https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114537
Bug ID: 114537
Summary: bit_cast does not work NSDMI of bitfields
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Keywords: FIXME
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: pinskia at gcc dot gnu.org
Target Milestone: ---
Take:
```
#include <bit>
struct A { signed char b:1 = 0; signed char b1:7 = 0; };
struct B { unsigned char b; };
static_assert( std::bit_cast<B>(A{}).b == 0 );
```
This should work as the bitfields span all of the char space but currently we
get a sorry:
```
<source>:9:40: error: non-constant condition for static assertion
9 | static_assert( std::bit_cast<B>(A{}).b == 0 );
| ~~~~~~~~~~~~~~~~~~~~~~~~^~~~
In file included from <source>:2:
<source>:9:32: in 'constexpr' expansion of 'std::bit_cast<B, A>(A{0, 0})'
/opt/compiler-explorer/gcc-trunk-20240331/include/c++/14.0.1/bit:94:33: sorry,
unimplemented: '__builtin_bit_cast' cannot be constant evaluated because the
argument cannot be encoded
94 | return __builtin_bit_cast(_To, __from);
| ^~~
```
I had been looking for a quick workaround for PR 114536 (for little-endian
only) but it looked like NSDMI for bitfields is not implemented fully.
