https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84031
Bug ID: 84031
Summary: structured binding unpacks nameless padding bitfields
Product: gcc
Version: 7.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: will at dash dot org
Target Milestone: ---
In g++ >= 7.1.0 nameless bitfields are unpacked in structured binding
declarations.
GCC compiles this code with no error or warning.
using uint8_t = unsigned char;
struct bits8 { uint8_t :1, b1:1, b23:2, :1, b567:3; };
static_assert(sizeof(bits8)==1);
int main()
{
static constexpr bits8 byte{1,2,3};
const auto& [b0,b1,b23,b4,b567] = byte;
static_assert(b1==1 && b23==2 && b567==3);
static_assert(b0==0 && b4==0);
}
Clang, on the other hand, does not accept this code - it chooses not to unpack
nameless bitfields (Clang error: type 'const bits8' decomposes into 3 elements,
but 5 names were provided).
GCC errors on this code which clang accepts:
int main()
{
static constexpr bits8 byte{1,2,3};
const auto& [b1,b23,b567] = byte;
static_assert(b1==1 && b23==2 && b567==3);
}
GCC error: only 3 names provided for structured binding
const auto& [b1,b23,b567] = byte;
^~~~~~~~~~~~~
note: while 'const bits8' decomposes into 5 elements
I believe that g++ behavior is incorrect. The intent is for the binding
declarations to match identifiers. This interpretation is symmetrical with the
braced-init-list where at most 3 values can be specified to initialize only the
named bitfield members. The nameless ones are passed over...
