Warn for type parameter defaults on impl blocks

src/test/compile-fail/type-parameter-invalid-lint.rs

@@ -14,4 +14,10 @@
 fn avg<T=i32>(_: T) {}
 //~^ ERROR defaults for type parameters are only allowed
 //~| WARNING hard error
+
+struct S<T>(T);
+impl<T=i32> S<T> {}
+//~^ ERROR defaults for type parameters are only allowed
+//~| WARNING hard error
+
 fn main() {}