+const f31: <T extends Box<number>>(a: T[]) => T[] = arrayFilter(x => x.value > 10);
+
+const f40: <A, B>(b: B, a: A) => [A, B] = flip(zip);
+
+// Repro from #16293
+
+type fn = <A>(a: A) => A;
+const fn: fn = a => a;
+
+
+//// [genericContextualTypes1.js]
+"use strict";
+var f00 = list;
+var f01 = function (x) { return [x]; };
+var f02 = wrap(list);
+var f03 = wrap(function (x) { return [x]; });
+var f10 = compose(function (a) { return list(a); }, function (b) { return box(b); });
+var f11 = compose(list, box);
+var f12 = compose(function (a) { return unbox(a); }, function (b) { return unlist(b); });
+var f13 = compose(unbox, unlist);
+var arrayMap = function (f) { return function (a) { return a.map(f); }; };
+var arrayFilter = function (f) { return function (a) { return a.filter(f); }; };
+var f20 = arrayMap(function (x) { return x.length; });
+var f21 = arrayMap(function (x) { return [x]; });
+var f22 = arrayMap(identity);
+var f23 = arrayMap(function (value) { return ({ value: value }); });
+var f30 = arrayFilter(function (x) { return x.length > 10; });
+var f31 = arrayFilter(function (x) { return x.value > 10; });
+var f40 = flip(zip);
+var fn = function (a) { return a; };
+
+
+//// [genericContextualTypes1.d.ts]
+declare type Box<T> = {
+    value: T;
+};
+declare function wrap<A, B>(f: (a: A) => B): (a: A) => B;
+declare function compose<A, B, C>(f: (a: A) => B, g: (b: B) => C): (a: A) => C;
+declare function list<T>(a: T): T[];
+declare function unlist<T>(a: T[]): T;
+declare function box<V>(x: V): Box<V>;
+declare function unbox<W>(x: Box<W>): W;
+declare function map<T, U>(a: T[], f: (x: T) => U): U[];
+declare function identity<T>(x: T): T;
+declare function zip<A, B>(a: A, b: B): [A, B];
+declare function flip<X, Y, Z>(f: (x: X, y: Y) => Z): (y: Y, x: X) => Z;
+declare const f00: <A>(x: A) => A[];
+declare const f01: <A>(x: A) => A[];
+declare const f02: <A>(x: A) => A[];
+declare const f03: <A>(x: A) => A[];
+declare const f10: <T>(x: T) => Box<T[]>;
+declare const f11: <T>(x: T) => Box<T[]>;
+declare const f12: <T>(x: Box<T[]>) => T;
+declare const f13: <T>(x: Box<T[]>) => T;
+declare const arrayMap: <T, U>(f: (x: T) => U) => (a: T[]) => U[];
+declare const arrayFilter: <T>(f: (x: T) => boolean) => (a: T[]) => T[];
+declare const f20: (a: string[]) => number[];
+declare const f21: <A>(a: A[]) => A[][];
+declare const f22: <A>(a: A[]) => A[];
+declare const f23: <A>(a: A[]) => Box<A>[];
+declare const f30: (a: string[]) => string[];
+declare const f31: <T extends Box<number>>(a: T[]) => T[];
+declare const f40: <A, B>(b: B, a: A) => [A, B];
+declare type fn = <A>(a: A) => A;
+declare const fn: fn;