refactor: add `[[noreturn]]` attributes #339

fanquake commented at 11:12 AM on August 10, 2026: member

These will be used downstream, see https://github.com/bitcoin/bitcoin/pull/35911.

DrahtBot commented at 11:12 AM on August 10, 2026: none

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in include/mp/proxy-types.h:223 in 82d5c4c070 outdated

 219 | @@ -220,7 +220,7 @@ void ThrowField(TypeList<LocalType>, InvokeContext& invoke_context, Input&& inpu
 220 |  {
 221 |      ReadField(
 222 |          TypeList<LocalType>(), invoke_context, input, ReadDestEmplace(TypeList<LocalType>(),
 223 | -            [](auto&& ...args) -> const LocalType& { throw LocalType{std::forward<decltype(args)>(args)...}; }));
 224 | +            [] [[noreturn]] (auto&& ...args) -> const LocalType& { throw LocalType{std::forward<decltype(args)>(args)...}; }));

ryanofsky commented at 7:50 PM on August 10, 2026:

In commit "refactor: add missing [[noreturn]] attributes" (82d5c4c07019902bd8eeced04cc298a97efed849)

It seems like this noreturn might not work because it requires c++23 lambda syntax. It is causing some CI jobs to fail with

/home/runner/work/libmultiprocess/libmultiprocess/include/mp/proxy-types.h:223:16: error: an attribute specifier sequence in this position is a C++23 extension [-Werror,-Wc++23-lambda-attributes]

https://github.com/bitcoin-core/libmultiprocess/actions/runs/31382467869/job/93435472734?pr=339#step:6:474

hebasto commented at 11:44 AM on August 11, 2026:

@fanquake

Was this change prompted by the -Wmissing-noreturn compiler flag? If so, could you provide the exact build configuration (system, compiler, extra flags) used to reproduce the warning?

fanquake commented at 3:10 PM on August 11, 2026:

At least with Apple Clang and -Wmissing-noreturn:

export CXXFLAGS="-Wmissing-noreturn"
cmake -B build -G Ninja          
-- The CXX compiler identification is AppleClang 21.0.0.21000101
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD - Success
-- Found Threads: TRUE
-- Found ZLIB: /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib/libz.tbd (found version "1.2.12")
-- Performing Test HAVE_PTHREAD_GETNAME_NP
-- Performing Test HAVE_PTHREAD_GETNAME_NP - Success
-- Performing Test HAVE_PTHREAD_THREADID_NP
-- Performing Test HAVE_PTHREAD_THREADID_NP - Success
-- Performing Test HAVE_PTHREAD_GETTHREADID_NP
-- Performing Test HAVE_PTHREAD_GETTHREADID_NP - Failed
-- Configuring done (0.9s)
-- Generating done (0.0s)
-- Build files have been written to: /Users/michael/libmultiprocess/build
cmake --build build --clean-first
[1/1] Cleaning all built files...
Cleaning... 0 files.
[5/7] Building CXX object CMakeFiles/multiprocess.dir/src/mp/proxy.cpp.o
In file included from /Users/michael/libmultiprocess/src/mp/proxy.cpp:8:
/Users/michael/libmultiprocess/include/mp/proxy-types.h:223:13: warning: function 'operator()' could be declared with attribute 'noreturn' [-Wmissing-noreturn]
  223 |             [](auto&& ...args) -> const LocalType& { throw LocalType{std::forward<decltype(args)>(args)...}; }));
      |             ^
1 warning generated.
[7/7] Linking CXX static library libmultiprocess.a

hebasto commented at 3:27 PM on August 11, 2026:

It appears that the behavior of -Wmissing-noreturn depends on the Clang version:

$ env CXX="clang++-21" CXXFLAGS="-Wmissing-noreturn" cmake -B build_clang21
$ cmake --build build_clang21
[6/7] Building CXX object CMakeFiles/multiprocess.dir/src/mp/proxy.cpp.o
In file included from /home/hebasto/dev/libmultiprocess/src/mp/proxy.cpp:8:
/home/hebasto/dev/libmultiprocess/include/mp/proxy-types.h:223:13: warning: function 'operator()' could be declared with attribute 'noreturn' [-Wmissing-noreturn]
  223 |             [](auto&& ...args) -> const LocalType& { throw LocalType{std::forward<decltype(args)>(args)...}; }));
      |             ^
1 warning generated.
[7/7] Linking CXX static library libmultiprocess.a

However,

$ env CXX="clang++-22" CXXFLAGS="-Wmissing-noreturn" cmake -B build_clang22
$ cmake --build build_clang22
[7/7] Linking CXX static library libmultiprocess.a

Could this be a regression in Clang?

ryanofsky commented at 3:27 PM on August 11, 2026:

Maybe we can just turn off the -Wc++23-lambda-attributes error. It seems like it might be less practically useful than the -Wmissing-noreturn one.

hebasto commented at 3:36 PM on August 11, 2026:

Could this be a regression in Clang?

Not a regression, but a deliberate change: https://github.com/llvm/llvm-project/commit/3baddbbb0a698102073635f7559336f92bc7fe83.

refactor: add missing [[noreturn]] attributes

These will be used downstream, see
https://github.com/bitcoin/bitcoin/pull/35911.

636aaff576

ci: add -Wmissing-noreturn

Suppress `-Wc++23-lambda-attributes` warnings in effected jobs.
```bash
/libmultiprocess/include/mp/proxy-types.h:223:16: warning: an attribute specifier sequence in this position is a C++23 extension [-Wc++23-lambda-attributes]
  223 |             [] [[noreturn]] (auto&& ...args) -> const LocalType& { throw LocalType{std::forward<decltype(args)>(args)...}; }));
      |                ^
1 warning generated.
```

a779a09764

fanquake force-pushed on Aug 12, 2026

hebasto approved

hebasto commented at 10:37 AM on August 12, 2026: member

ACK a779a09764c2d24d3a6a54e54126b5623384a020.

ryanofsky merged this on Aug 12, 2026

ryanofsky closed this on Aug 12, 2026

fanquake deleted the branch on Aug 12, 2026

ryanofsky commented at 3:05 PM on August 12, 2026: collaborator

It looks like we will also need to add -Wno-c++23-lambda-attributes to the bitcoin build after the next libmultiprocess subtree update. Implemented in https://github.com/ryanofsky/bitcoin/commit/c8aef32afc5a81e118b20aa021cb90ad8230f973

EDIT: That commit doesn't work for older versions of clang or for MSVC. Updated version is https://github.com/ryanofsky/bitcoin/commit/fd7da34fd80980ebbe633b8e79792742494818e5