Give more helpful error when trying to set default values on an interface.

src/compiler/checker.ts

@@ -532,7 +532,7 @@ namespace ts {
                             }
 
                             // Because of module/namespace merging, a module's exports are in scope,
-                            // yet we never want to treat an export specifier as putting a member in scope. 
+                            // yet we never want to treat an export specifier as putting a member in scope.
                             // Therefore, if the name we find is purely an export specifier, it is not actually considered in scope.
                             // Two things to note about this:
                             //     1. We have to check this without calling getSymbol. The problem with calling getSymbol
@@ -11398,7 +11398,7 @@ namespace ts {
                             // we can get here in two cases
                             // 1. mixed static and instance class members
                             // 2. something with the same name was defined before the set of overloads that prevents them from merging
-                            // here we'll report error only for the first case since for second we should already report error in binder 
+                            // here we'll report error only for the first case since for second we should already report error in binder
                             if (reportError) {
                                 const diagnostic = node.flags & NodeFlags.Static ? Diagnostics.Function_overload_must_be_static : Diagnostics.Function_overload_must_not_be_static;
                                 error(errorNode, diagnostic);
@@ -12065,8 +12065,8 @@ namespace ts {
                 const symbol = getSymbolOfNode(node);
                 const localSymbol = node.localSymbol || symbol;
 
-                // Since the javascript won't do semantic analysis like typescript, 
-                // if the javascript file comes before the typescript file and both contain same name functions, 
+                // Since the javascript won't do semantic analysis like typescript,
+                // if the javascript file comes before the typescript file and both contain same name functions,
                 // checkFunctionOrConstructorSymbol wouldn't be called if we didnt ignore javascript function.
                 const firstDeclaration = forEach(localSymbol.declarations,
                     // Get first non javascript function declaration
@@ -16227,11 +16227,17 @@ namespace ts {
                 if (checkGrammarForNonSymbolComputedProperty(node.name, Diagnostics.A_computed_property_name_in_an_interface_must_directly_refer_to_a_built_in_symbol)) {
                     return true;
                 }
+                if (node.initializer) {
+                    return grammarErrorOnNode(node.initializer, Diagnostics.An_interface_property_cannot_have_an_initializer);
+                }
             }
             else if (node.parent.kind === SyntaxKind.TypeLiteral) {
                 if (checkGrammarForNonSymbolComputedProperty(node.name, Diagnostics.A_computed_property_name_in_a_type_literal_must_directly_refer_to_a_built_in_symbol)) {
                     return true;
                 }
+                if (node.initializer) {
+                    return grammarErrorOnNode(node.initializer, Diagnostics.A_type_literal_property_cannot_have_an_initializer);
+                }
             }
 
             if (isInAmbientContext(node) && node.initializer) {

src/compiler/diagnosticMessages.json

@@ -783,6 +783,14 @@
         "category": "Error",
         "code": 1245
     },
+    "An interface property cannot have an initializer.": {
+        "category": "Error",
+        "code": 1246
+    },
+    "A type literal property cannot have an initializer.": {
+        "category": "Error",
+        "code": 1247
+    },
 
     "'with' statements are not allowed in an async function block.": {
         "category": "Error",
@@ -2418,7 +2426,7 @@
     "Not all code paths return a value.": {
         "category": "Error",
         "code": 7030
-    },    
+    },
     "You cannot rename this element.": {
         "category": "Error",
         "code": 8000

src/compiler/parser.ts

@@ -2247,6 +2247,14 @@ namespace ts {
                 property.name = name;
                 property.questionToken = questionToken;
                 property.type = parseTypeAnnotation();
+
+                if (token === SyntaxKind.EqualsToken) {
+                    // Although type literal properties cannot not have initializers, we attempt
+                    // to parse an initializer so we can report in the checker that an interface
+                    // property or type literal property cannot have an initializer.
+                    property.initializer = parseNonParameterInitializer();
+                }
+
                 parseTypeMemberSemicolon();
                 return finishNode(property);
             }

src/compiler/types.ts

@@ -586,6 +586,7 @@ namespace ts {
         name: PropertyName;                 // Declared property name
         questionToken?: Node;               // Present on optional property
         type?: TypeNode;                    // Optional type annotation
+        initializer?: Expression;           // Optional initializer
     }
 
     // @kind(SyntaxKind.PropertyDeclaration)

tests/baselines/reference/errorOnInitializerInInterfaceProperty.errors.txt

@@ -0,0 +1,10 @@
+tests/cases/compiler/errorOnInitializerInInterfaceProperty.ts(2,19): error TS1246: An interface property cannot have an initializer.
+
+
+==== tests/cases/compiler/errorOnInitializerInInterfaceProperty.ts (1 errors) ====
+    interface Foo {
+        bar: number = 5;
+                      ~
+!!! error TS1246: An interface property cannot have an initializer.
+    }
+

tests/baselines/reference/errorOnInitializerInInterfaceProperty.js

@@ -0,0 +1,7 @@
+//// [errorOnInitializerInInterfaceProperty.ts]
+interface Foo {
+    bar: number = 5;
+}
+
+
+//// [errorOnInitializerInInterfaceProperty.js]

tests/baselines/reference/errorOnInitializerInObjectTypeLiteralProperty.errors.txt

@@ -0,0 +1,17 @@
+tests/cases/compiler/errorOnInitializerInObjectTypeLiteralProperty.ts(2,19): error TS1247: A type literal property cannot have an initializer.
+tests/cases/compiler/errorOnInitializerInObjectTypeLiteralProperty.ts(6,19): error TS1247: A type literal property cannot have an initializer.
+
+
+==== tests/cases/compiler/errorOnInitializerInObjectTypeLiteralProperty.ts (2 errors) ====
+    var Foo: {
+        bar: number = 5;
+                      ~
+!!! error TS1247: A type literal property cannot have an initializer.
+    };
+
+    let Bar: {
+        bar: number = 5;
+                      ~
+!!! error TS1247: A type literal property cannot have an initializer.
+    };
+

tests/baselines/reference/errorOnInitializerInObjectTypeLiteralProperty.js

@@ -0,0 +1,13 @@
+//// [errorOnInitializerInObjectTypeLiteralProperty.ts]
+var Foo: {
+    bar: number = 5;
+};
+
+let Bar: {
+    bar: number = 5;
+};
+
+
+//// [errorOnInitializerInObjectTypeLiteralProperty.js]
+var Foo;
+var Bar;

tests/cases/compiler/errorOnInitializerInInterfaceProperty.ts

@@ -0,0 +1,3 @@
+interface Foo {
+    bar: number = 5;
+}

tests/cases/compiler/errorOnInitializerInObjectTypeLiteralProperty.ts

@@ -0,0 +1,7 @@
+var Foo: {
+    bar: number = 5;
+};
+
+let Bar: {
+    bar: number = 5;
+};