[Bug c++/99637] New: bit_cast doesn't work with padding bits and it should

["hanicka at hanicka dot net" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99637

            Bug ID: 99637
           Summary: bit_cast doesn't work with padding bits and it should
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hanicka at hanicka dot net
  Target Milestone: ---

Standard states in https://eel.is/c++draft/bit.cast#2 "Padding bits of the
result are unspecified."

Current trunk GCC (11.0.1 20210316) gives error in presence of padding bits:

https://compiler-explorer.com/z/MhKMb5

```
struct my_field
{
    // change size to 64 and it will work
    unsigned long long a : 63;
};

constexpr unsigned long long get_value(const auto & val) noexcept {
    static_assert(sizeof(val) == sizeof(unsigned long long));

    return __builtin_bit_cast(unsigned long long, val);
}

constexpr auto f = my_field{0};
constexpr auto v = get_value(f);
```

this is the error message GCC gives, but this shouldn't be a hard error

```
<source>:14:29:   in 'constexpr' expansion of 'get_value<my_field>(f)'
<source>:10:31: error: '__builtin_bit_cast' accessing uninitialized byte at
offset 7
   10 |     return __builtin_bit_cast(unsigned long long, val);
```