https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124457
Bug ID: 124457
Summary: spurious reflection cannot be applied to a placeholder
type error in constrained return types
Product: gcc
Version: 16.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: barry.revzin at gmail dot com
Target Milestone: ---
Slightly reduced example from Walter Brown:
template <class T, auto t = ^^T>
concept True = true;
template <class T>
concept AlsoTrue = requires (T t) {
{ t } -> True;
};
This fails with:
<source>:1:31: error: '^^' cannot be applied to a placeholder type
1 | template <class T, auto t = ^^T>
| ^
<source>:6:14: error: template argument 2 is invalid
6 | { t } -> True;
| ^~~~
I think this should actually work. Doing static_assert(True<int>) does succeed.
Of the other uses of type-constraint syntax:
void f1(True auto x); // ok
template <True T> void f2(T ); // ok
auto f3(int) -> True auto; // error
