This test now works

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -366,6 +366,7 @@ fn negative_impl_exists<'cx, 'tcx>(
         return true;
     }
 
+    // FIXME we should implement an `assemble_neg_candidates` fn
     for o in util::elaborate_predicates(infcx.tcx, iter::once(o.predicate)) {
         if resolve_negative_obligation(infcx, param_env, region_context, &o) {
             return true;

src/test/ui/coherence/coherence-overlap-negate-alias-strict.rs

@@ -1,3 +1,5 @@
+// check-pass
+
 #![feature(negative_impls)]
 #![feature(rustc_attrs)]
 #![feature(trait_alias)]
@@ -13,7 +15,5 @@ impl !A for u32 {}
 trait C {}
 impl<T: AB> C for T {}
 impl C for u32 {}
-//~^ ERROR: conflicting implementations of trait `C` for type `u32` [E0119]
-// FIXME this should work, we should implement an `assemble_neg_candidates` fn
 
 fn main() {}

src/test/ui/coherence/coherence-overlap-negate-alias-strict.stderr

@@ -1,11 +0,0 @@
-error[E0119]: conflicting implementations of trait `C` for type `u32`
-  --> $DIR/coherence-overlap-negate-alias-strict.rs:15:1
-   |
-LL | impl<T: AB> C for T {}
-   | ------------------- first implementation here
-LL | impl C for u32 {}
-   | ^^^^^^^^^^^^^^ conflicting implementation for `u32`
-
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0119`.