Don't try to lower ReEmpty in NLL

Author: matthewjasper

src/librustc_mir/borrow_check/nll/type_check/free_region_relations.rs

@@ -334,6 +334,13 @@ impl UniversalRegionRelationsBuilder<'cx, 'gcx, 'tcx> {
 
             match outlives_bound {
                 OutlivesBound::RegionSubRegion(r1, r2) => {
+                    // `where Type:` is lowered to `where Type: 'empty` so that
+                    // we check `Type` is well formed, but there's no use for
+                    // this bound here.
+                    if let ty::ReEmpty = r1 {
+                        return;
+                    }
+
                     // The bound says that `r1 <= r2`; we store `r2: r1`.
                     let r1 = self.universal_regions.to_region_vid(r1);
                     let r2 = self.universal_regions.to_region_vid(r2);

src/test/ui/nll/empty-type-predicate.rs

@@ -0,0 +1,11 @@
+// Regression test for #61315
+//
+// `dyn T:` is lowered to `dyn T: ReEmpty` - check that we don't ICE in NLL for
+// the unexpected region.
+
+// compile-pass
+
+trait T {}
+fn f() where dyn T: {}
+
+fn main() {}