add a test for nested type ascriptions of never type

Author: WaffleLapkin

tests/crashes/nested_type_ascription.rs

@@ -0,0 +1,20 @@
+// Regression test for <https://github.com/rust-lang/rust/issues/149542>.
+//
+// This checks that a nested type ascription doesn't cause a crash when the
+// compiler checks if it constitutes a read of the never type.
+//
+//@ known-bug: #149542
+
+#![feature(never_type)]
+#![feature(type_ascription)]
+#![deny(unreachable_code)]
+
+fn main() {
+    unsafe {
+        let _ = type_ascribe!(type_ascribe!(*std::ptr::null(), !), _);
+
+        // this is *not* unreachable, because previous line does not actually read the never type
+        ();
+    }
+}
+