Avoid an ICE in the presence of HKL

compiler/rustc_borrowck/src/region_infer/opaque_types.rs

@@ -66,6 +66,10 @@ impl<'tcx> RegionInferenceContext<'tcx> {
 
                 let mut subst_regions = vec![self.universal_regions.fr_static];
                 let universal_substs = infcx.tcx.fold_regions(substs, &mut false, |region, _| {
+                    if let ty::RePlaceholder(..) = region {
+                        // Higher kinded regions don't need remapping, they don't refer to anything outside of this the substs.
+                        return region;
+                    }
                     let vid = self.to_region_vid(region);
                     trace!(?vid);
                     let scc = self.constraint_sccs.scc(vid);

src/test/ui/impl-trait/issues/issue-88236.rs

@@ -0,0 +1,15 @@
+// check-pass
+
+// this used to cause stack overflows
+
+trait Hrtb<'a> {
+    type Assoc;
+}
+
+impl<'a> Hrtb<'a> for () {
+    type Assoc = ();
+}
+
+fn make_impl() -> impl for<'a> Hrtb<'a, Assoc = impl Send + 'a> {}
+
+fn main() {}