1 files changed, 43 insertions, 0 deletions
diff --git a/src/test/ui/consts/const-eval/union-ub.rs b/src/test/ui/consts/const-eval/union-ub.rs
new file mode 100644
index 000000000..c1bfe69a7
--- /dev/null
+++ b/src/test/ui/consts/const-eval/union-ub.rs
@@ -0,0 +1,43 @@
+// stderr-per-bitwidth
+#![allow(const_err)] // make sure we cannot allow away the errors tested here
+
+#[repr(C)]
+union DummyUnion {
+    unit: (),
+    u8: u8,
+    bool: bool,
+}
+
+#[repr(C)]
+#[derive(Copy, Clone)]
+enum Enum {
+    A,
+    B,
+    C,
+}
+
+#[derive(Copy, Clone)]
+#[repr(C)]
+union Foo {
+    a: bool,
+    b: Enum,
+}
+
+#[repr(C)]
+union Bar {
+    foo: Foo,
+    u8: u8,
+}
+
+// the value is not valid for bools
+const BAD_BOOL: bool = unsafe { DummyUnion { u8: 42 }.bool};
+//~^ ERROR it is undefined behavior to use this value
+const UNINIT_BOOL: bool = unsafe { DummyUnion { unit: () }.bool};
+//~^ ERROR it is undefined behavior to use this value
+
+// The value is not valid for any union variant, but that's fine
+// unions are just a convenient way to transmute bits around
+const BAD_UNION: Foo = unsafe { Bar { u8: 42 }.foo };
+
+
+fn main() {}