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CWG2978 [temp.deduct.call] Deduction and qualification conversions with references #651
Description
Full name of submitter: Brian Bi
Issue description: There is implementation divergence in the treatment of the following deduction:
#include <cstddef>
template <std::size_t N>
void f(const int* const (&)[N]);
int main() {
int* a[3];
f(a);
}
It is not clear whether bullet 4.1 of [temp.deduct.call] allows qualification conversions.
Suggested resolution: Edit [temp.deduct.call]/4.1 as follows:
If the original P is a reference type, the deduced A (i.e., the type referred to by the reference) can be
more cv-qualifieda type other than the transformed A that is reference-compatible ([dcl.init.ref]) with the transformedA.
(We might not need to say "transformed", actually. As far as I can tell, when the original P is a reference type, we transform A only in the case where P is a forwarding reference, and in that case no difference in cv-qualification is possible.)
Activity
eisenwave commentedon Dec 25, 2024
Note that the following code indirectly reproduces this implementation divergence, and that is how the issue was originally discovered:
GCC accepts this, and Clang (with libstdc++) rejects it. See https://stackoverflow.com/q/79264902/5740428
Clang fails to call this overload:
Any resolution should make this construction of
std::spanvalid, in my opinion.jensmaurer commentedon Dec 25, 2024
"reference-compatible" allows derived-to-base conversions, which we don't want here (I think).
Maybe "... can be a type similar to the transformed A that is reference-compatible with the transformed A".
jensmaurer commentedon Dec 30, 2024
CWG2978
[-][temp.deduct.call] Deduction and qualification conversions with references[/-][+]CWG2978 [temp.deduct.call] Deduction and qualification conversions with references[/+]