Tested powerpc64le-linux, pushed to trunk. Backport to gcc-11 to follow.
-- >8 -- Our std::bit_cast was relying on the compiler to check for errors inside __builtin_bit_cast, instead of checking them as constraints. That means std::bit_cast was not SFINAE-friendly. This fix uses a requires-clause, so for old versions of Clang without concepts support the function will still be unconstrained. At some point in future we can remove the #ifdef __cpp_concepts check and rely on all compilers having full concepts support in C++20 mode. libstdc++-v3/ChangeLog: PR libstdc++/105027 * include/std/bit (bit_cast): Add constraints. * testsuite/26_numerics/bit/bit.cast/105027.cc: New test. --- libstdc++-v3/include/std/bit | 4 ++++ .../26_numerics/bit/bit.cast/105027.cc | 18 ++++++++++++++++++ 2 files changed, 22 insertions(+) create mode 100644 libstdc++-v3/testsuite/26_numerics/bit/bit.cast/105027.cc diff --git a/libstdc++-v3/include/std/bit b/libstdc++-v3/include/std/bit index cfd98e24eb5..a40f1ce99df 100644 --- a/libstdc++-v3/include/std/bit +++ b/libstdc++-v3/include/std/bit @@ -73,6 +73,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION [[nodiscard]] constexpr _To bit_cast(const _From& __from) noexcept +#ifdef __cpp_concepts + requires (sizeof(_To) == sizeof(_From)) + && __is_trivially_copyable(_To) && __is_trivially_copyable(_From) +#endif { return __builtin_bit_cast(_To, __from); } diff --git a/libstdc++-v3/testsuite/26_numerics/bit/bit.cast/105027.cc b/libstdc++-v3/testsuite/26_numerics/bit/bit.cast/105027.cc new file mode 100644 index 00000000000..301d94ec575 --- /dev/null +++ b/libstdc++-v3/testsuite/26_numerics/bit/bit.cast/105027.cc @@ -0,0 +1,18 @@ +// { dg-options "-std=gnu++20" } +// { dg-do compile { target c++20 } } + +// PR libstdc++/105027 - Missing constraints on std::bit_cast + +#include <bit> + +template<class T, class U> +concept BitCastable = requires(const U& u) { std::bit_cast<T>(u); }; + +static_assert(BitCastable<int, unsigned>); // OK + +static_assert(!BitCastable<int, char>); // #1: different size + +struct A { A(A const&); int i; }; +static_assert(!BitCastable<int, A>); // #2: not trivially copyable + +static_assert(!BitCastable<long, int()>); // #3: sizeof(int()) is ill-formed -- 2.34.1