https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99599
Bug ID: 99599
Summary: Concepts requirement falsely reporting recursion,
breaks tag_invoke pattern
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: the_gamester28 at msn dot com
Target Milestone: ---
The following code compiles fine on GCC 10, Clang, MSVC, but fails in GCC
11/trunk:
### begin
struct foo_tag{};
struct bar_tag{};
template <class T>
concept fooable = requires(T it) {
invoke_tag(foo_tag{}, it); // <-- here
};
template<class T>
auto invoke_tag(foo_tag, T in) {
return in;
}
template<fooable T>
auto invoke_tag(bar_tag, T it) {
return it;
}
int main() {
// Neither line below compiles in GCC 11, independently of the other
return invoke_tag(foo_tag{}, 2);
return invoke_tag(bar_tag{}, 2);
}
### end
Produces the following compiler error:
### begin
<source>: In substitution of 'template<class T> requires fooable<T> auto
invoke_tag(bar_tag, T) [with T = int]':
<source>:6:15: required by substitution of 'template<class T> requires
fooable<T> auto invoke_tag(bar_tag, T) [with T = int]'
<source>:21:35: required from here
<source>:5:9: required for the satisfaction of 'fooable<T>' [with T = int]
<source>:5:19: in requirements with 'T it' [with T = int]
<source>:5:19: error: satisfaction of atomic constraint 'requires(T it)
{invoke_tag({}, it);} [with T = T]' depends on itself
5 | concept fooable = requires(T it) {
| ^~~~~~~~~~~~~~~~
6 | invoke_tag(foo_tag{}, it);
| ~~~~~~~~~~~~~~~~~~~~~~~~~~
7 | };
| ~
### end
It seems that the template requirements of invoke_tag(bar_tag, int) are
considered while evaluating line marked "here". Requirements of irrelevant
overloads should not be considered, as it can potentially lead to falsely
reporting a cyclic dependency.
This bug effectively prevents using the modern tag_invoke pattern, which is
increasingly used as a customisation point in many modern libraries.
