Test match underscore on void from union.

tests/pass/union-uninhabited-match-underscore.rs

@@ -0,0 +1,17 @@
+fn main() {
+    #[derive(Copy, Clone)]
+    enum Void {}
+    union Uninit<T: Copy> {
+        value: T,
+        uninit: (),
+    }
+    unsafe {
+        let x: Uninit<Void> = Uninit { uninit: () };
+        match x.value {
+            // rustc warns about un unreachable pattern,
+            // but is wrong in unsafe code.
+            #[allow(unreachable_patterns)]
+            _ => println!("hi from the void!"),
+        }
+    }
+}

tests/pass/union-uninhabited-match-underscore.stdout

@@ -0,0 +1 @@
+hi from the void!