Clang accepts the undefined behavior operation in a constant expression

[xmh0511]

enum A {a = 0};
constexpr A e = static_cast<A>(1024);

According to [expr.static.cast p10

If the enumeration type does not have a fixed underlying type, the value is unchanged if the original value is within the range of the enumeration values ([dcl.enum]), and otherwise, the behavior is undefined.

[dcl.enum] p8 defines the range of the enumeration values, which says

Otherwise, the values of the enumeration are the values representable by a hypothetical integer type with minimal width M such that all enumerators can be represented.

In this case, it is sufficient to represent the value 0 of the unique enumerator if M is 1. Apparently, the value 1024 cannot be representable in the hypothetical integer type with minimal width M. So, the full-expression of the initialization is not a constant expression since the violation of [expr.const] p5

an operation that would have undefined behavior as specified in [intro] through [cpp];

However, Clang accepts this example without any diagnosis.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[llvmbot]

@llvm/issue-subscribers-c-20

[shafik]

Looks like duplicate of #50055

[AaronBallman]

Agreed, closing this as a duplicate of #50055.