https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89481
Bug ID: 89481
Summary: constexpr function allows writing one active union
member and reading another
Product: gcc
Version: 8.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: mickey.veksler at gmail dot com
Target Milestone: ---
The following code should not compile, not when passed as a template argument
to std::intergral_constant<bool, all_zeros()> :
constexpr bool all_zeros()
{
union mix {
double numeric;
char bytes[sizeof(double)+1];
};
mix zero{-0.0}; // active is 'numeric'
// setting a different active member(?):
zero.bytes[sizeof(double)] = '\0';
for (unsigned i=0 ; i != sizeof(double) ; ++i)
{
// or this is illegal (since it was never initialized through 'bytes')
if (zero.bytes[i])
return false;
}
return true;
}
The puzzling thing is that the following two give different results:
int main()
{
// return all_zeros();
return std::integral_constant<bool, all_zeros()>::value;
}
Unlike gcc, clang-7.0.0 emits diagnostics:
<source>:3:16: error: constexpr function never produces a constant expression
[-Winvalid-constexpr]
constexpr bool all_zeros()
^
<source>:10:32: note: assignment to member 'bytes' of union with active
member 'numeric' is not allowed in a constant expression
zero.bytes[sizeof(double)] = '\0';
^
