[Bug libstdc++/105027] New: Missing constraints on std::bit_cast

[ed at catmur dot uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105027

            Bug ID: 105027
           Summary: Missing constraints on std::bit_cast
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ed at catmur dot uk
  Target Milestone: ---

[bit.cast] Constraints (nb. prior to P1719R2 these were specified as SFINAE,
but they have been there since the start):
(1.1) sizeof(To) == sizeof(From) is true;
(1.2) is_­trivially_­copyable_­v<To> is true; and
(1.3) is_­trivially_­copyable_­v<From> is true.

#include <bit>
template<class T, class U>
concept BitCastable = requires(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

#1 and #2 are erroneously accepted although the corresponding call to
std::bit_cast would fail internally; MSVC and clang (with libc++) correctly
reject.

static_assert(BitCastable<long, int()>); // #3: see below
int f();
long l = std::bit_cast<long>(f); // #4

By my reading of [temp.deduct] #3 should be a hard error, since sizeof(int())
is not only invalid; it is not SFINAE. However, compilers that allow it (gcc
everywhere, MSVC and clang as SFINAE) should then reject either by SFINAE, or
as having the incorrect size, or since function types are not trivially
copyable; gcc/libstdc++ does not and also accepts #4, decaying the function
reference to a function pointer; clang/libstdc++ rejects within its own
__builtin_bit_cast.