Type inference for isomorphic mapped types

Author: ahejlsberg

src/compiler/checker.ts

@@ -8425,7 +8425,7 @@ namespace ts {
         // results for union and intersection types for performance reasons.
         function couldContainTypeParameters(type: Type): boolean {
             const objectFlags = getObjectFlags(type);
-            return !!(type.flags & TypeFlags.TypeParameter ||
+            return !!(type.flags & (TypeFlags.TypeParameter | TypeFlags.IndexedAccess) ||
                 objectFlags & ObjectFlags.Reference && forEach((<TypeReference>type).typeArguments, couldContainTypeParameters) ||
                 objectFlags & ObjectFlags.Anonymous && type.symbol && type.symbol.flags & (SymbolFlags.Method | SymbolFlags.TypeLiteral | SymbolFlags.Class) ||
                 objectFlags & ObjectFlags.Mapped ||
@@ -8443,8 +8443,57 @@ namespace ts {
             return type === typeParameter || type.flags & TypeFlags.UnionOrIntersection && forEach((<UnionOrIntersectionType>type).types, t => isTypeParameterAtTopLevel(t, typeParameter));
         }
 
-        function inferTypes(context: InferenceContext, originalSource: Type, originalTarget: Type) {
-            const typeParameters = context.signature.typeParameters;
+        // Infer a suitable input type for an isomorphic mapped type { [P in keyof T]: X }. We construct
+        // an object type with the same set of properties as the source type, where the type of each
+        // property is computed by inferring from the source property type to X for a synthetic type
+        // parameter T[P] (i.e. we treat the type T[P] as the type parameter we're inferring for).
+        function inferTypeForIsomorphicMappedType(source: Type, target: MappedType): Type {
+            if (!isMappableType(source)) {
+                return source;
+            }
+            const typeParameter = getIndexedAccessType((<IndexType>getConstraintTypeFromMappedType(target)).type, getTypeParameterFromMappedType(target));
+            const typeParameterArray = [typeParameter];
+            const typeInferences = createTypeInferencesObject();
+            const typeInferencesArray = [typeInferences];
+            const templateType = getTemplateTypeFromMappedType(target);
+            const properties = getPropertiesOfType(source);
+            const members = createSymbolTable(properties);
+            let hasInferredTypes = false;
+            for (const prop of properties) {
+                const inferredPropType = inferTargetType(getTypeOfSymbol(prop));
+                if (inferredPropType) {
+                    const inferredProp = <TransientSymbol>createSymbol(SymbolFlags.Property | SymbolFlags.Transient | prop.flags & SymbolFlags.Optional, prop.name);
+                    inferredProp.declarations = prop.declarations;
+                    inferredProp.type = inferredPropType;
+                    inferredProp.isReadonly = isReadonlySymbol(prop);
+                    members[prop.name] = inferredProp;
+                    hasInferredTypes = true;
+                }
+            }
+            let indexInfo = getIndexInfoOfType(source, IndexKind.String);
+            if (indexInfo) {
+                const inferredIndexType = inferTargetType(indexInfo.type);
+                if (inferredIndexType) {
+                    indexInfo = createIndexInfo(inferredIndexType, indexInfo.isReadonly);
+                    hasInferredTypes = true;
+                }
+            }
+            return hasInferredTypes ? createAnonymousType(undefined, members, emptyArray, emptyArray, indexInfo, undefined) : source;
+
+            function inferTargetType(sourceType: Type): Type {
+                typeInferences.primary = undefined;
+                typeInferences.secondary = undefined;
+                inferTypes(typeParameterArray, typeInferencesArray, sourceType, templateType);
+                const inferences = typeInferences.primary || typeInferences.secondary;
+                return inferences && getUnionType(inferences, /*subtypeReduction*/ true);
+            }
+        }
+
+        function inferTypesWithContext(context: InferenceContext, originalSource: Type, originalTarget: Type) {
+            inferTypes(context.signature.typeParameters, context.inferences, originalSource, originalTarget);
+        }
+
+        function inferTypes(typeParameters: Type[], typeInferences: TypeInferences[], originalSource: Type, originalTarget: Type) {
             let sourceStack: Type[];
             let targetStack: Type[];
             let depth = 0;
@@ -8512,7 +8561,7 @@ namespace ts {
                         target = removeTypesFromUnionOrIntersection(<UnionOrIntersectionType>target, matchingTypes);
                     }
                 }
-                if (target.flags & TypeFlags.TypeParameter) {
+                if (target.flags & (TypeFlags.TypeParameter | TypeFlags.IndexedAccess)) {
                     // If target is a type parameter, make an inference, unless the source type contains
                     // the anyFunctionType (the wildcard type that's used to avoid contextually typing functions).
                     // Because the anyFunctionType is internal, it should not be exposed to the user by adding
@@ -8524,7 +8573,7 @@ namespace ts {
                     }
                     for (let i = 0; i < typeParameters.length; i++) {
                         if (target === typeParameters[i]) {
-                            const inferences = context.inferences[i];
+                            const inferences = typeInferences[i];
                             if (!inferences.isFixed) {
                                 // Any inferences that are made to a type parameter in a union type are inferior
                                 // to inferences made to a flat (non-union) type. This is because if we infer to
@@ -8538,7 +8587,7 @@ namespace ts {
                                 if (!contains(candidates, source)) {
                                     candidates.push(source);
                                 }
-                                if (!isTypeParameterAtTopLevel(originalTarget, <TypeParameter>target)) {
+                                if (target.flags & TypeFlags.TypeParameter && !isTypeParameterAtTopLevel(originalTarget, <TypeParameter>target)) {
                                     inferences.topLevel = false;
                                 }
                             }
@@ -8589,15 +8638,29 @@ namespace ts {
                     if (getObjectFlags(target) & ObjectFlags.Mapped) {
                         const constraintType = getConstraintTypeFromMappedType(<MappedType>target);
                         if (getObjectFlags(source) & ObjectFlags.Mapped) {
+                            // We're inferring from a mapped type to a mapped type, so simply infer from constraint type to
+                            // constraint type and from template type to template type.
                             inferFromTypes(getConstraintTypeFromMappedType(<MappedType>source), constraintType);
                             inferFromTypes(getTemplateTypeFromMappedType(<MappedType>source), getTemplateTypeFromMappedType(<MappedType>target));
                             return;
                         }
                         if (constraintType.flags & TypeFlags.TypeParameter) {
+                            // We're inferring from some source type S to a mapped type { [P in T]: X }, where T is a type
+                            // parameter. Infer from 'keyof S' to T and infer from a union of each property type in S to X.
                             inferFromTypes(getIndexType(source), constraintType);
                             inferFromTypes(getUnionType(map(getPropertiesOfType(source), getTypeOfSymbol)), getTemplateTypeFromMappedType(<MappedType>target));
                             return;
                         }
+                        if (constraintType.flags & TypeFlags.Index) {
+                            // We're inferring from some source type S to an isomorphic mapped type { [P in keyof T]: X },
+                            // where T is a type parameter. Use inferTypeForIsomorphicMappedType to infer a suitable source
+                            // type and then infer from that type to T.
+                            const index = indexOf(typeParameters, (<IndexType>constraintType).type);
+                            if (index >= 0 && !typeInferences[index].isFixed) {
+                                inferFromTypes(inferTypeForIsomorphicMappedType(source, <MappedType>target), typeParameters[index]);
+                            }
+                            return;
+                        }
                     }
                     source = getApparentType(source);
                     if (source.flags & TypeFlags.Object) {
@@ -12458,7 +12521,7 @@ namespace ts {
             const context = createInferenceContext(signature, /*inferUnionTypes*/ true);
             forEachMatchingParameterType(contextualSignature, signature, (source, target) => {
                 // Type parameters from outer context referenced by source type are fixed by instantiation of the source type
-                inferTypes(context, instantiateType(source, contextualMapper), target);
+                inferTypesWithContext(context, instantiateType(source, contextualMapper), target);
             });
             return getSignatureInstantiation(signature, getInferredTypes(context));
         }
@@ -12493,7 +12556,7 @@ namespace ts {
             if (thisType) {
                 const thisArgumentNode = getThisArgumentOfCall(node);
                 const thisArgumentType = thisArgumentNode ? checkExpression(thisArgumentNode) : voidType;
-                inferTypes(context, thisArgumentType, thisType);
+                inferTypesWithContext(context, thisArgumentType, thisType);
             }
 
             // We perform two passes over the arguments. In the first pass we infer from all arguments, but use
@@ -12515,7 +12578,7 @@ namespace ts {
                         argType = checkExpressionWithContextualType(arg, paramType, mapper);
                     }
 
-                    inferTypes(context, argType, paramType);
+                    inferTypesWithContext(context, argType, paramType);
                 }
             }
 
@@ -12530,7 +12593,7 @@ namespace ts {
                     if (excludeArgument[i] === false) {
                         const arg = args[i];
                         const paramType = getTypeAtPosition(signature, i);
-                        inferTypes(context, checkExpressionWithContextualType(arg, paramType, inferenceMapper), paramType);
+                        inferTypesWithContext(context, checkExpressionWithContextualType(arg, paramType, inferenceMapper), paramType);
                     }
                 }
             }
@@ -13617,7 +13680,7 @@ namespace ts {
                 for (let i = 0; i < len; i++) {
                     const declaration = <ParameterDeclaration>signature.parameters[i].valueDeclaration;
                     if (declaration.type) {
-                        inferTypes(mapper.context, getTypeFromTypeNode(declaration.type), getTypeAtPosition(context, i));
+                        inferTypesWithContext(mapper.context, getTypeFromTypeNode(declaration.type), getTypeAtPosition(context, i));
                     }
                 }
             }
@@ -13703,7 +13766,7 @@ namespace ts {
                 // T in the second overload so that we do not infer Base as a candidate for T
                 // (inferring Base would make type argument inference inconsistent between the two
                 // overloads).
-                inferTypes(mapper.context, links.type, instantiateType(contextualType, mapper));
+                inferTypesWithContext(mapper.context, links.type, instantiateType(contextualType, mapper));
             }
         }