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

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1305,7 +1305,7 @@ impl EncodeContext<'a, 'tcx> {
             if encode_const {
                 record!(self.tables.mir_for_ctfe[def_id.to_def_id()] <- self.tcx.mir_for_ctfe(def_id));
 
-                // FIXME this feels wrong to have in `encode_mir`
+                // FIXME(generic_const_exprs): this feels wrong to have in `encode_mir`
                 let abstract_const = self.tcx.thir_abstract_const(def_id);
                 if let Ok(Some(abstract_const)) = abstract_const {
                     record!(self.tables.thir_abstract_consts[def_id.to_def_id()] <- abstract_const);

compiler/rustc_trait_selection/src/traits/const_evaluatable.rs

@@ -267,10 +267,16 @@ impl<'a, 'tcx> AbstractConstBuilder<'a, 'tcx> {
 
             fn visit_expr(&mut self, expr: &thir::Expr<'tcx>) {
                 self.is_poly |= expr.ty.definitely_has_param_types_or_consts(self.tcx);
-                if self.is_poly {
-                    return;
+                if self.is_poly == false {
+                    visit::walk_expr(self, expr)
+                }
+            }
+
+            fn visit_pat(&mut self, pat: &thir::Pat<'tcx>) {
+                self.is_poly |= pat.ty.definitely_has_param_types_or_consts(self.tcx);
+                if self.is_poly == false {
+                    visit::walk_pat(self, pat);
                 }
-                visit::walk_expr(self, expr);
             }
 
             fn visit_const(&mut self, ct: &'tcx ty::Const<'tcx>) {
@@ -280,6 +286,7 @@ impl<'a, 'tcx> AbstractConstBuilder<'a, 'tcx> {
 
         let mut is_poly_vis = IsThirPolymorphic { is_poly: false, thir: body, tcx };
         visit::walk_expr(&mut is_poly_vis, &body[body_id]);
+        debug!("AbstractConstBuilder: is_poly={}", is_poly_vis.is_poly);
         if is_poly_vis.is_poly == false {
             return Ok(None);
         }

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() {}

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

@@ -0,0 +1,10 @@
+error: unconstrained generic constant
+  --> $DIR/unify-op-with-fn-call.rs:28:12
+   |
+LL |     bar2::<{ std::ops::Add::add(N, N) }>();
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: try adding a `where` bound using this expression: `where [(); { std::ops::Add::add(N, N) }]:`
+
+error: aborting due to previous error
+