Avoid expensive relationship checking in mapped type member resolution

Author: ahejlsberg

src/compiler/checker.ts

@@ -9589,7 +9589,7 @@ namespace ts {
                     // When creating an optional property in strictNullChecks mode, if 'undefined' isn't assignable to the
                     // type, we include 'undefined' in the type. Similarly, when creating a non-optional property in strictNullChecks
                     // mode, if the underlying property is optional we remove 'undefined' from the type.
-                    prop.type = strictNullChecks && isOptional && !isTypeAssignableTo(undefinedType, propType) ? getOptionalType(propType) :
+                    prop.type = strictNullChecks && isOptional && !maybeTypeOfKind(propType, TypeFlags.Undefined | TypeFlags.Void) ? getOptionalType(propType) :
                         strictNullChecks && !isOptional && modifiersProp && modifiersProp.flags & SymbolFlags.Optional ? getTypeWithFacts(propType, TypeFacts.NEUndefined) :
                         propType;
                     if (modifiersProp) {
@@ -13681,7 +13681,7 @@ namespace ts {
             const templateMapper = combineTypeMappers(mapper, createTypeMapper([getTypeParameterFromMappedType(type)], [key]));
             const propType = instantiateType(getTemplateTypeFromMappedType(<MappedType>type.target || type), templateMapper);
             const modifiers = getMappedTypeModifiers(type);
-            return strictNullChecks && modifiers & MappedTypeModifiers.IncludeOptional && !isTypeAssignableTo(undefinedType, propType) ? getOptionalType(propType) :
+            return strictNullChecks && modifiers & MappedTypeModifiers.IncludeOptional && !maybeTypeOfKind(propType, TypeFlags.Undefined | TypeFlags.Void) ? getOptionalType(propType) :
                 strictNullChecks && modifiers & MappedTypeModifiers.ExcludeOptional && isOptional ? getTypeWithFacts(propType, TypeFacts.NEUndefined) :
                 propType;
         }