src/diagnosticMessages.json

@@ -119,6 +119,7 @@
   "Decorators are not valid here.": 1206,
   "'abstract' modifier can only appear on a class, method, or property declaration.": 1242,
   "Method '{0}' cannot have an implementation because it is marked abstract.": 1245,
+  "An interface property cannot have an initializer.": 1246,
   "A definite assignment assertion '!' is not permitted in this context.": 1255,
   "A class may only extend another class.": 1311,
   "A parameter property cannot be declared using a rest parameter.": 1317,

src/program.ts

@@ -2671,6 +2671,10 @@ export class Program extends DiagnosticEmitter {
     /** Parent interface. */
     parent: InterfacePrototype
   ): void {
+    let initializer = declaration.initializer;
+    if (initializer) {
+      this.error(DiagnosticCode.An_interface_property_cannot_have_an_initializer, initializer.range);
+    }
     let typeNode = declaration.type;
     if (!typeNode) typeNode = Node.createOmittedType(declaration.name.range.atEnd);
     this.initializeProperty(

tests/compiler/interface-with-initializer.json

@@ -0,0 +1,7 @@
+{
+  "asc_flags": [
+  ],
+  "stderr": [
+    "TS1246: An interface property cannot have an initializer."
+  ]
+}

tests/compiler/interface-with-initializer.ts

@@ -0,0 +1,9 @@
+interface I {
+  v: string = "";
+}
+
+class C implements I {
+  v: string = "";
+}
+
+new C();