On 27/08/20 12:46 +0200, Jakub Jelinek wrote:
On Thu, Aug 27, 2020 at 12:06:13PM +0200, Jakub Jelinek via Gcc-patches wrote:Oops, rewrote the testcase from __builtin_bit_cast to std::bit_cast without adjusting the syntax properly. Also, let's not use bitfields in there, as clang doesn't support those. So, adjusted testcase below. clang++ rejects all 6 of those, but from what you said, I'd expect that u and z should be well defined. #include <bit> struct S { short a; int b; }; struct T { int a, b; }; constexpr int foo () { S a = S (); S b = { 0, 0 }; S c = a; S d; S e; d = a; e = S (); int u = std::bit_cast<T> (a).a; // Is this well defined due to value initialization of a?
The standard says that padding bits in the bit_cast result are unspecified, so I don't think they have to be copied even if the source object was zero-initialized and has all-zero padding bits.
