Regression test for issue 55608.

Author: pnkfelix

src/test/ui/impl-trait/issue-55608-captures-empty-region.rs

@@ -0,0 +1,22 @@
+// This used to ICE because it creates an `impl Trait` that captures a
+// hidden empty region.
+
+#![feature(conservative_impl_trait)]
+
+fn server() -> impl FilterBase2 { //~ ERROR [E0700]
+    segment2(|| { loop { } }).map2(|| "")
+}
+
+trait FilterBase2 {
+    fn map2<F>(self, _fn: F) -> Map2<F> where Self: Sized { loop { } }
+}
+
+struct Map2<F> { _func: F }
+
+impl<F> FilterBase2 for Map2<F> { }
+
+fn segment2<F>(_fn: F) -> Map2<F> where F: Fn() -> Result<(), ()> {
+    loop { }
+}
+
+fn main() { server(); }

src/test/ui/impl-trait/issue-55608-captures-empty-region.stderr

@@ -0,0 +1,11 @@
+error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
+  --> $DIR/issue-55608-captures-empty-region.rs:6:16
+   |
+LL | fn server() -> impl FilterBase2 { //~ ERROR [E0700]
+   |                ^^^^^^^^^^^^^^^^
+   |
+   = note: hidden type `Map2<[closure@$DIR/issue-55608-captures-empty-region.rs:7:36: 7:41]>` captures an empty lifetime
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0700`.