[Bug c++/111352] New: Inconsistent constructor behavior associated with auto lambda

[peter.fletcher at bandicootimaging dot com.au via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111352

            Bug ID: 111352
           Summary: Inconsistent constructor behavior associated with auto
                    lambda
           Product: gcc
           Version: 11.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: peter.fletcher at bandicootimaging dot com.au
  Target Milestone: ---

Created attachment 55861
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55861&action=edit
Output of gcc -v -save_temps

When a constructor involving an auto lambda is called in two different
contexts, one triggers a compilation error, one does not.

g++ test_bug3.cpp -o test_bug3 -Wall -Wextra
test_bug3.cpp: In function ‘int main(int, char**)’:
test_bug3.cpp:41:27: error: ‘auto’ parameter not permitted in this context
   41 |     AA aa(Constructor([&](auto& p) { p.x = 1; }));
      |                           ^~~~
Compilation exited abnormally with code 1 at Sat Sep  9 08:50:05

template <typename L> struct Constructor
{
    const L& lambda;

    Constructor(const L& lambda) :
        lambda(lambda)
    {
    }

    template <typename T> operator T () const
    {
        T out;

        lambda(out);

        return out;
    }
};

struct A
{
    int x;
};

struct AA
{
    int x{0};

    AA()
    {
    }

    AA(A op) :
        x(op.x)
    {
    }
};

int main(int argc, char* argv[])
{
    AA aa(Constructor([&](auto& p) { p.x = 1; }));
    AA bb = AA(Constructor([&](auto& p) { p.x = 1; }));

    return aa.x+bb.x+argc+argv[0][0];
}