Improve type relationships for generic mapped types

Author: ahejlsberg

src/compiler/checker.ts

@@ -4662,10 +4662,6 @@ namespace ts {
             return type.modifiersType;
         }
 
-        function getErasedTemplateTypeFromMappedType(type: MappedType) {
-            return instantiateType(getTemplateTypeFromMappedType(type), createTypeEraser([getTypeParameterFromMappedType(type)]));
-        }
-
         function isGenericMappedType(type: Type) {
             if (getObjectFlags(type) & ObjectFlags.Mapped) {
                 const constraintType = getConstraintTypeFromMappedType(<MappedType>type);
@@ -7765,25 +7761,24 @@ namespace ts {
                 return result;
             }
 
-            // A type [P in S]: X is related to a type [P in T]: Y if T is related to S and X is related to Y.
+            // A type [P in S]: X is related to a type [Q in T]: Y if T is related to S and X' is
+            // related to Y, where X' is an instantiation of X in which P is replaced with Q. Notice
+            // that S and T are contra-variant whereas X and Y are co-variant.
             function mappedTypeRelatedTo(source: Type, target: Type, reportErrors: boolean): Ternary {
                 if (isGenericMappedType(target)) {
                     if (isGenericMappedType(source)) {
-                        let result: Ternary;
-                        if (relation === identityRelation) {
-                            const readonlyMatches = !(<MappedType>source).declaration.readonlyToken === !(<MappedType>target).declaration.readonlyToken;
-                            const optionalMatches = !(<MappedType>source).declaration.questionToken === !(<MappedType>target).declaration.questionToken;
-                            if (readonlyMatches && optionalMatches) {
-                                if (result = isRelatedTo(getConstraintTypeFromMappedType(<MappedType>target), getConstraintTypeFromMappedType(<MappedType>source), reportErrors)) {
-                                    return result & isRelatedTo(getErasedTemplateTypeFromMappedType(<MappedType>source), getErasedTemplateTypeFromMappedType(<MappedType>target), reportErrors);
-                                }
-                            }
-                        }
-                        else {
-                            if (relation === comparableRelation || !(<MappedType>source).declaration.questionToken || (<MappedType>target).declaration.questionToken) {
-                                if (result = isRelatedTo(getConstraintTypeFromMappedType(<MappedType>target), getConstraintTypeFromMappedType(<MappedType>source), reportErrors)) {
-                                    return result & isRelatedTo(getTemplateTypeFromMappedType(<MappedType>source), getTemplateTypeFromMappedType(<MappedType>target), reportErrors);
-                                }
+                        const sourceReadonly = !!(<MappedType>source).declaration.readonlyToken;
+                        const sourceOptional = !!(<MappedType>source).declaration.questionToken;
+                        const targetReadonly = !!(<MappedType>target).declaration.readonlyToken;
+                        const targetOptional = !!(<MappedType>target).declaration.questionToken;
+                        const modifiersRelated = relation === identityRelation ?
+                            sourceReadonly === targetReadonly && sourceOptional === targetOptional :
+                            relation === comparableRelation || !sourceOptional || targetOptional;
+                        if (modifiersRelated) {
+                            let result: Ternary;
+                            if (result = isRelatedTo(getConstraintTypeFromMappedType(<MappedType>target), getConstraintTypeFromMappedType(<MappedType>source), reportErrors)) {
+                                const mapper = createTypeMapper([getTypeParameterFromMappedType(<MappedType>source)], [getTypeParameterFromMappedType(<MappedType>target)]);
+                                return result & isRelatedTo(instantiateType(getTemplateTypeFromMappedType(<MappedType>source), mapper), getTemplateTypeFromMappedType(<MappedType>target), reportErrors);
                             }
                         }
                     }