Fortify generic param default checks

tests/ui/generics/generic-extern.rs

@@ -1,7 +1,15 @@
+// Ensure that we reject generic parameters on foreign items.
+
 extern "C" {
     fn foo<T>(); //~ ERROR foreign items may not have type parameters
+
+    // Furthermore, check that type parameter defaults lead to a *hard* error,
+    // not just a lint error, for maximum forward compatibility.
+    #[allow(invalid_type_param_default)] // Should have no effect here.
+    fn bar<T = ()>(); //~ ERROR foreign items may not have type parameters
+    //~^ ERROR defaults for generic parameters are not allowed here
 }
 
 fn main() {
-    foo::<i32>(); //~ ERROR requires unsafe
+    unsafe { foo::<i32>() };
 }