Fix assoc-type test

compiler/rustc_trait_selection/src/traits/query/evaluate_obligation.rs

@@ -1,3 +1,6 @@
+use rustc_hir as hir;
+use rustc_middle::ty::PredicateKind;
+
 use crate::infer::canonical::OriginalQueryValues;
 use crate::infer::InferCtxt;
 use crate::traits::{
@@ -46,6 +49,12 @@ impl<'cx, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'cx, 'tcx> {
         &self,
         obligation: &PredicateObligation<'tcx>,
     ) -> bool {
+        if let PredicateKind::Trait(pred) = obligation.predicate.kind().skip_binder() {
+            if let hir::Constness::Const = pred.constness {
+                // do not evaluate to holds when we have a const predicate.
+                return false;
+            }
+        }
         self.evaluate_obligation_no_overflow(obligation).must_apply_considering_regions()
     }
 

src/test/ui/rfc-2632-const-trait-impl/assoc-type.rs

@@ -1,8 +1,4 @@
-// ignore-test
+// FIXME(fee1-dead): this should have a better error message
-//
-// FIXME: This test should fail since, within a const impl of `Foo`, the bound on `Foo::Bar` should
-// require a const impl of `Add` for the associated type.
-
 #![feature(const_trait_impl)]
 
 struct NonConstAdd(i32);
@@ -21,6 +17,7 @@ trait Foo {
 
 impl const Foo for NonConstAdd {
     type Bar = NonConstAdd;
+    //~^ ERROR
 }
 
 fn main() {}

src/test/ui/rfc-2632-const-trait-impl/assoc-type.stderr

@@ -0,0 +1,18 @@
+error[E0277]: cannot add `NonConstAdd` to `NonConstAdd`
+  --> $DIR/assoc-type.rs:19:5
+   |
+LL |     type Bar: std::ops::Add;
+   |               ------------- required by this bound in `Foo::Bar`
+...
+LL |     type Bar = NonConstAdd;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^ no implementation for `NonConstAdd + NonConstAdd`
+   |
+   = help: the trait `Add` is not implemented for `NonConstAdd`
+help: consider introducing a `where` bound, but there might be an alternative better way to express this requirement
+   |
+LL | impl const Foo for NonConstAdd where NonConstAdd: Add {
+   |                                ^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.