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);
```
