For 'T extends { [x: string]: XXX }' constraint of T[keyof T] is XXX

ahejlsberg · ahejlsberg · commit b14d38908520 · 2018-04-18T11:01:09.000-07:00
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
@@ -8230,8 +8230,9 @@ namespace ts {
                 maybeTypeOfKind(type, TypeFlags.InstantiableNonPrimitive) ? getIndexTypeForGenericType(<InstantiableType | UnionOrIntersectionType>type, includeDeclaredTypes) :
                 getObjectFlags(type) & ObjectFlags.Mapped ? getConstraintTypeFromMappedType(<MappedType>type) :
                 type === wildcardType ? wildcardType :
-                type.flags & TypeFlags.Any || getIndexInfoOfType(type, IndexKind.String) ? keyofConstraintType :
-                keyofStringsOnly ? getLiteralTypeFromPropertyNames(type, TypeFlags.StringLiteral) :
+                type.flags & TypeFlags.Any ? keyofConstraintType :
+                keyofStringsOnly ? getIndexInfoOfType(type, IndexKind.String) ? stringType : getLiteralTypeFromPropertyNames(type, TypeFlags.StringLiteral) :
+                getIndexInfoOfType(type, IndexKind.String) ? getUnionType([stringType, numberType, getLiteralTypeFromPropertyNames(type, TypeFlags.UniqueESSymbol)]) :
                 getNonEnumNumberIndexInfo(type) ? getUnionType([numberType, getLiteralTypeFromPropertyNames(type, TypeFlags.StringLiteral | TypeFlags.UniqueESSymbol)]) :
                 getLiteralTypeFromPropertyNames(type, TypeFlags.StringOrNumberLiteralOrUnique);
         }
@@ -8303,7 +8304,11 @@ namespace ts {
                     getIndexInfoOfType(objectType, IndexKind.String) ||
                     undefined;
                 if (indexInfo) {
-                    if (accessExpression && indexInfo.isReadonly && (isAssignmentTarget(accessExpression) || isDeleteTarget(accessExpression))) {
+                    if (accessNode && !isTypeAssignableToKind(indexType, TypeFlags.String | TypeFlags.Number)) {
+                        const indexNode = accessNode.kind === SyntaxKind.ElementAccessExpression ? accessNode.argumentExpression : accessNode.indexType;
+                        error(indexNode, Diagnostics.Type_0_cannot_be_used_as_an_index_type, typeToString(indexType));
+                    }
+                    else if (accessExpression && indexInfo.isReadonly && (isAssignmentTarget(accessExpression) || isDeleteTarget(accessExpression))) {
                         error(accessExpression, Diagnostics.Index_signature_in_type_0_only_permits_reading, typeToString(objectType));
                     }
                     return indexInfo.type;
@@ -20055,6 +20060,15 @@ namespace ts {
             return widened;
         }
 
+        function isTypeParameterWithKeyofConstraint(type: Type) {
+            if (type.flags & TypeFlags.TypeParameter) {
+                const constraintDeclaration = getConstraintDeclaration(<TypeParameter>type);
+                return constraintDeclaration && constraintDeclaration.kind === SyntaxKind.TypeOperator &&
+                    (<TypeOperatorNode>constraintDeclaration).operator === SyntaxKind.KeyOfKeyword;
+            }
+            return false;
+        }
+
         function isLiteralOfContextualType(candidateType: Type, contextualType: Type): boolean {
             if (contextualType) {
                 if (contextualType.flags & TypeFlags.UnionOrIntersection) {
@@ -20066,7 +20080,7 @@ namespace ts {
                     // this a literal context for literals of that primitive type. For example, given a
                     // type parameter 'T extends string', infer string literal types for T.
                     const constraint = getBaseConstraintOfType(contextualType) || emptyObjectType;
-                    return constraint === keyofConstraintType && maybeTypeOfKind(candidateType, TypeFlags.StringLiteral | TypeFlags.NumberLiteral | TypeFlags.UniqueESSymbol) ||
+                    return isTypeParameterWithKeyofConstraint(contextualType) && maybeTypeOfKind(candidateType, TypeFlags.StringLiteral | TypeFlags.NumberLiteral | TypeFlags.UniqueESSymbol) ||
                         constraint.flags & TypeFlags.String && maybeTypeOfKind(candidateType, TypeFlags.StringLiteral) ||
                         constraint.flags & TypeFlags.Number && maybeTypeOfKind(candidateType, TypeFlags.NumberLiteral) ||
                         constraint.flags & TypeFlags.Boolean && maybeTypeOfKind(candidateType, TypeFlags.BooleanLiteral) ||
