https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88815
Bug ID: 88815
Summary: [9 Regression] is_constexpr (based on narrowing
conversion and expression SFINAE) broken
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: programmer at posteo dot de
Target Milestone: ---
[note: following code adopted from
https://stackoverflow.com/a/50169108/2615118]
The following experiment of an "is_constexpr" implementation fails on GCC 9.
https://godbolt.org/z/xDiFet
// BEGIN CODE
struct true_type {
constexpr operator bool() const { return true; }
};
struct false_type {
constexpr operator bool() const { return false; }
};
template<int (*p)()>
true_type is_constexpr_impl(decltype(int{(p(), 0U)}));// narrowing conversion
template<int (*p)()>
false_type is_constexpr_impl(...);
template<int (*p)()>
using is_constexpr = decltype(is_constexpr_impl<p>(0));
constexpr int f() { return 0; }
int g() { return 0; }
static_assert(is_constexpr<f>());
static_assert(!is_constexpr<g>());// this one fails in GCC 9.0.0 20190109
// END CODE
