https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97665
--- Comment #4 from Luke Dalessandro <ldalessandro at gmail dot com> --- Hi Jakob, Thank you for looking at this. I restructured the code sample according to your suggestions and it is available here https://godbolt.org/z/P1bMEz. I don't understand a couple of things that you said, and ultimately I'm not sure if you are agreeing that this could be a bug. > You can do Foo foo = Foo(); and it compiles. 1. I can't do `Foo foo = Foo();` because the purpose of the union is to allocate uninitialized storage for the `Foo` during `constexpr` execution when `Foo` has no default constructor. I realize now I meant to write `constexpr Foo() = delete;` in the original code. I _can_ use the monostate to have an active member at initialization, but would prefer not to as it complicates the union. > clang++ rejects it too: > error: constexpr union constructor that does not initialize any member is a > C++20 extension [-Werror,-Wc++20-extensions] though only with > -pedantic-errors. 2. I can't get clang to emit the warning you describe even with the provided flags, perhaps I am doing it wrong or misinterpreting your comment. > P1331R2 support is there since > r10-5194-g7906797ebec6881d7d90165340f51efcf447d716 > (so without [1] it is accepted for -std=c++2a since that revision) 3. I think this means that, if the member is not an array, then it is accepted with `-std=c++2a`. I do observe this, however it's not my use case. Technically I have an array of some class template parameter `N`. The updated test case with the deleted constructor is. ``` struct Foo { constexpr Foo() = delete; }; union U { // struct {} monostate = {}; Foo foo; constexpr U() {} }; struct V { U storage[1]; }; constexpr V v; ```
