[Bug c++/103543] New: Potential compiler warning for return of temporary?

[Bug c++/103543] New: Potential compiler warning for return of temporary?

[gcc at hazlewoods dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103543

            Bug ID: 103543
           Summary: Potential compiler warning for return of temporary?
           Product: gcc
           Version: 11.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc at hazlewoods dot net
  Target Milestone: ---

A return of a temporary from a lambda assigned to `std::function<>` does not
cause a warning, unlike a regular function.
Would be useful if it trapped, but it may be too far down the deduction path.

<code>
#include <iostream>
#include <functional>

struct S
{
  int s{};
};

using F = std::function<S const& ()>;

S const& fn()
{
  return S{};  // Warning: returning ref to temporary
}

int main()
{
  F f = [](){ return S{}; };  // No warning
  std::cout << f().s << std::endl;  // Ouch
}
</code>


g++-11 -W -Wall -std=c++20 -O2 -pedantic-errors test.cpp --save-temps

g++-11 --version
g++-11 (GCC) 11.2.1 20211105

[Bug c++/103543] Potential compiler warning for return of temporary with std::function<const T&()> converting from T(*)()

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103543

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
            Summary|Potential compiler warning  |Potential compiler warning
                   |for return of temporary?    |for return of temporary
                   |                            |with std::function<const
                   |                            |T&()> converting from
                   |                            |T(*)()

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
That is because the lambda itself returns a S and not a const reference.

You do get a warning with -Wsystem-headers inside invoke.

With:
[]() -> S const&  { return S{}; };

You do get the correct warning.

Also this has nothing to do with lambdas really but really std::function as
shown by:
struct S
{
  int s{};
};
using F = std::function<S const& ()>;
auto fn()
{
  return S{};
}
int main()
{
  F f = fn; // No warning
  __builtin_printf("%d.\n", f().s);
}
---- CUT ----
The above code is the same as the lambda case really.

[Bug c++/103543] Potential compiler warning for return of temporary with std::function<const T&()> converting from T(*)()

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103543

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

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

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This is a dup of bug 70692.

*** This bug has been marked as a duplicate of bug 70692 ***