https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81525
Bug ID: 81525
Summary: [7 Regression] Invalid codegen with constexpr variable
template
Product: gcc
Version: 7.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ldionne.2 at gmail dot com
Target Milestone: ---
The following code prints
i = 0
N = 0
when it should print
i = 0
N = 1
This seems to be a regression since GCC 7, although I am not sure exactly
which minor version introduced it. The problem only triggers when 'N' is
constexpr, when it is assigned from a variable template, and when we go
through enough templates (randomly making functions not templates seems
to fix the issue). The code is (sorry, I reduced a lot but could not
reduce further):
#include <iostream>
template <int i>
struct integral_constant {
constexpr operator int() const { return i; }
};
template <int i>
constexpr integral_constant<i> int_{};
template <typename F>
void with_index(F f) {
f(integral_constant<0>{});
}
template <typename Dummy>
void print(Dummy) {
constexpr auto N = ::int_<1>;
(void)N; // otherwise we get [warning: variable 'N' set but not used]
// this may be a hint that something is wrong
auto f = [&](auto i) {
std::cout << "i = " << static_cast<int>(i) << std::endl;
std::cout << "N = " << static_cast<int>(N) << std::endl;
};
with_index(f);
}
int main() {
::print(0);
}
Live example with GCC 6.3.0: https://wandbox.org/permlink/lclToxN6mnRAS3ol
Live example with GCC 7.1.0: https://wandbox.org/permlink/AUqaHBYlsfwHLUrU
Live example with GCC trunk: https://wandbox.org/permlink/8wxi8MqrS03MbujE
This bug report was lifted from a failure in Boost.Hana's test suite [1],
with the relevant original code being [2]. This is quite worrisome as this
breaks mundane constexpr-utilizing code in a nasty way.
[1]: https://travis-ci.org/boostorg/hana/jobs/256518824#L7663
[2]:
https://github.com/boostorg/hana/blob/e50749/example/tutorial/tag_dispatching.cpp#L72-L85
