revert #107074

3 files changed, 14 insertions, 2 deletions

diff --git a/compiler/rustc_const_eval/src/transform/validate.rs b/compiler/rustc_const_eval/src/transform/validate.rs
index 76b316cdf0c..56c60d59d28 100644
--- a/compiler/rustc_const_eval/src/transform/validate.rs
+++ b/compiler/rustc_const_eval/src/transform/validate.rs
@@ -13,7 +13,7 @@ use rustc_middle::mir::{
     RetagKind, RuntimePhase, Rvalue, SourceScope, Statement, StatementKind, Terminator,
     TerminatorKind, UnOp, START_BLOCK,
 };
-use rustc_middle::ty::{self, InstanceDef, ParamEnv, Ty, TyCtxt};
+use rustc_middle::ty::{self, InstanceDef, ParamEnv, Ty, TyCtxt, TypeVisitable};
 use rustc_mir_dataflow::impls::MaybeStorageLive;
 use rustc_mir_dataflow::storage::always_storage_live_locals;
 use rustc_mir_dataflow::{Analysis, ResultsCursor};
@@ -231,6 +231,15 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             return true;
         }
 
+        // We sometimes have to use `defining_opaque_types` for subtyping
+        // to succeed here and figuring out how exactly that should work
+        // is annoying. It is harmless enough to just not validate anything
+        // in that case. We still check this after analysis as all opque
+        // types have been revealed at this point.
+        if (src, dest).has_opaque_types() {
+            return true;
+        }
+
         crate::util::is_subtype(self.tcx, self.param_env, src, dest)
     }
 }
diff --git a/tests/ui/impl-trait/nested-return-type2.rs b/tests/ui/impl-trait/nested-return-type2.rs
index cc1f1f4ec44..fe883ce6fc8 100644
--- a/tests/ui/impl-trait/nested-return-type2.rs
+++ b/tests/ui/impl-trait/nested-return-type2.rs
@@ -1,4 +1,7 @@
 // check-pass
+// compile-flags: -Zvalidate-mir
+
+// Using -Zvalidate-mir as a regression test for #107346.
 
 trait Duh {}
 
diff --git a/tests/ui/impl-trait/nested-return-type2.stderr b/tests/ui/impl-trait/nested-return-type2.stderr
index 3aed05ca132..09ad3bd05c1 100644
--- a/tests/ui/impl-trait/nested-return-type2.stderr
+++ b/tests/ui/impl-trait/nested-return-type2.stderr
@@ -1,5 +1,5 @@
 warning: opaque type `impl Trait<Assoc = impl Send>` does not satisfy its associated type bounds
-  --> $DIR/nested-return-type2.rs:25:24
+  --> $DIR/nested-return-type2.rs:28:24
    |
 LL |     type Assoc: Duh;
    |                 --- this associated type bound is unsatisfied for `impl Send`