Bug: undefined behavior of constexpr static_cast from int to unscoped enum without fixed type outside range compiles fine on C++17

[carlosgalvezp]

Bugzilla Link	50711
Version	trunk
OS	Linux
CC	@zygoloid

Extended Description

Hi,

The following code is undefined behavior in C++17. And undefined behavior is not allowed in constant expressions - the compiler is required to diagnose it. Adapted from cppreference:

enum foo { a = 0, b = 1 };
constexpr foo x = static_cast(123); // undefined behavior since C++17

However Clang happily compiles this code:

https://godbolt.org/z/bzhWMxqoE

[zygoloid]

Confirmed; it doesn't look like any major compiler diagnoses this.

[carlosgalvezp]

Yep:

https://stackoverflow.com/a/66723431

There's an open bug for GCC:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95701

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[jyknight]

Closed by shafik's commit b364535.