[Bug c++/77911] Comparing function pointers in a constexpr function can produce an error.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77911 Patrick Palka changed: What|Removed |Added Status|NEW |RESOLVED Resolution|--- |FIXED CC||ppalka at gcc dot gnu.org Target Milestone|--- |12.0 --- Comment #6 from Patrick Palka --- This one is also fixed for GCC 12 by r12-6188.
[Bug c++/77911] Comparing function pointers in a constexpr function can produce an error.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77911 Bug 77911 depends on bug 69681, which changed state. Bug 69681 Summary: C/C++ FEs do not consider comparisons of distinct function pointers to be constant expressions https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69681 What|Removed |Added Status|NEW |RESOLVED Resolution|--- |FIXED
[Bug c++/77911] Comparing function pointers in a constexpr function can produce an error.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77911 Andrew Pinski changed: What|Removed |Added CC||tadeus.prastowo at unitn dot it --- Comment #5 from Andrew Pinski --- *** Bug 86607 has been marked as a duplicate of this bug. ***
[Bug c++/77911] Comparing function pointers in a constexpr function can produce an error.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77911 Andrew Pinski changed: What|Removed |Added Last reconfirmed||2021-08-05 Depends on||69681 Ever confirmed|0 |1 Status|UNCONFIRMED |NEW --- Comment #4 from Andrew Pinski --- Confirmed, I suspect this is the same problem as PR 69681. Referenced Bugs: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69681 [Bug 69681] C/C++ FEs do not consider comparisons of distinct function pointers to be constant expressions
[Bug c++/77911] Comparing function pointers in a constexpr function can produce an error.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77911
--- Comment #3 from Dr Hilbert Swan ---
A generous fellow offered me this workaround for the time being:
void test1(){}
void test2(){}
void test3(){}
void test4(){}
#define FUNCTION_LIST test1, test2, test3, test4
template
struct SearchFun
{
static constexpr int Idx = SearchFun::Idx;
};
template
struct SearchFun
{
static constexpr int Idx = N;
};
template
using FindMatchingIdx = SearchFun<0, Fun, FUNCTION_LIST>;
// Tests
constexpr unsigned int loc1 = FindMatchingIdx::Idx;
static_assert(loc1 == 0, "");
constexpr unsigned int loc2 = FindMatchingIdx::Idx;
static_assert(loc2 == 1, "");
constexpr unsigned int loc3 = FindMatchingIdx::Idx;
static_assert(loc3 == 2, "");
// If the array is needed
typedef void (*func)();
func funcs[] = { FUNCTION_LIST };
// If desired the template alias can be wrapped in a macro
#define FindMatching(Fun) FindMatchingIdx::Idx
constexpr unsigned int loc4 = FindMatching(test2);
static_assert(loc4 == 1, "");
[Bug c++/77911] Comparing function pointers in a constexpr function can produce an error.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77911
--- Comment #2 from Dr Hilbert Swan ---
If a similar thing is done with ints the code would look like this:
constexpr int i[] = { 1, 2, 3 };
constexpr int FindMatchingIdx(const int w, const int idx) {
return (w == i[idx]) ? (idx) : (FindMatchingIdx(w, idx + 1));
}
constexpr unsigned int loc = FindMatchingIdx(2, 0);
int main() {
return loc;
}
Which compiles correctly. (https://godbolt.org/g/mYpxn9) I've also tried float
and doubles, which are fine, but there something about function pointers that
gcc doesn't like.
An alternative method using recursive template meta programming fails to
compile with a very similar error: (https://godbolt.org/g/zanwsd)
void test1(){}void test2(){}
void test3(){}void test4(){}
typedef void(*Type)();
constexpr Type i[] = { &test1, &test2, &test3 };
template
struct FindMatchingIdx2 {
enum { value = FindMatchingIdx2::value };
};
template
struct FindMatchingIdx2 {
enum { value = (idx) };
};
template
struct FindMatchingIdx {
enum { flag = FindMatchingIdx2::value };
};
int main() {
return FindMatchingIdx::flag;
}
[Bug c++/77911] Comparing function pointers in a constexpr function can produce an error.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77911 --- Comment #1 from Dr Hilbert Swan --- Created attachment 39777 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=39777&action=edit Version that compiles.
