[Bug c++/88631] New: CTAD cannot deduce from () value initialization

[lichray at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88631

            Bug ID: 88631
           Summary: CTAD cannot deduce from () value initialization
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lichray at gmail dot com
  Target Milestone: ---

#include <type_traits>

template<class T = void>
class A {};

// A() -> A<>;

int main()
{
    auto x = A();   // #1
    auto x2 = A{};  // #2
    A y;            // #3
}

#1 fails, #2 and #3 succeeds, adding or removing the deduction guide doesn't
matter (thankfully).  This causes users not able to write std::less().