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From: "milasudril at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/113754] New: GCC complains when using a type that depends
 on lambda
Date: Sun, 04 Feb 2024 09:38:08 +0000
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X-Bugzilla-Component: c++
X-Bugzilla-Version: 13.2.1
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113754

            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=20

template<typename T, auto DeleteFn>
using unique_ptr_deleter =3D
  std::unique_ptr<T, decltype([](T* obj) { DeleteFn(obj); })>;

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

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 warnin=
g=20

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=3Dsubobject-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 =3D 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"=