Use immediate constraint instead of base constraint in T[K] relations

Author: ahejlsberg

src/compiler/checker.ts

@@ -3973,7 +3973,7 @@ namespace ts {
                 return true;
             }
             if (type.flags & TypeFlags.TypeVariable) {
-                const constraint = getBaseConstraintOfType(<TypeVariable>type);
+                const constraint = getBaseConstraintOfType(type);
                 return constraint && isValidBaseType(constraint) && isMixinConstructorType(constraint);
             }
             return false;
@@ -4989,16 +4989,32 @@ namespace ts {
         }
 
         function getConstraintOfType(type: TypeVariable | UnionOrIntersectionType): Type {
-            return type.flags & TypeFlags.TypeParameter ? getConstraintOfTypeParameter(<TypeParameter>type) : getBaseConstraintOfType(type);
+            return type.flags & TypeFlags.TypeParameter ? getConstraintOfTypeParameter(<TypeParameter>type) :
+                type.flags & TypeFlags.IndexedAccess ? getConstraintOfIndexedAccess(<IndexedAccessType>type) :
+                getBaseConstraintOfType(type);
         }
 
         function getConstraintOfTypeParameter(typeParameter: TypeParameter): Type {
             return hasNonCircularBaseConstraint(typeParameter) ? getConstraintFromTypeParameter(typeParameter) : undefined;
         }
 
-        function getBaseConstraintOfType(type: TypeVariable | UnionOrIntersectionType): Type {
-            const constraint = getResolvedBaseConstraint(type);
-            return constraint !== noConstraintType && constraint !== circularConstraintType ? constraint : undefined;
+        function getConstraintOfIndexedAccess(type: IndexedAccessType) {
+            const baseObjectType = getBaseConstraintOfType(type.objectType);
+            const baseIndexType = getBaseConstraintOfType(type.indexType);
+            return baseObjectType || baseIndexType ? getIndexedAccessType(baseObjectType || type.objectType, baseIndexType || type.indexType) : undefined;
+        }
+
+        function getBaseConstraintOfType(type: Type): Type {
+            if (type.flags & (TypeFlags.TypeVariable | TypeFlags.UnionOrIntersection)) {
+                const constraint = getResolvedBaseConstraint(<TypeVariable | UnionOrIntersectionType>type);
+                if (constraint !== noConstraintType && constraint !== circularConstraintType) {
+                    return constraint;
+                }
+            }
+            else if (type.flags & TypeFlags.Index) {
+                return stringType;
+            }
+            return undefined;
         }
 
         function hasNonCircularBaseConstraint(type: TypeVariable): boolean {
@@ -5096,7 +5112,7 @@ namespace ts {
          * 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.TypeVariable ? getBaseConstraintOfType(<TypeVariable>type) || emptyObjectType : type;
+            const t = type.flags & TypeFlags.TypeVariable ? getBaseConstraintOfType(type) || emptyObjectType : type;
             return t.flags & TypeFlags.Intersection ? getApparentTypeOfIntersectionType(<IntersectionType>t) :
                 t.flags & TypeFlags.StringLike ? globalStringType :
                 t.flags & TypeFlags.NumberLike ? globalNumberType :
@@ -7921,7 +7937,7 @@ namespace ts {
                     }
                     // 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 = getBaseConstraintOfType(<IndexedAccessType>target);
+                    const constraint = getBaseConstraintOfType(target);
                     if (constraint) {
                         if (result = isRelatedTo(source, constraint, reportErrors)) {
                             errorInfo = saveErrorInfo;
@@ -7961,7 +7977,7 @@ 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 = getBaseConstraintOfType(<IndexedAccessType>source);
+                    const constraint = getConstraintOfType(<IndexedAccessType>source);
                     if (constraint) {
                         if (result = isRelatedTo(constraint, target, reportErrors)) {
                             errorInfo = saveErrorInfo;
@@ -9874,7 +9890,7 @@ namespace ts {
                 return strictNullChecks ? TypeFacts.ObjectStrictFacts : TypeFacts.ObjectFacts;
             }
             if (flags & TypeFlags.TypeVariable) {
-                return getTypeFacts(getBaseConstraintOfType(<TypeVariable>type) || emptyObjectType);
+                return getTypeFacts(getBaseConstraintOfType(type) || emptyObjectType);
             }
             if (flags & TypeFlags.UnionOrIntersection) {
                 return getTypeFactsOfTypes((<UnionOrIntersectionType>type).types);
@@ -10686,7 +10702,7 @@ namespace ts {
                             return targetType;
                         }
                         if (type.flags & TypeFlags.TypeVariable) {
-                            const constraint = getBaseConstraintOfType(<TypeVariable>type) || anyType;
+                            const constraint = getBaseConstraintOfType(type) || anyType;
                             if (isTypeSubtypeOf(targetType, constraint)) {
                                 return getIntersectionType([type, targetType]);
                             }
@@ -16234,7 +16250,7 @@ namespace ts {
         function isLiteralContextualType(contextualType: Type) {
             if (contextualType) {
                 if (contextualType.flags & TypeFlags.TypeVariable) {
-                    const constraint = getBaseConstraintOfType(<TypeVariable>contextualType) || emptyObjectType;
+                    const constraint = getBaseConstraintOfType(contextualType) || emptyObjectType;
                     // 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.
@@ -17124,7 +17140,7 @@ namespace ts {
             // Check if we're indexing with a numeric type and the object type is a generic
             // type with a constraint that has a numeric index signature.
             if (maybeTypeOfKind(objectType, TypeFlags.TypeVariable) && isTypeOfKind(indexType, TypeFlags.NumberLike)) {
-                const constraint = getBaseConstraintOfType(<TypeVariable | UnionOrIntersectionType>objectType);
+                const constraint = getBaseConstraintOfType(objectType);
                 if (constraint && getIndexInfoOfType(constraint, IndexKind.Number)) {
                     return type;
                 }