2 files changed, 69 insertions, 0 deletions
diff --git a/tests/ui/pattern/usefulness/unions.rs b/tests/ui/pattern/usefulness/unions.rs
new file mode 100644
index 00000000000..80a7f36a09a
--- /dev/null
+++ b/tests/ui/pattern/usefulness/unions.rs
@@ -0,0 +1,35 @@
+fn main() {
+    #[derive(Copy, Clone)]
+    union U8AsBool {
+        n: u8,
+        b: bool,
+    }
+
+    let x = U8AsBool { n: 1 };
+    unsafe {
+        match x {
+            // exhaustive
+            U8AsBool { n: 2 } => {}
+            U8AsBool { b: true } => {}
+            U8AsBool { b: false } => {}
+        }
+        match x {
+            // exhaustive
+            U8AsBool { b: true } => {}
+            U8AsBool { n: 0 } => {}
+            U8AsBool { n: 1.. } => {}
+        }
+        match x {
+            //~^ ERROR non-exhaustive patterns: `U8AsBool { n: 0_u8 }` and `U8AsBool { b: false }` not covered
+            U8AsBool { b: true } => {}
+            U8AsBool { n: 1.. } => {}
+        }
+        // Our approach can report duplicate witnesses sometimes.
+        match (x, true) {
+            //~^ ERROR non-exhaustive patterns: `(U8AsBool { n: 0_u8 }, false)`, `(U8AsBool { b: false }, false)`, `(U8AsBool { n: 0_u8 }, false)` and 1 more not covered
+            (U8AsBool { b: true }, true) => {}
+            (U8AsBool { b: false }, true) => {}
+            (U8AsBool { n: 1.. }, true) => {}
+        }
+    }
+}
diff --git a/tests/ui/pattern/usefulness/unions.stderr b/tests/ui/pattern/usefulness/unions.stderr
new file mode 100644
index 00000000000..4b397dc25db
--- /dev/null
+++ b/tests/ui/pattern/usefulness/unions.stderr
@@ -0,0 +1,34 @@
+error[E0004]: non-exhaustive patterns: `U8AsBool { n: 0_u8 }` and `U8AsBool { b: false }` not covered
+  --> $DIR/unions.rs:22:15
+   |
+LL |         match x {
+   |               ^ patterns `U8AsBool { n: 0_u8 }` and `U8AsBool { b: false }` not covered
+   |
+note: `U8AsBool` defined here
+  --> $DIR/unions.rs:3:11
+   |
+LL |     union U8AsBool {
+   |           ^^^^^^^^
+   = note: the matched value is of type `U8AsBool`
+help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern, a match arm with multiple or-patterns as shown, or multiple match arms
+   |
+LL ~             U8AsBool { n: 1.. } => {},
+LL +             U8AsBool { n: 0_u8 } | U8AsBool { b: false } => todo!()
+   |
+
+error[E0004]: non-exhaustive patterns: `(U8AsBool { n: 0_u8 }, false)`, `(U8AsBool { b: false }, false)`, `(U8AsBool { n: 0_u8 }, false)` and 1 more not covered
+  --> $DIR/unions.rs:28:15
+   |
+LL |         match (x, true) {
+   |               ^^^^^^^^^ patterns `(U8AsBool { n: 0_u8 }, false)`, `(U8AsBool { b: false }, false)`, `(U8AsBool { n: 0_u8 }, false)` and 1 more not covered
+   |
+   = note: the matched value is of type `(U8AsBool, bool)`
+help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern as shown, or multiple match arms
+   |
+LL ~             (U8AsBool { n: 1.. }, true) => {},
+LL +             _ => todo!()
+   |
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0004`.