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gccとclangで挙動が違うC++コード。多分UBを踏んでいるが、どこで踏んでいるかはいまいちよくわかっていない。

@anqou C++20 (n4868) §7.6.1.9/10
https://timsong-cpp.github.io/cppwp/n4868/expr.static.cast#10

> If the enumeration type has a fixed underlying type, the value is first converted to that type by integral conversion, if necessary, and then to the enumeration type.
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.

ここに該当して未定義動作ですね

http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1766

ははー、 C++11/14 では unspecified だったのが undefined になったんか

std::byte - cppreference.com
https://en.cppreference.com/w/cpp/types/byte

さっきのやつ <https://mstdn.anqou.net/@anqou/107190942386550421>、構図としては std::byte とほぼ同じなので、 -1 を bool に変化した true にならないとおかしい気がするんだよな……

https://timsong-cpp.github.io/cppwp/n4868/dcl.enum#8
[dcl.enum]/8:
> For an enumeration whose underlying type is fixed, the values of the enumeration are the values of the underlying type.

> The resulting value is the same as converting the original value to the underlying type of the enumeration ([conv.fpint]), and subsequently to the enumeration type.

https://timsong-cpp.github.io/cppwp/n4868/expr.static.cast#10
[expr.static.cast]/10:
> If the enumeration type has a fixed underlying type, the value is first converted to that type by integral conversion, if necessary, and then to the enumeration type.

https://timsong-cpp.github.io/cppwp/n4868/conv.bool#1
[conv.bool]/1:
> A prvalue of arithmetic, unscoped enumeration, pointer, or pointer-to-member type can be converted to a prvalue of type bool.
A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true.

だよなぁ……

https://timsong-cpp.github.io/cppwp/n4868/dcl.init.list#3.8
[dcl.init.list]/3.8:
> Otherwise, if T is an enumeration with a fixed underlying type ([dcl.enum]) U, the initializer-list has a single element v, v can be implicitly converted to U, and the initialization is direct-list-initialization, the object is initialized with the value T(v) ([expr.type.conv]); if a narrowing conversion is required to convert v to U, the program is ill-formed.

これは braced initializer の話で、実際 Bit{-2} したら ill-formed なのでエラーになる:

https://wandbox.org/permlink/FJcw7RxBBa8dFWGB

https://timsong-cpp.github.io/cppwp/n4868/basic.fundamental#10
[basic.fundamental]/10:
> Type bool is a distinct type that has the same object representation, value representation, and alignment requirements as an implementation-defined unsigned integer type.
The values of type bool are true and false.

これもしかして関係あるのでは

https://timsong-cpp.github.io/cppwp/n4868/expr.cast#4
[expr.cast]/4
> If a conversion can be interpreted in more than one of the ways listed above, the interpretation that appears first in the list is used, even if a cast resulting from that interpretation is ill-formed.

で、 Bit(-2) は、 [expr.cast]/4.1 の const_cast ([expr.const.cast], §7.6.1.11) が該当しないから [expr.cast]/4.2 の static_cast として扱われることになるはず

https://mastodon.cardina1.red/@lo48576/107191091043504503

この最後の "The resulting value is ..." という文は直前の "floating-point type can be also ..." にかかっていて整数から enum への場合には関係ないと考えるべきだな

https://mstdn.anqou.net/@anqou/107190942386550421

まだこれのことを考えているのですが、規格追いまくってるけど implementation-defined なやつかコンパイラのバグか確定しきれずにいます (少なくとも UB ではなさそう)