Don't mention IMPLIED_BOUNDS_ENTAILMENT if signatures reference error

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -302,7 +302,7 @@ fn compare_method_predicate_entailment<'tcx>(
         return Err(emitted);
     }
 
-    if check_implied_wf == CheckImpliedWfMode::Check {
+    if check_implied_wf == CheckImpliedWfMode::Check && !(impl_sig, trait_sig).references_error() {
         // We need to check that the impl's args are well-formed given
         // the hybrid param-env (impl + trait method where-clauses).
         ocx.register_obligation(traits::Obligation::new(

tests/ui/implied-bounds/references-err.rs

@@ -0,0 +1,22 @@
+trait Identity {
+    type Identity;
+}
+impl<T> Identity for T {
+    type Identity = T;
+}
+
+trait Trait {
+    type Assoc: Identity;
+    fn tokenize(&self) -> <Self::Assoc as Identity>::Identity;
+}
+
+impl Trait for () {
+    type Assoc = DoesNotExist;
+    //~^ ERROR cannot find type `DoesNotExist` in this scope
+
+    fn tokenize(&self) -> <Self::Assoc as Identity>::Identity {
+        unimplemented!()
+    }
+}
+
+fn main() {}

tests/ui/implied-bounds/references-err.stderr

@@ -0,0 +1,9 @@
+error[E0412]: cannot find type `DoesNotExist` in this scope
+  --> $DIR/references-err.rs:14:18
+   |
+LL |     type Assoc = DoesNotExist;
+   |                  ^^^^^^^^^^^^ not found in this scope
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0412`.