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

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

[peter.fletcher at bandicootimaging dot com.au]

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];
}

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

[peter.fletcher at bandicootimaging dot com.au]

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

Peter Fletcher <peter.fletcher at bandicootimaging dot com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |peter.fletcher@bandicootima
                   |                            |ging.com.au

--- Comment #1 from Peter Fletcher <peter.fletcher at bandicootimaging dot com.au> ---
Created attachment 55862
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55862&action=edit
Source file

Source code

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

[pinskia at gcc dot gnu.org]

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

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
99% sure this is because we start parsing as a function declaration and then
see it is not but we errored out already.

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

[pinskia at gcc dot gnu.org]

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

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2023-09-09
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |rejects-valid

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
You can also use this as a workaround:
```
AA aa{Constructor([](auto& p) { p.x = 1; })};
```

Confirmed.
The problem is GCC starts parsing as a function declaration for aa that takes a
type of arrays of function pointers that has a function argument named
Constructor and then noticed the `{` and decides whoops we need to back up. But
it is too late as GCC already error'd for the auto too early on.

Basically GCC errors out on auto too soon (during the parsing) rather
afterwards ...

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

[pinskia at gcc dot gnu.org]

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

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
also see https://en.wikipedia.org/wiki/Most_vexing_parse for more explanation
on why this mistake in GCC happened.