Infer contextual types from generic return types

src/compiler/checker.ts

@@ -14591,18 +14591,18 @@ namespace ts {
             }
 
             function inferFromProperties(source: Type, target: Type) {
-                if (isTupleType(source)) {
+                if (isArrayType(source) || isTupleType(source)) {
                     if (isTupleType(target)) {
-                        const sourceLength = getLengthOfTupleType(source);
+                        const sourceLength = isTupleType(source) ? getLengthOfTupleType(source) : 0;
                         const targetLength = getLengthOfTupleType(target);
-                        const sourceRestType = getRestTypeOfTupleType(source);
+                        const sourceRestType = isTupleType(source) ? getRestTypeOfTupleType(source) : getElementTypeOfArrayType(source);
                         const targetRestType = getRestTypeOfTupleType(target);
                         const fixedLength = targetLength < sourceLength || sourceRestType ? targetLength : sourceLength;
                         for (let i = 0; i < fixedLength; i++) {
-                            inferFromTypes(i < sourceLength ? source.typeArguments![i] : sourceRestType!, target.typeArguments![i]);
+                            inferFromTypes(i < sourceLength ? (<TypeReference>source).typeArguments![i] : sourceRestType!, target.typeArguments![i]);
                         }
                         if (targetRestType) {
-                            const types = fixedLength < sourceLength ? source.typeArguments!.slice(fixedLength, sourceLength) : [];
+                            const types = fixedLength < sourceLength ? (<TypeReference>source).typeArguments!.slice(fixedLength, sourceLength) : [];
                             if (sourceRestType) {
                                 types.push(sourceRestType);
                             }
@@ -17694,19 +17694,49 @@ namespace ts {
         // Return the contextual type for a given expression node. During overload resolution, a contextual type may temporarily
         // be "pushed" onto a node using the contextualType property.
         function getApparentTypeOfContextualType(node: Expression): Type | undefined {
-            let contextualType = getContextualType(node);
-            contextualType = contextualType && mapType(contextualType, getApparentType);
-            if (contextualType && contextualType.flags & TypeFlags.Union) {
-                if (isObjectLiteralExpression(node)) {
-                    return discriminateContextualTypeByObjectMembers(node, contextualType as UnionType);
+            const contextualType = instantiateContextualType(getContextualType(node), node);
+            if (contextualType) {
+                const apparentType = mapType(contextualType, getApparentType);
+                if (apparentType.flags & TypeFlags.Union) {
+                    if (isObjectLiteralExpression(node)) {
+                        return discriminateContextualTypeByObjectMembers(node, apparentType as UnionType);
+                    }
+                    else if (isJsxAttributes(node)) {
+                        return discriminateContextualTypeByJSXAttributes(node, apparentType as UnionType);
+                    }
                 }
-                else if (isJsxAttributes(node)) {
-                    return discriminateContextualTypeByJSXAttributes(node, contextualType as UnionType);
+                return apparentType;
+            }
+        }
+
+        // If the given contextual type constains instantiable types and if a mapper representing
+        // return type inferences is available, instantiate those types using that mapper.
+        function instantiateContextualType(contextualType: Type | undefined, node: Expression): Type | undefined {
+            if (contextualType && maybeTypeOfKind(contextualType, TypeFlags.Instantiable)) {
+                const returnMapper = (<InferenceContext>getContextualMapper(node)).returnMapper;
+                if (returnMapper) {
+                    return instantiateInstantiableTypes(contextualType, returnMapper);
                 }
             }
             return contextualType;
         }
 
+        // This function is similar to instantiateType, except (a) that it only instantiates types that
+        // are classified as instantiable (i.e. it doesn't instantiate object types), and (b) it performs
+        // no reductions on instantiated union types.
+        function instantiateInstantiableTypes(type: Type, mapper: TypeMapper): Type {
+            if (type.flags & TypeFlags.Instantiable) {
+                return instantiateType(type, mapper);
+            }
+            if (type.flags & TypeFlags.Union) {
+                return getUnionType(map((<UnionType>type).types, t => instantiateInstantiableTypes(t, mapper)), UnionReduction.None);
+            }
+            if (type.flags & TypeFlags.Intersection) {
+                return getIntersectionType(map((<IntersectionType>type).types, t => instantiateInstantiableTypes(t, mapper)));
+            }
+            return type;
+        }
+
         /**
          * Woah! Do you really want to use this function?
          *
@@ -19847,6 +19877,9 @@ namespace ts {
                     const inferenceTargetType = getReturnTypeOfSignature(signature);
                      // Inferences made from return types have lower priority than all other inferences.
                     inferTypes(context.inferences, inferenceSourceType, inferenceTargetType, InferencePriority.ReturnType);
+                    // Create a type mapper for instantiating generic contextual types using the inferences made
+                    // from the return type.
+                    context.returnMapper = cloneTypeMapper(context);
                 }
             }
 
@@ -22945,7 +22978,12 @@ namespace ts {
             context.contextualMapper = contextualMapper;
             const checkMode = contextualMapper === identityMapper ? CheckMode.SkipContextSensitive :
                 contextualMapper ? CheckMode.Inferential : CheckMode.Contextual;
-            const result = checkExpression(node, checkMode);
+            const type = checkExpression(node, checkMode);
+            // We strip literal freshness when an appropriate contextual type is present such that contextually typed
+            // literals always preserve their literal types (otherwise they might widen during type inference). An alternative
+            // here would be to not mark contextually typed literals as fresh in the first place.
+            const result = maybeTypeOfKind(type, TypeFlags.Literal) && isLiteralOfContextualType(type, instantiateContextualType(contextualType, node)) ?
+                getRegularTypeOfLiteralType(type) : type;
             context.contextualType = saveContextualType;
             context.contextualMapper = saveContextualMapper;
             return result;
@@ -23022,12 +23060,9 @@ namespace ts {
         }
 
         function checkExpressionForMutableLocation(node: Expression, checkMode: CheckMode | undefined, contextualType?: Type, forceTuple?: boolean): Type {
-            if (arguments.length === 2) {
-                contextualType = getContextualType(node);
-            }
             const type = checkExpression(node, checkMode, forceTuple);
             return isTypeAssertion(node) ? type :
-                getWidenedLiteralLikeTypeForContextualType(type, contextualType);
+                getWidenedLiteralLikeTypeForContextualType(type, instantiateContextualType(arguments.length === 2 ? getContextualType(node) : contextualType, node));
         }
 
         function checkPropertyAssignment(node: PropertyAssignment, checkMode?: CheckMode): Type {

src/compiler/types.ts

@@ -4364,6 +4364,7 @@ namespace ts {
         inferences: InferenceInfo[];                  // Inferences made for each type parameter
         flags: InferenceFlags;                        // Inference flags
         compareTypes: TypeComparer;                   // Type comparer function
+        returnMapper?: TypeMapper;                    // Type mapper for inferences from return types (if any)
     }
 
     /* @internal */

tests/baselines/reference/errorMessagesIntersectionTypes02.errors.txt

@@ -1,6 +1,6 @@
-tests/cases/compiler/errorMessagesIntersectionTypes02.ts(14,5): error TS2322: Type '{ fooProp: string; } & Bar' is not assignable to type 'FooBar'.
+tests/cases/compiler/errorMessagesIntersectionTypes02.ts(14,5): error TS2322: Type '{ fooProp: "frizzlebizzle"; } & Bar' is not assignable to type 'FooBar'.
   Types of property 'fooProp' are incompatible.
-    Type 'string' is not assignable to type '"hello" | "world"'.
+    Type '"frizzlebizzle"' is not assignable to type '"hello" | "world"'.
 
 
 ==== tests/cases/compiler/errorMessagesIntersectionTypes02.ts (1 errors) ====
@@ -19,8 +19,8 @@ tests/cases/compiler/errorMessagesIntersectionTypes02.ts(14,5): error TS2322: Ty
 
     let fooBar: FooBar = mixBar({
         ~~~~~~
-!!! error TS2322: Type '{ fooProp: string; } & Bar' is not assignable to type 'FooBar'.
+!!! error TS2322: Type '{ fooProp: "frizzlebizzle"; } & Bar' is not assignable to type 'FooBar'.
 !!! error TS2322:   Types of property 'fooProp' are incompatible.
-!!! error TS2322:     Type 'string' is not assignable to type '"hello" | "world"'.
+!!! error TS2322:     Type '"frizzlebizzle"' is not assignable to type '"hello" | "world"'.
         fooProp: "frizzlebizzle"
     });

tests/baselines/reference/errorMessagesIntersectionTypes02.types

@@ -18,12 +18,12 @@ declare function mixBar<T>(obj: T): T & Bar;
 
 let fooBar: FooBar = mixBar({
 >fooBar : FooBar
->mixBar({    fooProp: "frizzlebizzle"}) : { fooProp: string; } & Bar
+>mixBar({    fooProp: "frizzlebizzle"}) : { fooProp: "frizzlebizzle"; } & Bar
 >mixBar : <T>(obj: T) => T & Bar
->{    fooProp: "frizzlebizzle"} : { fooProp: string; }
+>{    fooProp: "frizzlebizzle"} : { fooProp: "frizzlebizzle"; }
 
     fooProp: "frizzlebizzle"
->fooProp : string
+>fooProp : "frizzlebizzle"
 >"frizzlebizzle" : "frizzlebizzle"
 
 });

tests/baselines/reference/inferFromGenericFunctionReturnTypes3.errors.txt

@@ -0,0 +1,177 @@
+tests/cases/compiler/inferFromGenericFunctionReturnTypes3.ts(28,30): error TS2345: Argument of type 'string' is not assignable to parameter of type '"bar"'.
+
+
+==== tests/cases/compiler/inferFromGenericFunctionReturnTypes3.ts (1 errors) ====
+    // Repros from #5487
+
+    function truePromise(): Promise<true> {
+        return Promise.resolve(true);
+    }
+
+    interface Wrap<T> {
+        value: T;
+    }
+
+    function wrap<T>(value: T): Wrap<T> {
+        return { value };
+    }
+
+    function wrappedFoo(): Wrap<'foo'> {
+        return wrap('foo');
+    }
+
+    function wrapBar(value: 'bar'): Wrap<'bar'> {
+        return { value };
+    }
+
+    function wrappedBar(): Wrap<'bar'> {
+        const value = 'bar';
+        const inferred = wrapBar(value);
+        const literal = wrapBar('bar');
+        const value2: string = 'bar';
+        const literal2 = wrapBar(value2);  // Error
+                                 ~~~~~~
+!!! error TS2345: Argument of type 'string' is not assignable to parameter of type '"bar"'.
+        return wrap(value);
+    }
+
+    function wrappedBaz(): Wrap<'baz'> {
+        const value: 'baz' = 'baz';
+        return wrap(value);
+    }
+
+    // Repro from #11152
+
+    interface FolderContentItem {
+        type: 'folder' | 'file';
+    }
+
+    let a: FolderContentItem[] = [];
+    a = [1, 2, 3, 4, 5].map(v => ({ type: 'folder' }));
+
+    // Repro from #11312
+
+    let arr: Array<[number, number]> = [[1, 2]]
+
+    let mappedArr: Array<[number, number]> = arr.map(([x, y]) => {
+        return [x, y];
+    })
+
+    // Repro from #13594
+
+    export namespace DiagnosticSeverity {
+    	export const Error = 1;
+    	export const Warning = 2;
+    	export const Information = 3;
+    	export const Hint = 4;
+    }
+
+    export type DiagnosticSeverity = 1 | 2 | 3 | 4;
+
+    export interface Diagnostic {
+    	severity?: DiagnosticSeverity;
+    	code?: number | string;
+    	source?: string;
+    	message: string;
+    }
+
+    function bug(): Diagnostic[] {
+    	let values: any[] = [];
+    	return values.map((value) => {
+    		return {
+    			severity: DiagnosticSeverity.Error,
+    			message: 'message'
+    		}
+    	});
+    }
+
+    // Repro from #22870
+
+    function objectToMap(obj: any) {
+    	return new Map(Object.keys(obj).map(key => [key, obj[key]]));
+    };
+
+    // Repro from #24352
+
+    interface Person {
+      phoneNumbers: {
+        __typename: 'PhoneNumber';
+      }[];
+    }
+
+    function createPerson(): Person {
+      return {
+        phoneNumbers: [1].map(() => ({
+          __typename: 'PhoneNumber'
+        }))
+      };
+    }
+
+    // Repro from #26621
+
+    type Box<T> = { value: T };
+    declare function box<T>(value: T): Box<T>;
+
+    type WinCondition =
+        | { type: 'win', player: string }
+        | { type: 'draw' };
+
+    let zz: Box<WinCondition> = box({ type: 'draw' });
+
+    type WinType = 'win' | 'draw';
+
+    let yy: Box<WinType> = box('draw');
+
+    // Repro from #27074
+
+    interface OK<T> {
+        kind: "OK";
+        value: T;
+    }
+    export function ok<T>(value: T): OK<T> {
+        return {
+            kind: "OK",
+            value: value
+        };
+    }
+
+    let result: OK<[string, number]> = ok(["hello", 12]);
+
+    // Repro from #25889
+
+    interface I {
+        code: 'mapped',
+        name: string,
+    }
+
+    const a3: I[] = ['a', 'b'].map(name => {
+        return {
+            code: 'mapped',
+            name,
+        }
+    });
+
+    // Repro from https://www.memsql.com/blog/porting-30k-lines-of-code-from-flow-to-typescript/
+
+    type Player = {
+        name: string;
+        age: number;
+        position: "STRIKER" | "GOALKEEPER",
+    };
+
+    type F = () => Promise<Array<Player>>;
+
+    const f1: F = () => {
+        return Promise.all([
+            {
+                name: "David Gomes",
+                age: 23,
+                position: "GOALKEEPER",
+            }, {
+                name: "Cristiano Ronaldo",
+                age: 33,
+                position: "STRIKER",
+            }
+        ]);
+    };
+