rzarzynski/20220118_gcc12_dout_need_dynamic.md Secret

## 20220118_gcc12_dout_need_dynamic.md

      
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              20220118_gcc12_dout_need_dynamic.md
            
          
    dout & need_dynamic on GCC 12

The bug report


 rzarzynski:  question about need_dynamic stuff in dout.h?  see https://paste.centos.org/view/e8c376ec.
 full log at https://kojipkgs.fedoraproject.org//work/tasks/2255/81372255/build.log

Sandbox

The following sippet is available on goldbot.org.
#include <iostream>
#include <utility>

namespace ceph::dout {

template<typename T>
struct dynamic_marker_t {
  T value;
  operator T() const { return value; }
};

template<typename T>
dynamic_marker_t<T> need_dynamic(T&& t) {
  return dynamic_marker_t<T>{ std::forward<T>(t) };
}

template<typename T>
struct is_dynamic : public std::false_type {};

template<typename T>
struct is_dynamic<dynamic_marker_t<T>> : public std::true_type {};

} // ceph::dout

#define dout_impl(cct, sub, v)                                          \
  do {                                                                  \
  const bool dyn_or_stat = [&](const auto cctX) {                     \
    if constexpr (ceph::dout::is_dynamic<decltype(sub)>::value ||       \
                  ceph::dout::is_dynamic<decltype(v)>::value) {         \
      return true;                 \
    } else {                                                            \
      /* The parentheses are **essential** because commas in angle      \
       * brackets are NOT ignored on macro expansion! A language's      \
       * limitation, sorry. */                                          \
      return false;    \
    }                                                                   \
  }(cct);                                                               \
                                                                        \
  if (true) {                                                  \
    std::cout << (dyn_or_stat ? "dynamic " : "static ")

#define dendl_impl std::endl;                                          \
  }                                                                     \
  } while (0)


#define ldpp_dout(dpp, v)                                               \
  if (decltype(auto) pdpp = (dpp); pdpp) /* workaround -Wnonnull-compare for 'this' */ \
    dout_impl(nullptr, ceph::dout::need_dynamic(42), v)

int main (void)
{
  ldpp_dout(42, 20) << "OK!" << dendl_impl;
}
Fix?

It is unkwnown whether the following patch fixes the issue or not. Unfortunately, an environment with GCC-12 onboard isn't easily available yet.
Under x86-64 gcc (trunk) on goldbot.org the problem does not replicate.
diff --git a/src/common/dout.h b/src/common/dout.h
index 421222d535f..3a76a842704 100644
--- a/src/common/dout.h
+++ b/src/common/dout.h
@@ -99,11 +99,14 @@ namespace ceph::dout {
 template<typename T>
 struct dynamic_marker_t {
   T value;
-  operator T() const { return value; }
+  // constexpr ctor isn't needed as it's an aggregate type
+  constexpr operator T() const { return value; }
 };
 
 template<typename T>
-dynamic_marker_t<T> need_dynamic(T&& t) {
+constexpr dynamic_marker_t<T> need_dynamic(T&& t) {
+  // deprecated in C++17 but that's fine for testing
+  static_assert(std::is_literal_type_v<T>);
+  static_assert(std::is_literal_type_v<dynamic_marker_t<T>>);
   return dynamic_marker_t<T>{ std::forward<T>(t) };
 }