Compilation fails with gcc trunk (futur gcc 11), protobuf 3.15.0 and -std=gnu++20: const_cast is not accepted in a constexpr expression

[Romain-Geissler-1A]

Hi,

All is in the title: I tried to compile protobuf 3.15.0 with gcc trunk and -std=gnu++20, and it fails with this error:

google/protobuf/unittest_lite.pb.cc:155:86: error: 'constinit' variable 'protobuf_unittest::_TestAllTypesLite_default_instance_' does not have a constant initializer
  155 | PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT TestAllTypesLiteDefaultTypeInternal _TestAllTypesLite_default_instance_;
      |                                                                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from ./google/protobuf/unittest_lite.pb.h:25,
                 from google/protobuf/unittest_lite.pb.cc:4:
google/protobuf/unittest_lite.pb.cc:155:86:   in 'constexpr' expansion of 'protobuf_unittest::TestAllTypesLiteDefaultTypeInternal()'
google/protobuf/unittest_lite.pb.cc:149:7:   in 'constexpr' expansion of '((protobuf_unittest::TestAllTypesLiteDefaultTypeInternal*)this)->protobuf_unittest::TestAllTypesLiteDefaultTypeInternal::<anonymous>.protobuf_unittest::TestAllTypesLiteDefaultTypeInternal::<unnamed union>::_instance.protobuf_unittest::TestAllTypesLite::TestAllTypesLite((google::protobuf::internal::ConstantInitialized(), google::protobuf::internal::ConstantInitialized()))'
google/protobuf/unittest_lite.pb.cc:97:5:   in 'constexpr' expansion of '((protobuf_unittest::TestAllTypesLite*)this)->protobuf_unittest::TestAllTypesLite::default_string_.google::protobuf::internal::ArenaStringPtr::ArenaStringPtr(0)'
../../src/google/protobuf/arenastring.h:179:9:   in 'constexpr' expansion of '((google::protobuf::internal::ArenaStringPtr*)this)->google::protobuf::internal::ArenaStringPtr::tagged_ptr_.google::protobuf::internal::TaggedPtr<std::__cxx11::basic_string<char> >::TaggedPtr(default_value)'
../../src/google/protobuf/arenastring.h:95:14: error: conversion of 'const google::protobuf::internal::ExplicitlyConstructed<std::__cxx11::basic_string<char> >*' null pointer to 'google::protobuf::internal::ExplicitlyConstructed<std::__cxx11::basic_string<char> >*' is not a constant expression
   95 |       : ptr_(const_cast<ExplicitlyConstructed<std::string>*>(ptr)) {}
      |              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It seems that const_cast can't be used in a constexpr expression, so the constinit instantiation failed.

Note: one way to avoid reinterpret_cast or other kind of forbidden casts in constexpr expression might be to use some union struct having the multiple types you want to cast things into.

Cheers,
Romain

[acozzette]

@Romain-Geissler-1A Thanks for the bug report. I am going to disable our use of constinit for GCC until we can figure out a better solution.

[dlj-NaN]

This no longer appears to be an issue as of gcc-12-4218-g57c7ec62ee0.