error[E0080]: it is undefined behavior to use this value
  --> $DIR/union-ub.rs:31:1
   |
LL | const BAD_BOOL: bool = unsafe { DummyUnion { u8: 42 }.bool};
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 42, but expected something less or equal to 1
   |
   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.

error: aborting due to previous error

For more information about this error, try `rustc --explain E0080`.