Add a test with a negative impl that used to fail to pass coherence but now does

Author: jakubadamw

tests/ui/coherence/negative-coherence-ref-negative-impl.rs

@@ -0,0 +1,15 @@
+//! Regression test for <https://github.com/rust-lang/rust/issues/112588>.
+
+//@ check-pass
+
+#![feature(negative_impls, with_negative_coherence)]
+
+trait Trait {}
+impl<T: ?Sized> !Trait for &T {}
+
+trait OtherTrait<T> {}
+
+impl<T: Trait> OtherTrait<T> for T {}
+impl<T, U> OtherTrait<&U> for &T {}
+
+fn main() {}