Defer indexed access T[K] where T is generic and K is non-generic

Author: ahejlsberg

src/compiler/checker.ts

@@ -4667,33 +4667,24 @@ namespace ts {
          * The apparent type of a type parameter is the base constraint instantiated with the type parameter
          * as the type argument for the 'this' type.
          */
-        function getApparentTypeOfTypeParameter(type: TypeParameter) {
+        function getApparentTypeOfTypeVariable(type: TypeVariable) {
             if (!type.resolvedApparentType) {
-                let constraintType = getConstraintOfTypeParameter(type);
+                let constraintType = getConstraintOfTypeVariable(type);
                 while (constraintType && constraintType.flags & TypeFlags.TypeParameter) {
-                    constraintType = getConstraintOfTypeParameter(<TypeParameter>constraintType);
+                    constraintType = getConstraintOfTypeVariable(<TypeVariable>constraintType);
                 }
                 type.resolvedApparentType = getTypeWithThisArgument(constraintType || emptyObjectType, type);
             }
             return type.resolvedApparentType;
         }
 
-        /**
-         * The apparent type of an indexed access T[K] is the type of T's string index signature, if any.
-         */
-        function getApparentTypeOfIndexedAccess(type: IndexedAccessType) {
-            return getIndexTypeOfType(getApparentType(type.objectType), IndexKind.String) || type;
-        }
-
         /**
          * For a type parameter, return the base constraint of the type parameter. For the string, number,
          * boolean, and symbol primitive types, return the corresponding object types. Otherwise return the
          * type itself. Note that the apparent type of a union type is the union type itself.
          */
         function getApparentType(type: Type): Type {
-            const t = type.flags & TypeFlags.TypeParameter ? getApparentTypeOfTypeParameter(<TypeParameter>type) :
-                type.flags & TypeFlags.IndexedAccess ? getApparentTypeOfIndexedAccess(<IndexedAccessType>type) :
-                type;
+            const t = type.flags & TypeFlags.TypeVariable ? getApparentTypeOfTypeVariable(<TypeVariable>type) : type;
             return t.flags & TypeFlags.StringLike ? globalStringType :
                 t.flags & TypeFlags.NumberLike ? globalNumberType :
                 t.flags & TypeFlags.BooleanLike ? globalBooleanType :
@@ -5279,6 +5270,31 @@ namespace ts {
             return typeParameter.constraint === noConstraintType ? undefined : typeParameter.constraint;
         }
 
+        function getConstraintOfIndexedAccess(type: IndexedAccessType): Type {
+            // The constraint of T[K], where T is an object, union, or intersection type,
+            // is the type of the string index signature of T, if any.
+            if (type.objectType.flags & TypeFlags.StructuredType) {
+                return getIndexTypeOfType(type.objectType, IndexKind.String);
+            }
+            // The constraint of T[K], where T is a type variable, is A[K], where A is the
+            // apparent type of T.
+            if (type.objectType.flags & TypeFlags.TypeVariable) {
+                const apparentType = getApparentTypeOfTypeVariable(<TypeVariable>type.objectType);
+                if (apparentType !== emptyObjectType) {
+                    return isTypeOfKind((<IndexedAccessType>type).indexType, TypeFlags.StringLike) ?
+                        getIndexedAccessType(apparentType, (<IndexedAccessType>type).indexType) :
+                        getIndexTypeOfType(apparentType, IndexKind.String);
+                }
+            }
+            return undefined;
+        }
+
+        function getConstraintOfTypeVariable(type: TypeVariable): Type {
+            return type.flags & TypeFlags.TypeParameter ? getConstraintOfTypeParameter(<TypeParameter>type) :
+                type.flags & TypeFlags.IndexedAccess ? getConstraintOfIndexedAccess(<IndexedAccessType>type) :
+                undefined;
+        }
+
         function getParentSymbolOfTypeParameter(typeParameter: TypeParameter): Symbol {
             return getSymbolOfNode(getDeclarationOfKind(typeParameter.symbol, SyntaxKind.TypeParameter).parent);
         }
@@ -6032,11 +6048,16 @@ namespace ts {
         }
 
         function getIndexedAccessType(objectType: Type, indexType: Type, accessNode?: ElementAccessExpression | IndexedAccessTypeNode) {
-            if (maybeTypeOfKind(indexType, TypeFlags.TypeVariable | TypeFlags.Index) || isGenericMappedType(objectType)) {
-                // If the index type is generic or if the object type is a mapped type with a generic constraint,
-                // we are performing a higher-order index access where we cannot meaningfully access the properties
-                // of the object type. In those cases, we first check that the index type is assignable to 'keyof T'
-                // for the object type.
+            // If the index type is generic, if the object type is generic and doesn't originate in an expression,
+            // or if the object type is a mapped type with a generic constraint, we are performing a higher-order
+            // index access where we cannot meaningfully access the properties of the object type. Note that for a
+            // generic T and a non-generic K, we eagerly resolve T[K] if it originates in an expression. This is to
+            // preserve backwards compatibility. For example, an element access 'this["foo"]' has always been resolved
+            // eagerly using the constraint type of 'this' at the given location.
+            if (maybeTypeOfKind(indexType, TypeFlags.TypeVariable | TypeFlags.Index) ||
+                maybeTypeOfKind(objectType, TypeFlags.TypeVariable) && !(accessNode && accessNode.kind === SyntaxKind.ElementAccessExpression) ||
+                isGenericMappedType(objectType)) {
+                // We first check that the index type is assignable to 'keyof T' for the object type.
                 if (accessNode) {
                     if (!isTypeAssignableTo(indexType, getIndexType(objectType))) {
                         error(accessNode, Diagnostics.Type_0_cannot_be_used_to_index_type_1, typeToString(indexType), typeToString(objectType));
@@ -6053,6 +6074,7 @@ namespace ts {
                 const id = objectType.id + "," + indexType.id;
                 return indexedAccessTypes[id] || (indexedAccessTypes[id] = createIndexedAccessType(objectType, indexType));
             }
+            // In the following we resolve T[K] to the type of the property in T selected by K.
             const apparentObjectType = getApparentType(objectType);
             if (indexType.flags & TypeFlags.Union && !(indexType.flags & TypeFlags.Primitive)) {
                 const propTypes: Type[] = [];
@@ -7240,8 +7262,7 @@ namespace ts {
                         return result;
                     }
                 }
-
-                if (target.flags & TypeFlags.TypeParameter) {
+                else if (target.flags & TypeFlags.TypeParameter) {
                     // A source type { [P in keyof T]: X } is related to a target type T if X is related to T[P].
                     if (getObjectFlags(source) & ObjectFlags.Mapped && getConstraintTypeFromMappedType(<MappedType>source) === getIndexType(target)) {
                         if (!(<MappedType>source).declaration.questionToken) {
@@ -7270,10 +7291,10 @@ namespace ts {
                             return result;
                         }
                     }
-                    // Given a type parameter T with a constraint C, a type S is assignable to
+                    // Given a type variable T with a constraint C, a type S is assignable to
                     // keyof T if S is assignable to keyof C.
-                    if ((<IndexType>target).type.flags & TypeFlags.TypeParameter) {
-                        const constraint = getConstraintOfTypeParameter(<TypeParameter>(<IndexType>target).type);
+                    if ((<IndexType>target).type.flags & TypeFlags.TypeVariable) {
+                        const constraint = getConstraintOfTypeVariable(<TypeVariable>(<IndexType>target).type);
                         if (constraint) {
                             if (result = isRelatedTo(source, getIndexType(constraint), reportErrors)) {
                                 return result;
@@ -7289,6 +7310,15 @@ namespace ts {
                             return result;
                         }
                     }
+                    // A type S is related to a type T[K] if S is related to A[K], where K is string-like and
+                    // A is the apparent type of S.
+                    const constraint = getConstraintOfIndexedAccess(<IndexedAccessType>target);
+                    if (constraint) {
+                        if (result = isRelatedTo(source, constraint, reportErrors)) {
+                            errorInfo = saveErrorInfo;
+                            return result;
+                        }
+                    }
                 }
 
                 if (source.flags & TypeFlags.TypeParameter) {
@@ -7297,6 +7327,7 @@ namespace ts {
                         const indexedAccessType = getIndexedAccessType(source, getTypeParameterFromMappedType(<MappedType>target));
                         const templateType = getTemplateTypeFromMappedType(<MappedType>target);
                         if (result = isRelatedTo(indexedAccessType, templateType, reportErrors)) {
+                            errorInfo = saveErrorInfo;
                             return result;
                         }
                     }
@@ -7318,6 +7349,17 @@ namespace ts {
                         }
                     }
                 }
+                else if (source.flags & TypeFlags.IndexedAccess) {
+                    // A type S[K] is related to a type T if A[K] is related to T, where K is string-like and
+                    // A is the apparent type of S.
+                    const constraint = getConstraintOfIndexedAccess(<IndexedAccessType>source);
+                    if (constraint) {
+                        if (result = isRelatedTo(constraint, target, reportErrors)) {
+                            errorInfo = saveErrorInfo;
+                            return result;
+                        }
+                    }
+                }
                 else {
                     if (getObjectFlags(source) & ObjectFlags.Reference && getObjectFlags(target) & ObjectFlags.Reference && (<TypeReference>source).target === (<TypeReference>target).target) {
                         // We have type references to same target type, see if relationship holds for all type arguments
@@ -14978,8 +15020,8 @@ namespace ts {
 
         function isLiteralContextualType(contextualType: Type) {
             if (contextualType) {
-                if (contextualType.flags & TypeFlags.TypeParameter) {
-                    const apparentType = getApparentTypeOfTypeParameter(<TypeParameter>contextualType);
+                if (contextualType.flags & TypeFlags.TypeVariable) {
+                    const apparentType = getApparentTypeOfTypeVariable(<TypeVariable>contextualType);
                     // If the type parameter is constrained to the base primitive type we're checking for,
                     // consider this a literal context. For example, given a type parameter 'T extends string',
                     // this causes us to infer string literal types for T.
@@ -15814,7 +15856,7 @@ namespace ts {
             checkSourceElement(node.type);
             const type = <MappedType>getTypeFromMappedTypeNode(node);
             const constraintType = getConstraintTypeFromMappedType(type);
-            const keyType = constraintType.flags & TypeFlags.TypeParameter ? getApparentTypeOfTypeParameter(<TypeParameter>constraintType) : constraintType;
+            const keyType = constraintType.flags & TypeFlags.TypeVariable ? getApparentTypeOfTypeVariable(<TypeVariable>constraintType) : constraintType;
             checkTypeAssignableTo(keyType, stringType, node.typeParameter.constraint);
         }
 

src/compiler/types.ts

@@ -2967,6 +2967,8 @@ namespace ts {
     }
 
     export interface TypeVariable extends Type {
+        /* @internal */
+        resolvedApparentType: Type;
         /* @internal */
         resolvedIndexType: IndexType;
     }
@@ -2979,8 +2981,6 @@ namespace ts {
         /* @internal */
         mapper?: TypeMapper;     // Instantiation mapper
         /* @internal */
-        resolvedApparentType: Type;
-        /* @internal */
         isThisType?: boolean;
     }