[Bug c++/102401] New: std::bit_cast falls over, seemingly due to some invisible alignment requirements

["krax.powah at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102401

            Bug ID: 102401
           Summary: std::bit_cast falls over, seemingly due to some
                    invisible alignment requirements
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: krax.powah at gmail dot com
  Target Milestone: ---

GCC Version: The current truck (12.0.0 20210917), but also present on all
versions which support std::bit_cast.

Repro case (https://godbolt.org/z/zWbMnMfvx):

```
    #include <bit>

    struct Item
    {
        float a = 0;
        float b = 0;
        char  c = 0;  // Removing this or changing it to type with a size of 4
    };                // fixes it.

    struct M
    {
        struct Data { char d[sizeof(Item)]; };
        static constexpr Data build(Item item)
        {
            Data d {};
            d = std::bit_cast<Data>(item);
            return d;
        }
    };

    int main(void)
    {
        constexpr auto data = M::build({}); // Only occurs if this is a
constexpr assignment
    }
```

Result:

```
    In file included from <source>:1:
    /opt/compiler-explorer/gcc-trunk-20210917/include/c++/12.0.0/bit: In
function 'int main()':
    <source>:26:35:   in 'constexpr' expansion of 'M::build(Item{(float)0,
(float)0, '\000'})'
    <source>:18:32:   in 'constexpr' expansion of 'std::bit_cast<M::Data,
Item>(item)'
    /opt/compiler-explorer/gcc-trunk-20210917/include/c++/12.0.0/bit:77:33:
error: '__builtin_bit_cast' accessing uninitialized byte at offset 9
       77 |       return __builtin_bit_cast(_To, __from);
          |                                 ^~~
    ASM generation compiler returned: 1
    In file included from <source>:1:
    /opt/compiler-explorer/gcc-trunk-20210917/include/c++/12.0.0/bit: In
function 'int main()':
    <source>:26:35:   in 'constexpr' expansion of 'M::build(Item{(float)0,
(float)0, '\000'})'
    <source>:18:32:   in 'constexpr' expansion of 'std::bit_cast<M::Data,
Item>(item)'
    /opt/compiler-explorer/gcc-trunk-20210917/include/c++/12.0.0/bit:77:33:
error: '__builtin_bit_cast' accessing uninitialized byte at offset 9
       77 |       return __builtin_bit_cast(_To, __from);
          |                                 ^~~
    Execution build compiler returned: 1
```