Implementation for weak types

src/compiler/checker.ts

@@ -4785,9 +4785,14 @@ namespace ts {
                 let result = Ternary.True;
                 let properties = getPropertiesOfObjectType(target);
                 let requireOptionalProperties = relation === subtypeRelation && !(source.flags & TypeFlags.ObjectLiteral);
+                let foundMatchingProperty = !isWeak(target);
                 for (let targetProp of properties) {
                     let sourceProp = getPropertyOfType(source, targetProp.name);
 
+                    if (sourceProp) {
+                        foundMatchingProperty = true;
+                    }
+
                     if (sourceProp !== targetProp) {
                         if (!sourceProp) {
                             if (!(targetProp.flags & SymbolFlags.Optional) || requireOptionalProperties) {
@@ -4859,6 +4864,14 @@ namespace ts {
                         }
                     }
                 }
+
+                if (!foundMatchingProperty && getPropertiesOfType(source).length > 0) {
+                    if (reportErrors) {
+                        reportError(Diagnostics.Weak_type_0_has_no_properties_in_common_with_1, typeToString(target), typeToString(source));
+                    }
+                    return Ternary.False;
+                }
+
                 return result;
             }
 
@@ -5121,6 +5134,17 @@ namespace ts {
             return false;
         }
 
+        // A type is 'weak' if it is an object type with at least one optional property
+        // and no required properties or index signatures
+        function isWeak(type: Type) {
+            let props = getPropertiesOfType(type);
+            return type.flags & TypeFlags.ObjectType &&
+                props.length > 0 &&
+                !forEach(props, p => !(p.flags & SymbolFlags.Optional)) &&
+                !getIndexTypeOfType(type, IndexKind.String) &&
+                !getIndexTypeOfType(type, IndexKind.Number);
+        }
+
         function isPropertyIdenticalTo(sourceProp: Symbol, targetProp: Symbol): boolean {
             return compareProperties(sourceProp, targetProp, compareTypes) !== Ternary.False;
         }

src/compiler/diagnosticInformationMap.generated.ts

@@ -414,6 +414,7 @@ namespace ts {
         The_arguments_object_cannot_be_referenced_in_an_async_arrow_function_Consider_using_a_standard_async_function_expression: { code: 2522, category: DiagnosticCategory.Error, key: "The 'arguments' object cannot be referenced in an async arrow function. Consider using a standard async function expression." },
         yield_expressions_cannot_be_used_in_a_parameter_initializer: { code: 2523, category: DiagnosticCategory.Error, key: "'yield' expressions cannot be used in a parameter initializer." },
         await_expressions_cannot_be_used_in_a_parameter_initializer: { code: 2524, category: DiagnosticCategory.Error, key: "'await' expressions cannot be used in a parameter initializer." },
+        Weak_type_0_has_no_properties_in_common_with_1: { code: 2525, category: DiagnosticCategory.Error, key: "Weak type '{0}' has no properties in common with '{1}'." },
         JSX_element_attributes_type_0_must_be_an_object_type: { code: 2600, category: DiagnosticCategory.Error, key: "JSX element attributes type '{0}' must be an object type." },
         The_return_type_of_a_JSX_element_constructor_must_return_an_object_type: { code: 2601, category: DiagnosticCategory.Error, key: "The return type of a JSX element constructor must return an object type." },
         JSX_element_implicitly_has_type_any_because_the_global_type_JSX_Element_does_not_exist: { code: 2602, category: DiagnosticCategory.Error, key: "JSX element implicitly has type 'any' because the global type 'JSX.Element' does not exist." },

src/compiler/diagnosticMessages.json

@@ -1645,6 +1645,10 @@
         "category": "Error",
         "code": 2524
     },
+    "Weak type '{0}' has no properties in common with '{1}'.": {
+        "category": "Error",
+        "code": 2525
+    },
     "JSX element attributes type '{0}' must be an object type.": {
         "category": "Error",
         "code": 2600

src/server/editorServices.ts

@@ -912,7 +912,7 @@ namespace ts.server {
             var dirPath = ts.getDirectoryPath(configFilename);
             var rawConfig: { config?: ProjectOptions; error?: Diagnostic; } = ts.readConfigFile(configFilename);
             if (rawConfig.error) {
-                return rawConfig.error;
+                return { errorMsg: ts.flattenDiagnosticMessageText(rawConfig.error.messageText, '\n') };
             }
             else {
                 var parsedCommandLine = ts.parseConfigFile(rawConfig.config, this.host, dirPath);