[v30.0-rc2 regression] `src/google/protobuf/compiler/java/context.h` causes compile error C2079 in MSVC with `/std:c++20` and `/Zc:__cplusplus`

[yukawa]

What version of protobuf and what language are you using?
Version: 40ec76e
Language: C++

What operating system (Linux, Windows, ...) and version?
Windows 11 24H2

What runtime / compiler are you using (e.g., python version or gcc version)
Microsoft (R) C/C++ Optimizing Compiler Version 19.42.34438 for x64

What did you do?
Steps to reproduce the behavior:

git clone https://github.com/protocolbuffers/protobuf.git
cd protobuf
git checkout 40ec76ef674edb4cca75411d26b1984a8bf1eab6
cd examples
bazelisk --output_base=C:/x --ignore_all_rc_files --output_base=C:/x build addressbook_cc_proto --define=protobuf_allow_msvc=true --cxxopt "/std:c++20" --host_cxxopt "/std:c++20" --cxxopt "/Zc:__cplusplus" --host_cxxopt "/Zc:__cplusplus" --verbose_failures

What did you expect to see
The build succeeds.

What did you see instead?

INFO: Analyzed target //:addressbook_cc_proto (0 packages loaded, 0 targets configured).
ERROR: C:/x/external/protobuf~/src/google/protobuf/compiler/java/BUILD.bazel:68:11: Compiling src/google/protobuf/compiler/java/internal_helpers.cc [for tool] failed: (Exit 2): cl.exe failed: error executing CppCompile command (from target @@protobuf~//src/google/protobuf/compiler/java:internal_helpers)
  cd /d C:/x/execroot/_main
  SET INCLUDE=C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.42.34433\include;C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.42.34433\ATLMFC\include;C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Auxiliary\VS\include;C:\Program Files (x86)\Windows Kits\10\include\10.0.26100.0\ucrt;C:\Program Files (x86)\Windows Kits\10\\include\10.0.26100.0\\um;C:\Program Files (x86)\Windows Kits\10\\include\10.0.26100.0\\shared;C:\Program Files (x86)\Windows Kits\10\\include\10.0.26100.0\\winrt;C:\Program Files (x86)\Windows Kits\10\\include\10.0.26100.0\\cppwinrt;C:\Program Files (x86)\Windows Kits\NETFXSDK\4.8\include\um
    SET PATH=C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.42.34433\bin\HostX64\x64;C:\Program Files\Microsoft Visual Studio\2022\Community\Common7\IDE\VC\VCPackages;C:\Program Files\Microsoft Visual Studio\2022\Community\Common7\IDE\CommonExtensions\Microsoft\TestWindow;C:\Program Files\Microsoft Visual Studio\2022\Community\Common7\IDE\CommonExtensions\Microsoft\TeamFoundation\Team Explorer;C:\Program Files\Microsoft Visual Studio\2022\Community\MSBuild\Current\bin\Roslyn;C:\Program Files (x86)\Microsoft SDKs\Windows\v10.0A\bin\NETFX 4.8 Tools\x64\;C:\Program Files\Microsoft Visual Studio\2022\Community\Common7\IDE\CommonExtensions\Microsoft\FSharp\Tools;C:\Program Files\Microsoft Visual Studio\2022\Community\Team Tools\DiagnosticsHub\Collector;C:\Program Files (x86)\Windows Kits\10\bin\10.0.26100.0\\x64;C:\Program Files (x86)\Windows Kits\10\bin\\x64;C:\Program Files\Microsoft Visual Studio\2022\Community\\MSBuild\Current\Bin\amd64;C:\Windows\Microsoft.NET\Framework64\v4.0.30319;C:\Program Files\Microsoft Visual Studio\2022\Community\Common7\IDE\;C:\Program Files\Microsoft Visual Studio\2022\Community\Common7\Tools\;;C:\WINDOWS\system32;C:\Program Files\Microsoft Visual Studio\2022\Community\Common7\IDE\CommonExtensions\Microsoft\CMake\CMake\bin;C:\Program Files\Microsoft Visual Studio\2022\Community\Common7\IDE\CommonExtensions\Microsoft\CMake\Ninja;C:\Program Files\Microsoft Visual Studio\2022\Community\Common7\IDE\VC\Linux\bin\ConnectionManagerExe;C:\Program Files\Microsoft Visual Studio\2022\Community\VC\vcpkg
    SET PWD=/proc/self/cwd
    SET RUNFILES_MANIFEST_ONLY=1
    SET TEMP=C:\Users\yukawa\AppData\Local\Temp
    SET TMP=C:\Users\yukawa\AppData\Local\Temp
    SET VSLANG=1033
  C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.42.34433\bin\HostX64\x64\cl.exe @bazel-out/x64_windows-opt-exec-ST-13d3ddad9198/bin/external/protobuf~/src/google/protobuf/compiler/java/_objs/internal_helpers/internal_helpers.obj.params
# Configuration: f7e5f1179875076cb7b4f3ddd6965b5716e54d99ae929d3d122a387b659f99f3
# Execution platform: @@local_config_platform//:host
C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.42.34433\include\utility(470): error C2079: 'std::pair<const google::protobuf::FieldDescriptor *,google::protobuf::compiler::java::FieldGeneratorInfo>::second' uses undefined struct 'google::protobuf::compiler::java::FieldGeneratorInfo'
C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.42.34433\include\utility(470): note: the template instantiation context (the oldest one first) is
bazel-out/x64_windows-opt-exec-ST-13d3ddad9198/bin/external/protobuf~/src/google/protobuf/compiler/java/_virtual_includes/helpers\google/protobuf/compiler/java/context.h(81): note: see reference to class template instantiation 'absl::lts_20250127::flat_hash_map<const google::protobuf::FieldDescriptor *,google::protobuf::compiler::java::FieldGeneratorInfo,absl::lts_20250127::container_internal::HashEq<K,void>::Hash,absl::lts_20250127::container_internal::HashEq<K,void>::Eq,std::allocator<std::pair<const K,V>>>' being compiled
        with
        [
            K=const google::protobuf::FieldDescriptor *,
            V=google::protobuf::compiler::java::FieldGeneratorInfo
        ]
external/abseil-cpp~\absl/container/flat_hash_map.h(129): note: see reference to class template instantiation 'absl::lts_20250127::container_internal::raw_hash_map<absl::lts_20250127::container_internal::FlatHashMapPolicy<K,V>,Hash,Eq,Allocator>' being compiled
        with
        [
            K=const google::protobuf::FieldDescriptor *,
            V=google::protobuf::compiler::java::FieldGeneratorInfo,
            Hash=absl::lts_20250127::container_internal::HashEq<const google::protobuf::FieldDescriptor *,void>::Hash,
            Eq=absl::lts_20250127::container_internal::HashEq<const google::protobuf::FieldDescriptor *,void>::Eq,
            Allocator=std::allocator<std::pair<const google::protobuf::FieldDescriptor *const ,google::protobuf::compiler::java::FieldGeneratorInfo>>
        ]
external/abseil-cpp~\absl/container/internal/raw_hash_map.h(33): note: see reference to class template instantiation 'absl::lts_20250127::container_internal::raw_hash_set<Policy,Hash,Eq,Alloc>' being compiled
        with
        [
            Policy=absl::lts_20250127::container_internal::FlatHashMapPolicy<const google::protobuf::FieldDescriptor *,google::protobuf::compiler::java::FieldGeneratorInfo>,
            Hash=absl::lts_20250127::container_internal::HashEq<const google::protobuf::FieldDescriptor *,void>::Hash,
            Eq=absl::lts_20250127::container_internal::HashEq<const google::protobuf::FieldDescriptor *,void>::Eq,
            Alloc=std::allocator<std::pair<const google::protobuf::FieldDescriptor *const ,google::protobuf::compiler::java::FieldGeneratorInfo>>
        ]
external/abseil-cpp~\absl/container/internal/raw_hash_set.h(3030): note: see reference to class template instantiation 'absl::lts_20250127::conjunction<std::integral_constant<bool,true>,absl::lts_20250127::type_traits_internal::IsOwner<std::pair<const google::protobuf::FieldDescriptor *,google::protobuf::compiler::java::FieldGeneratorInfo>>,absl::lts_20250127::type_traits_internal::IsView<std::pair<const google::protobuf::FieldDescriptor *,google::protobuf::compiler::java::FieldGeneratorInfo>>>' being compiled
external/abseil-cpp~\absl/meta/type_traits.h(147): note: see reference to class template instantiation 'absl::lts_20250127::conjunction<absl::lts_20250127::type_traits_internal::IsOwner<std::pair<const google::protobuf::FieldDescriptor *,google::protobuf::compiler::java::FieldGeneratorInfo>>,absl::lts_20250127::type_traits_internal::IsView<std::pair<const google::protobuf::FieldDescriptor *,google::protobuf::compiler::java::FieldGeneratorInfo>>>' being compiled
external/abseil-cpp~\absl/meta/type_traits.h(147): note: see reference to class template instantiation 'absl::lts_20250127::type_traits_internal::IsOwner<std::pair<const google::protobuf::FieldDescriptor *,google::protobuf::compiler::java::FieldGeneratorInfo>>' being compiled
external/abseil-cpp~\absl/meta/type_traits.h(607): note: see reference to class template instantiation 'std::pair<const google::protobuf::FieldDescriptor *,google::protobuf::compiler::java::FieldGeneratorInfo>' being compiled
C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.42.34433\include\utility(470): error C2079: 'std::pair<const google::protobuf::OneofDescriptor *,google::protobuf::compiler::java::OneofGeneratorInfo>::second' uses undefined struct 'google::protobuf::compiler::java::OneofGeneratorInfo'
C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.42.34433\include\utility(470): note: the template instantiation context (the oldest one first) is
bazel-out/x64_windows-opt-exec-ST-13d3ddad9198/bin/external/protobuf~/src/google/protobuf/compiler/java/_virtual_includes/helpers\google/protobuf/compiler/java/context.h(83): note: see reference to class template instantiation 'absl::lts_20250127::flat_hash_map<const google::protobuf::OneofDescriptor *,google::protobuf::compiler::java::OneofGeneratorInfo,absl::lts_20250127::container_internal::HashEq<K,void>::Hash,absl::lts_20250127::container_internal::HashEq<K,void>::Eq,std::allocator<std::pair<const K,V>>>' being compiled
        with
        [
            K=const google::protobuf::OneofDescriptor *,
            V=google::protobuf::compiler::java::OneofGeneratorInfo
        ]
external/abseil-cpp~\absl/container/flat_hash_map.h(129): note: see reference to class template instantiation 'absl::lts_20250127::container_internal::raw_hash_map<absl::lts_20250127::container_internal::FlatHashMapPolicy<K,V>,Hash,Eq,Allocator>' being compiled
        with
        [
            K=const google::protobuf::OneofDescriptor *,
            V=google::protobuf::compiler::java::OneofGeneratorInfo,
            Hash=absl::lts_20250127::container_internal::HashEq<const google::protobuf::OneofDescriptor *,void>::Hash,
            Eq=absl::lts_20250127::container_internal::HashEq<const google::protobuf::OneofDescriptor *,void>::Eq,
            Allocator=std::allocator<std::pair<const google::protobuf::OneofDescriptor *const ,google::protobuf::compiler::java::OneofGeneratorInfo>>
        ]
external/abseil-cpp~\absl/container/internal/raw_hash_map.h(33): note: see reference to class template instantiation 'absl::lts_20250127::container_internal::raw_hash_set<Policy,Hash,Eq,Alloc>' being compiled
        with
        [
            Policy=absl::lts_20250127::container_internal::FlatHashMapPolicy<const google::protobuf::OneofDescriptor *,google::protobuf::compiler::java::OneofGeneratorInfo>,
            Hash=absl::lts_20250127::container_internal::HashEq<const google::protobuf::OneofDescriptor *,void>::Hash,
            Eq=absl::lts_20250127::container_internal::HashEq<const google::protobuf::OneofDescriptor *,void>::Eq,
            Alloc=std::allocator<std::pair<const google::protobuf::OneofDescriptor *const ,google::protobuf::compiler::java::OneofGeneratorInfo>>
        ]
external/abseil-cpp~\absl/container/internal/raw_hash_set.h(3030): note: see reference to class template instantiation 'absl::lts_20250127::conjunction<std::integral_constant<bool,true>,absl::lts_20250127::type_traits_internal::IsOwner<std::pair<const google::protobuf::OneofDescriptor *,google::protobuf::compiler::java::OneofGeneratorInfo>>,absl::lts_20250127::type_traits_internal::IsView<std::pair<const google::protobuf::OneofDescriptor *,google::protobuf::compiler::java::OneofGeneratorInfo>>>' being compiled
external/abseil-cpp~\absl/meta/type_traits.h(147): note: see reference to class template instantiation 'absl::lts_20250127::conjunction<absl::lts_20250127::type_traits_internal::IsOwner<std::pair<const google::protobuf::OneofDescriptor *,google::protobuf::compiler::java::OneofGeneratorInfo>>,absl::lts_20250127::type_traits_internal::IsView<std::pair<const google::protobuf::OneofDescriptor *,google::protobuf::compiler::java::OneofGeneratorInfo>>>' being compiled
external/abseil-cpp~\absl/meta/type_traits.h(147): note: see reference to class template instantiation 'absl::lts_20250127::type_traits_internal::IsOwner<std::pair<const google::protobuf::OneofDescriptor *,google::protobuf::compiler::java::OneofGeneratorInfo>>' being compiled
external/abseil-cpp~\absl/meta/type_traits.h(607): note: see reference to class template instantiation 'std::pair<const google::protobuf::OneofDescriptor *,google::protobuf::compiler::java::OneofGeneratorInfo>' being compiled
Target //:addressbook_cc_proto failed to build
INFO: Elapsed time: 7.979s, Critical Path: 7.80s
INFO: 33 processes: 33 internal.
ERROR: Build did NOT complete successfully

Anything else we should know about your project / environment

Here is the relevant error message.

error C2079: 'std::pair<const google::protobuf::FieldDescriptor *,google::protobuf::compiler::java::FieldGeneratorInfo>::second' uses undefined struct 'google::protobuf::compiler::java::FieldGeneratorInfo'

Here are the lines in question.

protobuf/src/google/protobuf/compiler/java/context.h

Lines 39 to 40 in 40ec76e

	struct FieldGeneratorInfo;
	struct OneofGeneratorInfo;

protobuf/src/google/protobuf/compiler/java/context.h

Lines 80 to 83 in 40ec76e

	absl::flat_hash_map<const FieldDescriptor*, FieldGeneratorInfo>
	field_generator_info_map_;
	absl::flat_hash_map<const OneofDescriptor*, OneofGeneratorInfo>
	oneof_generator_info_map_;

src/google/protobuf/compiler/java/context.h looks to be trying to pass a forward declared type to the second type parameter of absl::flat_hash_map, which I'm not quite sure if it's really supported or not.

Other observations

• The compile failure is not reproducible with abseil-cpp 20240722.1. Looks like the issue is triggered by some recent change in abseil.
• The compile failure is not reproducible with abseil-cpp 20250127.0 if you do not specify /Zc:__cplusplus compiler option.

[shaod2]

@sbenzaquen I think using forward declaration should be fine right? Is there something we can do here?

[sbenzaquen]

No, we should not use incomplete types in the container.
We should #include the right header instead of a forward declaration.
Or change the container declaration to do something else that does not require putting incomplete types directly on it.