Add constraints to indexed access types in conditional types

Author: ahejlsberg

src/compiler/checker.ts

@@ -3033,7 +3033,7 @@ namespace ts {
                     return createConditionalTypeNode(checkTypeNode, extendsTypeNode, trueTypeNode, falseTypeNode);
                 }
                 if (type.flags & TypeFlags.Substitution) {
-                    return typeToTypeNodeHelper((<SubstitutionType>type).typeParameter, context);
+                    return typeToTypeNodeHelper((<SubstitutionType>type).typeVariable, context);
                 }
 
                 Debug.fail("Should be unreachable.");
@@ -7303,7 +7303,7 @@ namespace ts {
             const res = tryGetDeclaredTypeOfSymbol(symbol);
             if (res) {
                 return checkNoTypeArguments(node, symbol) ?
-                    res.flags & TypeFlags.TypeParameter ? getConstrainedTypeParameter(<TypeParameter>res, node) : res :
+                    res.flags & TypeFlags.TypeParameter ? getConstrainedTypeVariable(<TypeParameter>res, node) : res :
                     unknownType;
             }
 
@@ -7342,25 +7342,25 @@ namespace ts {
             }
         }
 
-        function getSubstitutionType(typeParameter: TypeParameter, substitute: Type) {
+        function getSubstitutionType(typeVariable: TypeVariable, substitute: Type) {
             const result = <SubstitutionType>createType(TypeFlags.Substitution);
-            result.typeParameter = typeParameter;
+            result.typeVariable = typeVariable;
             result.substitute = substitute;
             return result;
         }
 
-        function getConstrainedTypeParameter(typeParameter: TypeParameter, node: Node) {
+        function getConstrainedTypeVariable(typeVariable: TypeVariable, node: Node) {
             let constraints: Type[];
             while (isPartOfTypeNode(node)) {
                 const parent = node.parent;
                 if (parent.kind === SyntaxKind.ConditionalType && node === (<ConditionalTypeNode>parent).trueType) {
-                    if (getTypeFromTypeNode((<ConditionalTypeNode>parent).checkType) === typeParameter) {
+                    if (getActualTypeVariable(getTypeFromTypeNode((<ConditionalTypeNode>parent).checkType)) === typeVariable) {
                         constraints = append(constraints, getTypeFromTypeNode((<ConditionalTypeNode>parent).extendsType));
                     }
                 }
                 node = parent;
             }
-            return constraints ? getSubstitutionType(typeParameter, getIntersectionType(append(constraints, typeParameter))) : typeParameter;
+            return constraints ? getSubstitutionType(typeVariable, getIntersectionType(append(constraints, typeVariable))) : typeVariable;
         }
 
         function isJSDocTypeReference(node: TypeReferenceType): node is TypeReferenceNode {
@@ -8256,7 +8256,13 @@ namespace ts {
         function getTypeFromIndexedAccessTypeNode(node: IndexedAccessTypeNode) {
             const links = getNodeLinks(node);
             if (!links.resolvedType) {
-                links.resolvedType = getIndexedAccessType(getTypeFromTypeNode(node.objectType), getTypeFromTypeNode(node.indexType), node);
+                const objectType = getTypeFromTypeNode(node.objectType);
+                const indexType = getTypeFromTypeNode(node.indexType);
+                const resolved = getIndexedAccessType(objectType, indexType, node);
+                links.resolvedType = resolved.flags & TypeFlags.IndexedAccess &&
+                    (<IndexedAccessType>resolved).objectType === objectType &&
+                    (<IndexedAccessType>resolved).indexType === indexType ?
+                    getConstrainedTypeVariable(<IndexedAccessType>resolved, node) : resolved;
             }
             return links.resolvedType;
         }
@@ -8276,8 +8282,8 @@ namespace ts {
             return links.resolvedType;
         }
 
-        function getActualTypeParameter(type: Type) {
-            return type.flags & TypeFlags.Substitution ? (<SubstitutionType>type).typeParameter : type;
+        function getActualTypeVariable(type: Type) {
+            return type.flags & TypeFlags.Substitution ? (<SubstitutionType>type).typeVariable : type;
         }
 
         function getConditionalType(root: ConditionalRoot, mapper: TypeMapper): Type {
@@ -8321,7 +8327,7 @@ namespace ts {
                 }
             }
             // Return a deferred type for a check that is neither definitely true nor definitely false
-            const erasedCheckType = getActualTypeParameter(checkType);
+            const erasedCheckType = getActualTypeVariable(checkType);
             const result = <ConditionalType>createType(TypeFlags.Conditional);
             result.root = root;
             result.checkType = erasedCheckType;
@@ -9041,7 +9047,7 @@ namespace ts {
                     return getConditionalTypeInstantiation(<ConditionalType>type, combineTypeMappers((<ConditionalType>type).mapper, mapper));
                 }
                 if (type.flags & TypeFlags.Substitution) {
-                    return mapper((<SubstitutionType>type).typeParameter);
+                    return instantiateType((<SubstitutionType>type).typeVariable, mapper);
                 }
             }
             return type;
@@ -9647,10 +9653,10 @@ namespace ts {
                     target = (<LiteralType>target).regularType;
                 }
                 if (source.flags & TypeFlags.Substitution) {
-                    source = relation === definitelyAssignableRelation ? (<SubstitutionType>source).typeParameter : (<SubstitutionType>source).substitute;
+                    source = relation === definitelyAssignableRelation ? (<SubstitutionType>source).typeVariable : (<SubstitutionType>source).substitute;
                 }
                 if (target.flags & TypeFlags.Substitution) {
-                    target = (<SubstitutionType>target).typeParameter;
+                    target = (<SubstitutionType>target).typeVariable;
                 }
 
                 // both types are the same - covers 'they are the same primitive type or both are Any' or the same type parameter cases

src/compiler/types.ts

@@ -3844,6 +3844,8 @@ namespace ts {
         constraint?: Type;
     }
 
+    export type TypeVariable = TypeParameter | IndexedAccessType;
+
     // keyof T types (TypeFlags.Index)
     export interface IndexType extends InstantiableType {
         type: InstantiableType | UnionOrIntersectionType;
@@ -3875,14 +3877,14 @@ namespace ts {
     }
 
     // Type parameter substitution (TypeFlags.Substitution)
-    // Substitution types are created for type parameter references that occur in the true branch
-    // of a conditional type. For example, in 'T extends string ? Foo<T> : Bar<T>', the reference to
-    // T in Foo<T> is resolved as a substitution type that substitutes 'string & T' for T. Thus, if
-    // Foo has a 'string' constraint on its type parameter, T will satisfy it. Substitution types
-    // disappear upon instantiation (just like type parameters).
+    // Substitution types are created for type parameters or indexed access types that occur in the
+    // true branch of a conditional type. For example, in 'T extends string ? Foo<T> : Bar<T>', the
+    // reference to T in Foo<T> is resolved as a substitution type that substitutes 'string & T' for T.
+    // Thus, if Foo has a 'string' constraint on its type parameter, T will satisfy it. Substitution
+    // types disappear upon instantiation (just like type parameters).
     export interface SubstitutionType extends InstantiableType {
-        typeParameter: TypeParameter;  // Target type parameter
-        substitute: Type;              // Type to substitute for type parameter
+        typeVariable: TypeVariable;  // Target type variable
+        substitute: Type;            // Type to substitute for type parameter
     }
 
     export const enum SignatureKind {