[Bug c++/97256] New: auto function return different result

[Bug c++/97256] New: auto function return different result

[tangyixuan at mail dot dlut.edu.cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97256

            Bug ID: 97256
           Summary: auto function return different result
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tangyixuan at mail dot dlut.edu.cn
  Target Milestone: ---

Hi, when executing the following code, GCC reports 32765 while clang reports 1. 
Maybe there is something wrong?
$ cat s.cpp

#include <iostream>
auto func(const int a) {
  const int b = a;
  return [&](){ return b; };
}


int main() {
  auto c = func(1);
  std::cout << c() ;
  return 0;
}

$ g++ -std=c++2a s.cpp
$ ./a.out
32765


$ clang++ -std=c++2a s.cpp
$ ./a.out
1

[Bug c++/97256] auto function return different result

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97256

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
possibly this returns a reference to a local variable which is undefined?

[Bug c++/97256] auto function return different result

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97256

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Yes, the lambda captures a local variable by value, and then when you invoke
the lambda it refers to a dead variable.

[Bug c++/97256] auto function return different result

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97256

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #2)
> Yes, the lambda captures a local variable by value,

Duh, sorry, I meant captures a local variables BY REFERENCE.

> and then when you invoke
> the lambda it refers to a dead variable.

[Bug c++/97256] auto function return different result

[richard-gccbugzilla at metafoo dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97256

Richard Smith <richard-gccbugzilla at metafoo dot co.uk> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |richard-gccbugzilla@metafoo
                   |                            |.co.uk

--- Comment #4 from Richard Smith <richard-gccbugzilla at metafoo dot co.uk> ---
The reference to 'b' from within the lambda is not an odr-use, so is not
transformed into a use of the lambda's capture. This program has defined
behavior and is being miscompiled.

[Bug c++/97256] auto function return different result

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97256

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mpolacek at gcc dot gnu.org
     Ever confirmed|0                           |1
         Resolution|INVALID                     |---
             Status|RESOLVED                    |NEW
   Last reconfirmed|                            |2020-10-02

--- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Re-opening then, thanks.

[Bug c++/97256] auto function return different result

[richard-gccbugzilla at metafoo dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97256

--- Comment #6 from Richard Smith <richard-gccbugzilla at metafoo dot co.uk> ---
My apologies, I misread the testcase. Yes, this is UB.

[Bug c++/97256] auto function return different result

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97256

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|NEW                         |RESOLVED

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
And closing again.