add a FIXME related to the non-free-region case

I don't think it would actually be harmful to just ignore such cases
but I'm inclined not to take chances.

src/test/ui/impl-trait/multiple-lifetimes/ordinary-bounds-pick-original-elided.rs

@@ -0,0 +1,27 @@
+// edition:2018
+// run-pass
+// revisions: migrate mir
+//[mir]compile-flags: -Z borrowck=mir
+
+trait Trait<'a, 'b> { }
+impl<T> Trait<'_, '_> for T { }
+
+// Test case where we have elision in the impl trait and we have to
+// pick the right region.
+
+// Ultimately `Trait<'x, 'static>`.
+fn upper_bounds1(a: &u8) -> impl Trait<'_, 'static> {
+    (a, a)
+}
+
+// Ultimately `Trait<'x, 'x>`, so not really multiple bounds.
+fn upper_bounds2(a: &u8) -> impl Trait<'_, '_> {
+    (a, a)
+}
+
+// Kind of a weird annoying case.
+fn upper_bounds3<'b>(a: &u8) -> impl Trait<'_, 'b> {
+    (a, a)
+}
+
+fn main() { }