constexpr `__builtin_bit_cast` involving bit-field

[fsb4000]

#include <bit>
#include <type_traits>

template <typename Derived, typename Type>
struct alignas(sizeof(Type)) bit_field
{
};

struct S : bit_field<S, int>
{
    bool a : 1;
 
    constexpr S() noexcept = default;
    constexpr S(int i) noexcept: S( std::bit_cast<S>( i ) ) {}
};

static_assert(sizeof(S) == sizeof(int));
static_assert(std::is_trivially_copyable_v<S>);
static_assert(std::is_trivially_copyable_v<int>);

constexpr S sa = 0x00000001; // <source>(21)

https://godbolt.org/z/z99v39n1Y

bit_cast.h:31:12: note: constexpr bit_cast involving bit-field is not yet supported
    return __builtin_bit_cast(_ToType, __from);
           ^

Well, because of the note, I think this is a known issue.

But I couldn't find it via github issues, so I decided to create a new one.

https://github.com/llvm/llvm-project/issues?q=__builtin_bit_cast+is%3Aopen

https://github.com/llvm/llvm-project/issues?q=bit_cast+is%3Aopen

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[0-wiz-0]

I see this error message when I try to build libcxxabi 14.0.6 on NetBSD 9.99.98/amd64 (which comes with gcc 10.3.0 in case it matters):

[ 23%] Building CXX object src/CMakeFiles/cxxabi_static.dir/cxa_virtual.cpp.o                      
In file included from /scratch/lang/libcxxabi/work/libcxx-14.0.6.src/include/__compare/strong_order.h:12,                                                                                              
                 from /scratch/lang/libcxxabi/work/libcxx-14.0.6.src/include/__compare/weak_order.h:14,
                 from /scratch/lang/libcxxabi/work/libcxx-14.0.6.src/include/__compare/partial_order.h:14,
                 from /scratch/lang/libcxxabi/work/libcxx-14.0.6.src/include/__compare/compare_partial_order_fallback.h:13,
                 from /scratch/lang/libcxxabi/work/libcxx-14.0.6.src/include/compare:144,                                                                                                              
                 from /scratch/lang/libcxxabi/work/libcxx-14.0.6.src/include/utility:236,                                                                                                              
                 from /scratch/lang/libcxxabi/work/libcxx-14.0.6.src/include/__functional_base:26,
                 from /scratch/lang/libcxxabi/work/libcxx-14.0.6.src/include/memory:808,
                 from /scratch/lang/libcxxabi/work/libcxxabi-14.0.6.src/../libcxx/src/include/atomic_support.h:13,
                 from /scratch/lang/libcxxabi/work/libcxxabi-14.0.6.src/src/cxa_default_handlers.cpp:19:
/scratch/lang/libcxxabi/work/libcxx-14.0.6.src/include/__bit/bit_cast.h: In function 'constexpr _ToType std::__1::bit_cast(const _FromType&)':
/scratch/lang/libcxxabi/work/libcxx-14.0.6.src/include/__bit/bit_cast.h:31:38: error: expected primary-expression before ',' token
   31 |     return __builtin_bit_cast(_ToType, __from);                                            
      |                                      ^                                                     

[0323pin]

Hi,

Sorry for jumping in uninvited but, would this just compile with gcc >= 11?

[fsb4000]

Hi @0323pin !

My code sample works for gcc 11 and MSVC 2022: https://godbolt.org/z/s1ETKj5ET

[0323pin]

@fsb4000 Thanks!

[tbaederr]

FWIW this is tested:

https://github.com/llvm/llvm-project/blob/main/clang/test/SemaCXX/constexpr-builtin-bit-cast.cpp#L131-L145

[JohelEGP]

Unfortunately, the presence of the error message and the test don't imply that this is being actively worked on. Just that it was known at the time it was implemented.