add test for builtin types N + N unifying with fn call

src/test/ui/const-generics/generic_const_exprs/unify-op-with-fn-call.rs

@@ -1,7 +1,7 @@
-// check-pass
 #![feature(generic_const_exprs, adt_const_params, const_trait_impl)]
 #![allow(incomplete_features)]
 
+// test `N + N` unifies with explicit function calls for non-builtin-types
 #[derive(PartialEq, Eq)]
 struct Foo(u8);
 
@@ -21,4 +21,15 @@ fn foo<const N: Foo>(a: Evaluatable<{ N + N }>) {
 
 fn bar<const N: Foo>() {}
 
+// test that `N + N` unifies with explicit function calls for builin-types
+struct Evaluatable2<const N: usize>;
+
+fn foo2<const N: usize>(a: Evaluatable2<{ N + N }>) {
+    bar2::<{ std::ops::Add::add(N, N) }>();
+    //~^ error: unconstrained generic constant
+    // FIXME(generic_const_exprs) make this not an error
+}
+
+fn bar2<const N: usize>() {}
+
 fn main() {}