add tests and comment

compiler/rustc_traits/src/evaluate_obligation.rs

@@ -16,6 +16,8 @@ fn evaluate_obligation<'tcx>(
     canonical_goal: CanonicalPredicateGoal<'tcx>,
 ) -> Result<EvaluationResult, OverflowError> {
     debug!("evaluate_obligation(canonical_goal={:#?})", canonical_goal);
+    // HACK This bubble is required for this tests to pass:
+    // impl-trait/issue99642.rs
     tcx.infer_ctxt().with_opaque_type_inference(DefiningAnchor::Bubble).enter_with_canonical(
         DUMMY_SP,
         &canonical_goal,

compiler/rustc_traits/src/type_op.rs

@@ -258,6 +258,8 @@ fn type_op_prove_predicate<'tcx>(
     tcx: TyCtxt<'tcx>,
     canonicalized: Canonical<'tcx, ParamEnvAnd<'tcx, ProvePredicate<'tcx>>>,
 ) -> Result<&'tcx Canonical<'tcx, QueryResponse<'tcx, ()>>, NoSolution> {
+    // HACK This bubble is required for this test to pass:
+    // impl-trait/issue-99642.rs
     tcx.infer_ctxt().with_opaque_type_inference(DefiningAnchor::Bubble).enter_canonical_trait_query(
         &canonicalized,
         |infcx, fulfill_cx, key| {

src/test/ui/impl-trait/issue-99642-2.rs

@@ -0,0 +1,8 @@
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+type Opq = impl Sized;
+fn test() -> impl Iterator<Item = Opq> {
+    Box::new(0..) as Box<dyn Iterator<Item = _>>
+}
+fn main(){}

src/test/ui/impl-trait/issue-99642.rs

@@ -0,0 +1,7 @@
+// check-pass
+
+fn test() -> impl Iterator<Item = impl Sized> {
+    Box::new(0..) as Box<dyn Iterator<Item = _>>
+}
+
+fn main() {}