[Bug c++/113754] New: GCC complains when using a type that depends on lambda

[Bug c++/113754] New: GCC complains when using a type that depends on lambda

[milasudril at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113754

            Bug ID: 113754
           Summary: GCC complains when using a type that depends on lambda
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: milasudril at gmail dot com
  Target Milestone: ---

I tried to use 

template<typename T, auto DeleteFn>
using unique_ptr_deleter =
  std::unique_ptr<T, decltype([](T* obj) { DeleteFn(obj); })>;

As described by
https://andreasfertig.blog/2022/08/cpp-insights-lambdas-in-unevaluated-contexts/

It works well as long as I do not try define it in a header file. If gcc
detects a from a header file that uses unique_ptr_deleter, I get the warning 

lib/array_classes/./memory_block.hpp:12:8: error: 'memory_block' has a field
'unique_ptr_deleter<void, free> terraformer::memory_block::m_pointer' whose
type has internal linkage [-Werror=subobject-linkage]

Clang do generate any warnings.

Stand-alone snippet to reproduce the issue:

#include <memory>
#include <cstdlib>

# 7 "lib/array_classes/./memory_block.hpp"
 template<class T, auto DeleteFn>
 using unique_ptr_deleter = std::unique_ptr<T, decltype([](T* obj) {
DeleteFn(obj); })>;

 class memory_block
 {
 public:

 private:
  unique_ptr_deleter<void, free> m_pointer;
 };

# 4 "lib/array_classes/memory_block.test.cpp"

[Bug c++/113754] GCC complains when using a type that depends on lambda

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113754

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Blocks|                            |107430
           Keywords|                            |c++-lambda

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
lambda inside a decltype has many known issues, maybe this is a dup of one of
those.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107430
[Bug 107430] [meta-bug] lambda in decltype

[Bug c++/113754] GCC complains when using a type that depends on lambda

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113754

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2024-04-13
           Keywords|                            |diagnostic
         Depends on|                            |109367
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed. Very similar issue to PR 109367 .


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109367
[Bug 109367] bogus -Wunused-function warning with decltype of a lambda as an
argument