Continue folding in query normalizer on weak aliases

compiler/rustc_trait_selection/src/traits/query/normalize.rs

@@ -322,8 +322,12 @@ impl<'cx, 'tcx> FallibleTypeFolder<TyCtxt<'tcx>> for QueryNormalizer<'cx, 'tcx>
                 };
                 // `tcx.normalize_projection_ty` may normalize to a type that still has
                 // unevaluated consts, so keep normalizing here if that's the case.
-                if res != ty && res.has_type_flags(ty::TypeFlags::HAS_CT_PROJECTION) {
+                // Similarly, `tcx.normalize_weak_ty` will only unwrap one layer of type
-                    res.try_super_fold_with(self)?
+                // and we need to continue folding it to reveal the TAIT behind it.
+                if res != ty
+                    && (res.has_type_flags(ty::TypeFlags::HAS_CT_PROJECTION) || kind == ty::Weak)
+                {
+                    res.try_fold_with(self)?
                 } else {
                     res
                 }

tests/ui/type-alias-impl-trait/debug-ty-with-weak.rs

@@ -0,0 +1,12 @@
+// compile-flags: --crate-type=lib -Cdebuginfo=2
+// build-pass
+
+#![feature(type_alias_impl_trait)]
+
+type Debuggable = impl core::fmt::Debug;
+
+static mut TEST: Option<Debuggable> = None;
+
+fn foo() -> Debuggable {
+    0u32
+}