Specific errors by clang:

note: construction of subobject of member '_M_local_buf' of union with no active member is not allowed in a constant expression

error: accessing ‘std::__cxx11::basic_string<char>::<unnamed union>::_M_allocated_capacity’ member instead of initialized ‘std::__cxx11::basic_string<char>::<unnamed union>::_M_local_buf’ member in constant expression

Specific error by GCC:

error: accessing ‘std::__cxx11::basic_string<char>::<unnamed union>::_M_allocated_capacity’ member instead of initialized ‘std::__cxx11::basic_string<char>::<unnamed union>::_M_local_buf’ member in constant expression

Full errors:

Here's the clang 17 error:

/usr/include/c++/12/bits/stl_construct.h:97:14: note: construction of subobject of member '_M_local_buf' of union with no active member is not allowed in a constant expression
   97 |     { return ::new((void*)__location) _Tp(std::forward<_Args>(__args)...); }
      |              ^
/usr/include/c++/12/bits/char_traits.h:262:6: note: in call to 'construct_at(&[]() {
    std::string acc;
    sus::Array<char, 5>::with('a', 'b', 'c', 'd', 'e').into_iter().for_each([&](char v) {
        acc.push_back(v);
    });
    return acc;
}().._M_local_buf[0], acc.._M_local_buf[0])'
  262 |             std::construct_at(__s1 + __i, __s2[__i]);
      |             ^
/usr/include/c++/12/bits/char_traits.h:429:11: note: in call to 'copy(&[]() {
    std::string acc;
    sus::Array<char, 5>::with('a', 'b', 'c', 'd', 'e').into_iter().for_each([&](char v) {
        acc.push_back(v);
    });
    return acc;
}().._M_local_buf[0], &acc.._M_local_buf[0], 6)'
  429 |           return __gnu_cxx::char_traits<char_type>::copy(__s1, __s2, __n);
      |                  ^
/usr/include/c++/12/bits/basic_string.h:675:6: note: in call to 'copy(&[]() {
    std::string acc;
    sus::Array<char, 5>::with('a', 'b', 'c', 'd', 'e').into_iter().for_each([&](char v) {
        acc.push_back(v);
    });
    return acc;
}().._M_local_buf[0], &acc.._M_local_buf[0], 6)'
  675 |             traits_type::copy(_M_local_buf, __str._M_local_buf,
      |             ^
/home/runner/work/subspace/subspace/sus/iter/iterator_unittest.cc:1864:12: note: in call to 'basic_string(acc)'
 1864 |     return acc;
      |            ^
/home/runner/work/subspace/subspace/sus/iter/iterator_unittest.cc:1859:17: note: in call to '[]() {
    std::string acc;
    sus::Array<char, 5>::with('a', 'b', 'c', 'd', 'e').into_iter().for_each([&](char v) {
        acc.push_back(v);
    });
    return acc;
}.operator()()'
 1859 |   static_assert([]() {
      |                 ^

Here's the g++ 13 error:
/home/runner/work/subspace/subspace/sus/iter/iterator_unittest.cc:1792:24: error: non-constant condition for static assertion
 1787 |   static_assert(sus::Array<char, 5>::with('a', 'b', 'c', 'd', 'e')
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1788 |                     .into_iter()
      |                     ~~~~~~~~~~~~
 1789 |                     .fold(std::string(), [](std::string acc, char v) {
      |                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1790 |                       acc.push_back(v);
      |                       ~~~~~~~~~~~~~~~~~
 1791 |                       return acc;
      |                       ~~~~~~~~~~~
 1792 |                     }) == "abcde");
      |                     ~~~^~~~~~~~~~
/home/runner/work/subspace/subspace/sus/iter/iterator_unittest.cc:1792:35:   in ‘constexpr’ expansion of ‘sus::iter::IteratorBase<Iter, Item>::fold(B, F) && [with B = std::__cxx11::basic_string<char>; F = {anonymous}::Iterator_Fold_Test::TestBody()::<lambda(std::string, char)>; Iter = sus::containers::ArrayIntoIter<char, 5>; ItemT = char](std::__cxx11::basic_string<char>(), (<lambda closure object>{anonymous}::Iterator_Fold_Test::TestBody()::<lambda(std::string, char)>(), {anonymous}::Iterator_Fold_Test::TestBody()::<lambda(std::string, char)>()))’
/home/runner/work/subspace/subspace/sus/iter/iterator_unittest.cc:1792:35:   in ‘constexpr’ expansion of ‘sus::fn::call_mut(F&&, Args&& ...) [with F = {anonymous}::Iterator_Fold_Test::TestBody()::<lambda(std::string, char)>&; Args = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, char}]((* & sus::mem::move<std::__cxx11::basic_string<char>&>(init)), (& sus::mem::move<sus::option::Option<char>&>(o))->sus::option::Option<char>::unwrap())’
/home/runner/work/subspace/subspace/sus/iter/iterator_unittest.cc:1792:35:   in ‘constexpr’ expansion of ‘std::invoke(_Callable&&, _Args&& ...) [with _Callable = {anonymous}::Iterator_Fold_Test::TestBody()::<lambda(string, char)>&; _Args = {__cxx11::basic_string<char, char_traits<char>, allocator<char> >, char}; invoke_result_t<_Fn, _Args ...> = __cxx11::basic_string<char>]((* & sus::mem::forward<std::__cxx11::basic_string<char> >((* & args#0))), (* & sus::mem::forward<char>((* & args#1))))’
/home/runner/work/subspace/subspace/sus/iter/iterator_unittest.cc:1792:35:   in ‘constexpr’ expansion of ‘std::__invoke(_Callable&&, _Args&& ...) [with _Callable = {anonymous}::Iterator_Fold_Test::TestBody()::<lambda(string, char)>&; _Args = {__cxx11::basic_string<char, char_traits<char>, allocator<char> >, char}; typename __invoke_result<_Functor, _ArgTypes>::type = __cxx11::basic_string<char>]((* & std::forward<__cxx11::basic_string<char> >((* & __args#0))), (* & std::forward<char>((* & __args#1))))’
/home/runner/work/subspace/subspace/sus/iter/iterator_unittest.cc:1792:35:   in ‘constexpr’ expansion of ‘std::__invoke_impl(__invoke_other, _Fn&&, _Args&& ...) [with _Res = __cxx11::basic_string<char>; _Fn = {anonymous}::Iterator_Fold_Test::TestBody()::<lambda(string, char)>&; _Args = {__cxx11::basic_string<char, char_traits<char>, allocator<char> >, char}]((* & std::forward<{anonymous}::Iterator_Fold_Test::TestBody()::<lambda(string, char)>&>((* & __fn))), (* & std::forward<__cxx11::basic_string<char> >((* & __args#0))), (* & std::forward<char>((* & __args#1))))’
/home/runner/work/subspace/subspace/sus/iter/iterator_unittest.cc:1792:35:   in ‘constexpr’ expansion of ‘std::__cxx11::basic_string<char>((* & std::forward<__cxx11::basic_string<char> >((* & __args#0))))’
/home/runner/work/subspace/subspace/sus/iter/iterator_unittest.cc:1792:24: error: accessing ‘std::__cxx11::basic_string<char>::<unnamed union>::_M_allocated_capacity’ member instead of initialized ‘std::__cxx11::basic_string<char>::<unnamed union>::_M_local_buf’ member in constant expression

I don't have a great minimal repo atm, and I don't have a local machine that uses libstdc++, but the Iterator.Fold unittest in iterator_unittest.cc is an example that causes it here: https://github.com/danakj/subspace/blob/experiment-iters/sus/iter/iterator_unittest.cc#L1787-L1792

With gcc-13:

git clone --recurse-submodules https://github.com/danakj/subspace
cd subspace
git checkout test origin/experiment-iters
CXX=path/to/gcc-13 cmake -B out -DSUBSPACE_BUILD_TESTS=ON
cmake --build out -j 20