https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110594
Bug ID: 110594
Summary: std::variant's converting constructor does not resolve
alternative correctly
Product: gcc
Version: 13.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: xgao at nvidia dot com
Target Milestone: ---
Created attachment 55502
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55502&action=edit
minimum repro
According to https://en.cppreference.com/w/cpp/utility/variant/variant, I would
expect the attached program to output:
int64_t: 1
variant is int
However, I am getting:
int64_t: 1
variant is complex
gcc-9 seems to have the correct behavior for the attached program, however, the
implementation of std::variant in gcc-9 has another problem that the following
example in that cppreference.com page does not work:
std::variant<float, long, double> z = 0; // OK, holds long
// float and double are not candidates
Thanks to the help of jjsjann123 (https://github.com/jjsjann123), this is
potentially related to the SFINAE in the "variant" header:
// Helper used to check for valid conversions that don't involve narrowing.
template<typename _Ti> struct _Arr { _Ti _M_x[1]; };
seems that this will block int64_t from being selected as shown in
https://godbolt.org/z/Gcr4j53rd. However, the following code is totally valid:
int64_t x[] = {size_t(1)};
so probably that SFINAE might be wrong.
