https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102065
Patrick Palka <ppalka at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |DUPLICATE
CC| |ppalka at gcc dot gnu.org
Status|UNCONFIRMED |RESOLVED
--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Thanks for the bug report! Looks like this apparent regression started with
r11-2774 (which implements the changes in wg21.link/cwg2369).
But I think GCC's behavior is correct here, according to the latest standard.
The associated constraints of a function template are checked immediately after
deducing all template arguments, rather than after substituting the deduced
template arguments into the function type, so your use of enable_if_t as a
function parameter doesn't actually prevent the std::invocable constraint from
getting checked (and for the hard error to occur when this constraint check
causes the body of the lambda to get instantiated with int*).
One work around in your case is to give the lambda a non-deduced return type of
'void' so that we don't need to instantiate the body of the lambda during the
std::invocable check:
[](auto e) -> void
{
elem_func(e);
}
Another more general workaround is to instead encode the enable_if_t test as an
additional constraint that guards the std::invocable constraint, something
like:
#include <functional>
void elem_func(int) {}
template <
typename Sequence,
typename SequenceHandler
>
requires std::is_pointer_v<Sequence> && std::invocable<SequenceHandler, int>
void func(
const Sequence& values,
SequenceHandler&& handler)
{
}
template <
typename T,
typename ValueHandler
>
requires (!std::is_pointer_v<T>) && std::invocable<ValueHandler, T>
void func(
T value,
ValueHandler&& handler)
{
}
int main()
{
func(
new int[1], // pointer => 1st func should be called
[](auto e) // lambda should be std::invocable<int> => decltype(e)==int
{
elem_func(e); // cannot call elem_func if decltype(e)==int* (2nd
func)
}
);
}
*** This bug has been marked as a duplicate of bug 99599 ***
