Inherit generics for impl-trait.

2 files changed, 23 insertions, 3 deletions

diff --git a/compiler/rustc_middle/src/ty/mod.rs b/compiler/rustc_middle/src/ty/mod.rs
index 18eb06b83c9..bf000e4fd0e 100644
--- a/compiler/rustc_middle/src/ty/mod.rs
+++ b/compiler/rustc_middle/src/ty/mod.rs
@@ -1338,8 +1338,8 @@ impl<'tcx> OpaqueHiddenType<'tcx> {
             // HACK: The HIR lowering for async fn does not generate
             // any `+ Captures<'x>` bounds for the `impl Future<...>`, so all async fns with lifetimes
             // would now fail to compile. We should probably just make hir lowering fill this in properly.
-            OpaqueTyOrigin::AsyncFn(_) => map.collect(),
-            OpaqueTyOrigin::FnReturn(_) | OpaqueTyOrigin::TyAlias => {
+            OpaqueTyOrigin::FnReturn(_) | OpaqueTyOrigin::AsyncFn(_) => map.collect(),
+            OpaqueTyOrigin::TyAlias => {
                 // Opaque types may only use regions that are bound. So for
                 // ```rust
                 // type Foo<'a, 'b, 'c> = impl Trait<'a> + 'b;
diff --git a/compiler/rustc_middle/src/ty/relate.rs b/compiler/rustc_middle/src/ty/relate.rs
index c083a405e3c..42a1314b14d 100644
--- a/compiler/rustc_middle/src/ty/relate.rs
+++ b/compiler/rustc_middle/src/ty/relate.rs
@@ -170,6 +170,26 @@ pub fn relate_substs_with_variances<'tcx, R: TypeRelation<'tcx>>(
     tcx.mk_substs(params)
 }
 
+#[instrument(level = "trace", skip(relation), ret)]
+fn relate_opaque_item_substs<'tcx, R: TypeRelation<'tcx>>(
+    relation: &mut R,
+    def_id: DefId,
+    a_subst: SubstsRef<'tcx>,
+    b_subst: SubstsRef<'tcx>,
+) -> RelateResult<'tcx, SubstsRef<'tcx>> {
+    let tcx = relation.tcx();
+    let variances = tcx.variances_of(def_id);
+    debug!(?variances);
+
+    let params = iter::zip(a_subst, b_subst).enumerate().map(|(i, (a, b))| {
+        let variance = variances[i];
+        let variance_info = ty::VarianceDiagInfo::default();
+        relation.relate_with_variance(variance, variance_info, a, b)
+    });
+
+    tcx.mk_substs(params)
+}
+
 impl<'tcx> Relate<'tcx> for ty::FnSig<'tcx> {
     fn relate<R: TypeRelation<'tcx>>(
         relation: &mut R,
@@ -561,7 +581,7 @@ pub fn super_relate_tys<'tcx, R: TypeRelation<'tcx>>(
         (&ty::Opaque(a_def_id, a_substs), &ty::Opaque(b_def_id, b_substs))
             if a_def_id == b_def_id =>
         {
-            let substs = relate_substs(relation, a_substs, b_substs)?;
+            let substs = relate_opaque_item_substs(relation, a_def_id, a_substs, b_substs)?;
             Ok(tcx.mk_opaque(a_def_id, substs))
         }