Test generalization during coherence

compiler/rustc_infer/src/infer/combine.rs

@@ -548,7 +548,7 @@ impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
     }
 
     fn mark_ambiguous(&mut self) {
-        self.infcx.tcx.sess.delay_span_bug(self.cause.span, "we only generalize opaque types in situations where we already error for them elsewhere in coherence");
+        span_bug!(self.cause.span, "opaque types are handled in `tys`");
     }
 
     fn binders<T>(
@@ -675,6 +675,10 @@ impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
                 // relatable.
                 Ok(t)
             }
+            ty::Opaque(def_id, substs) => {
+                let s = self.relate(substs, substs)?;
+                Ok(if s == substs { t } else { self.infcx.tcx.mk_opaque(def_id, s) })
+            }
             _ => relate::super_relate_tys(self, t, t),
         }?;
 

src/test/ui/type-alias-impl-trait/coherence_generalization.rs

@@ -0,0 +1,13 @@
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+trait Trait {}
+type Opaque<T> = impl Sized;
+fn foo<T>() -> Opaque<T> {
+    ()
+}
+
+impl<T, V> Trait for (T, V, V, u32) {}
+impl<U, V> Trait for (Opaque<U>, V, i32, V) {}
+
+fn main() {}