Merge pull request #29478 from Microsoft/fixContextualReturnTypes

Infer contextual types from generic return types

Author: ahejlsberg

src/compiler/checker.ts

@@ -14636,18 +14636,18 @@ 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);
                             }
@@ -17759,19 +17759,49 @@ 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, /*noReductions*/ true);
+                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 contains 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 that (a) 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?
          *
@@ -19910,6 +19940,9 @@ 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);
                 }
             }
 
@@ -23020,7 +23053,12 @@ 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;
@@ -23104,13 +23142,10 @@ 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 isConstContext(node) ? getRegularTypeOfLiteralType(type) :
                 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

@@ -4370,6 +4370,7 @@ 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/contextualTypeShouldBeLiteral.js

@@ -93,7 +93,34 @@ let xyz: LikeA | LikeB = {
     }
 };
 
-xyz;
+xyz;
+
+// Repro from #29168
+
+interface TestObject {
+  type?: 'object';
+  items: {
+    [k: string]: TestGeneric;
+  };
+}
+
+interface TestString {
+  type: 'string';
+}
+
+type TestGeneric = (TestString | TestObject) & { [k: string]: any; };
+
+const test: TestGeneric = {
+  items: {
+    hello: { type: 'string' },
+    world: {
+      items: {
+        nested: { type: 'string' }
+      }
+    }
+  }
+};
+
 
 //// [contextualTypeShouldBeLiteral.js]
 "use strict";
@@ -134,3 +161,13 @@ var xyz = {
     }
 };
 xyz;
+var test = {
+    items: {
+        hello: { type: 'string' },
+        world: {
+            items: {
+                nested: { type: 'string' }
+            }
+        }
+    }
+};

tests/baselines/reference/contextualTypeShouldBeLiteral.symbols

@@ -227,3 +227,59 @@ let xyz: LikeA | LikeB = {
 xyz;
 >xyz : Symbol(xyz, Decl(contextualTypeShouldBeLiteral.ts, 82, 3))
 
+// Repro from #29168
+
+interface TestObject {
+>TestObject : Symbol(TestObject, Decl(contextualTypeShouldBeLiteral.ts, 94, 4))
+
+  type?: 'object';
+>type : Symbol(TestObject.type, Decl(contextualTypeShouldBeLiteral.ts, 98, 22))
+
+  items: {
+>items : Symbol(TestObject.items, Decl(contextualTypeShouldBeLiteral.ts, 99, 18))
+
+    [k: string]: TestGeneric;
+>k : Symbol(k, Decl(contextualTypeShouldBeLiteral.ts, 101, 5))
+>TestGeneric : Symbol(TestGeneric, Decl(contextualTypeShouldBeLiteral.ts, 107, 1))
+
+  };
+}
+
+interface TestString {
+>TestString : Symbol(TestString, Decl(contextualTypeShouldBeLiteral.ts, 103, 1))
+
+  type: 'string';
+>type : Symbol(TestString.type, Decl(contextualTypeShouldBeLiteral.ts, 105, 22))
+}
+
+type TestGeneric = (TestString | TestObject) & { [k: string]: any; };
+>TestGeneric : Symbol(TestGeneric, Decl(contextualTypeShouldBeLiteral.ts, 107, 1))
+>TestString : Symbol(TestString, Decl(contextualTypeShouldBeLiteral.ts, 103, 1))
+>TestObject : Symbol(TestObject, Decl(contextualTypeShouldBeLiteral.ts, 94, 4))
+>k : Symbol(k, Decl(contextualTypeShouldBeLiteral.ts, 109, 50))
+
+const test: TestGeneric = {
+>test : Symbol(test, Decl(contextualTypeShouldBeLiteral.ts, 111, 5))
+>TestGeneric : Symbol(TestGeneric, Decl(contextualTypeShouldBeLiteral.ts, 107, 1))
+
+  items: {
+>items : Symbol(items, Decl(contextualTypeShouldBeLiteral.ts, 111, 27))
+
+    hello: { type: 'string' },
+>hello : Symbol(hello, Decl(contextualTypeShouldBeLiteral.ts, 112, 10))
+>type : Symbol(type, Decl(contextualTypeShouldBeLiteral.ts, 113, 12))
+
+    world: {
+>world : Symbol(world, Decl(contextualTypeShouldBeLiteral.ts, 113, 30))
+
+      items: {
+>items : Symbol(items, Decl(contextualTypeShouldBeLiteral.ts, 114, 12))
+
+        nested: { type: 'string' }
+>nested : Symbol(nested, Decl(contextualTypeShouldBeLiteral.ts, 115, 14))
+>type : Symbol(type, Decl(contextualTypeShouldBeLiteral.ts, 116, 17))
+      }
+    }
+  }
+};
+

tests/baselines/reference/contextualTypeShouldBeLiteral.types

@@ -222,3 +222,59 @@ let xyz: LikeA | LikeB = {
 xyz;
 >xyz : LikeA
 
+// Repro from #29168
+
+interface TestObject {
+  type?: 'object';
+>type : "object" | undefined
+
+  items: {
+>items : { [k: string]: TestGeneric; }
+
+    [k: string]: TestGeneric;
+>k : string
+
+  };
+}
+
+interface TestString {
+  type: 'string';
+>type : "string"
+}
+
+type TestGeneric = (TestString | TestObject) & { [k: string]: any; };
+>TestGeneric : TestGeneric
+>k : string
+
+const test: TestGeneric = {
+>test : TestGeneric
+>{  items: {    hello: { type: 'string' },    world: {      items: {        nested: { type: 'string' }      }    }  }} : { items: { hello: { type: "string"; }; world: { items: { nested: { type: "string"; }; }; }; }; }
+
+  items: {
+>items : { hello: { type: "string"; }; world: { items: { nested: { type: "string"; }; }; }; }
+>{    hello: { type: 'string' },    world: {      items: {        nested: { type: 'string' }      }    }  } : { hello: { type: "string"; }; world: { items: { nested: { type: "string"; }; }; }; }
+
+    hello: { type: 'string' },
+>hello : { type: "string"; }
+>{ type: 'string' } : { type: "string"; }
+>type : "string"
+>'string' : "string"
+
+    world: {
+>world : { items: { nested: { type: "string"; }; }; }
+>{      items: {        nested: { type: 'string' }      }    } : { items: { nested: { type: "string"; }; }; }
+
+      items: {
+>items : { nested: { type: "string"; }; }
+>{        nested: { type: 'string' }      } : { nested: { type: "string"; }; }
+
+        nested: { type: 'string' }
+>nested : { type: "string"; }
+>{ type: 'string' } : { type: "string"; }
+>type : "string"
+>'string' : "string"
+      }
+    }
+  }
+};
+

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"
 
 });