clang-tidy fails with modernize diagnostics and standard C++ 20 or newer

[srpgilles]

clang-tidy v21.1.5 fails on macOS (Homebrew version) when modernize diagnostics are set and standard is more recent than C++17.

Let's consider this hello.cpp:

#include <cstddef>
#include <iostream>

int main([[maybe_unused]] int argc, [[maybe_unused]] char** argv)
{
    std::cout << "Hello world!" << std::endl;

    return EXIT_SUCCESS;
}

With C++ 17 it works:

➜ clang-tidy hello.cpp -checks="-*,modernize-*" -- -std=c++17 -x c++    

11204 warnings generated.
/Users/sgilles/Codes/Sandbox/hello.cpp:3:5: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
    3 | int main(int argc, char** argv)
      | ~~~ ^                          
      | auto                            -> int
Suppressed 11204 warnings (11203 in non-user code, 1 NOLINT).
Use -header-filter=.* to display errors from all non-system headers. Use -system-headers to display errors from system headers as well.

but it fails with more recent standards:

➜ clang-tidy hello.cpp -checks="-*,modernize-*" -- -std=c++20 -x c++
13289 warnings and 10 errors generated.
Error while processing /Users/sgilles/Codes/Sandbox/hello.cpp.
error: missing '(' after '__has_feature' [clang-diagnostic-error]
/Users/sgilles/Codes/Sandbox/hello.cpp:3:5: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
    3 | int main(int argc, char** argv)
      | ~~~ ^                          
      | auto                            -> int
Suppressed 13290 warnings (13288 in non-user code, 2 NOLINT).
Use -header-filter=.* to display errors from all non-system headers. Use -system-headers to display errors from system headers as well.
Found compiler error(s).

This is recent (the exact same code worked with the diagnostic and C++ 23 standard few months ago) and it is related specifically to modernize diagnostics.

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: None (srpgilles)

clang-tidy v21.1.5 fails on macOS (Homebrew version) when modernize diagnostics are set and standard is more recent than C++17.

Let's consider this hello.cpp:

#include <cstddef>
#include <iostream>

int main([[maybe_unused]] int argc, [[maybe_unused]] char** argv)
{
    std::cout << "Hello world!" << std::endl;

    return EXIT_SUCCESS;
}

With C++ 17 it works:

➜ clang-tidy hello.cpp -checks="-*,modernize-*" -- -std=c++17 -x c++    

11204 warnings generated.
/Users/sgilles/Codes/Sandbox/hello.cpp:3:5: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
    3 | int main(int argc, char** argv)
      | ~~~ ^                          
      | auto                            -> int
Suppressed 11204 warnings (11203 in non-user code, 1 NOLINT).
Use -header-filter=.* to display errors from all non-system headers. Use -system-headers to display errors from system headers as well.

but it fails with more recent standards:

➜ clang-tidy hello.cpp -checks="-*,modernize-*" -- -std=c++20 -x c++
13289 warnings and 10 errors generated.
Error while processing /Users/sgilles/Codes/Sandbox/hello.cpp.
error: missing '(' after '__has_feature' [clang-diagnostic-error]
/Users/sgilles/Codes/Sandbox/hello.cpp:3:5: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
    3 | int main(int argc, char** argv)
      | ~~~ ^                          
      | auto                            -> int
Suppressed 13290 warnings (13288 in non-user code, 2 NOLINT).
Use -header-filter=.* to display errors from all non-system headers. Use -system-headers to display errors from system headers as well.
Found compiler error(s).

This is recent (the exact same code worked with the diagnostic and C++ 23 standard few months ago) and it is related specifically to modernize diagnostics.

[vbvictor]

Hi, this issue doesn't seem to reproduce on my Ubuntu, tried 21.1.2 and main branch.
clang-diagnostic-error is a generic error from clang compiler which indicated it failed to build AST, so could you try running other checks and see if it breaks like this too? This problem should not depend on what checks you run.

[srpgilles]

I forgot to tell I was on macOS, with clang-tidy built from Homebrew.

The warning seems really related to modernize diagnostic, as shown by these two rather extreme commands (in the first I activate everything except modernize, and in the second only modernize):

~/Codes/Sandbox 
➜ clang-tidy --checks="*,-modernize-*" hello.cpp -- -std=c++20 -x c++ 
127408 warnings generated.
/Users/sgilles/Codes/Sandbox/hello.cpp:1:1: warning: system include cstddef not allowed [llvmlibc-restrict-system-libc-headers]
    1 | #include <cstddef>
      | ^~~~~~~~~~~~~~~~~~
    2 | #include <iostream>
/Users/sgilles/Codes/Sandbox/hello.cpp:2:1: warning: system include iostream not allowed [llvmlibc-restrict-system-libc-headers]
    2 | #include <iostream>
      | ^~~~~~~~~~~~~~~~~~~
    3 | 
/Users/sgilles/Codes/Sandbox/hello.cpp:4:5: warning: declaration must be enclosed within the 'LIBC_NAMESPACE_DECL' namespace [llvmlibc-implementation-in-namespace]
    4 | int main([[maybe_unused]] int argc, [[maybe_unused]] char** argv)
      |     ^
/Users/sgilles/Codes/Sandbox/hello.cpp:6:15: warning: 'operator<<<std::char_traits<char>>' must resolve to a function declared within the namespace defined by the 'LIBC_NAMESPACE' macro [llvmlibc-callee-namespace]
    6 |     std::cout << "Hello world!" << std::endl;
      |               ^
/opt/homebrew/Cellar/llvm/21.1.5/bin/../include/c++/v1/__ostream/basic_ostream.h:437:53: note: resolves to this declaration
  437 | _LIBCPP_HIDE_FROM_ABI basic_ostream<char, _Traits>& operator<<(basic_ostream<char, _Traits>& __os, const char* __str) {
      |                                                     ^
/Users/sgilles/Codes/Sandbox/hello.cpp:6:33: warning: 'operator<<' must resolve to a function declared within the namespace defined by the 'LIBC_NAMESPACE' macro [llvmlibc-callee-namespace]
    6 |     std::cout << "Hello world!" << std::endl;
      |                                 ^
/opt/homebrew/Cellar/llvm/21.1.5/bin/../include/c++/v1/__ostream/basic_ostream.h:79:56: note: resolves to this declaration
   79 |   inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1 basic_ostream& operator<<(basic_ostream& (*__pf)(basic_ostream&)) {
      |                                                        ^
/Users/sgilles/Codes/Sandbox/hello.cpp:6:36: warning: 'endl<char, std::char_traits<char>>' must resolve to a function declared within the namespace defined by the 'LIBC_NAMESPACE' macro [llvmlibc-callee-namespace]
    6 |     std::cout << "Hello world!" << std::endl;
      |                                    ^
/opt/homebrew/Cellar/llvm/21.1.5/bin/../include/c++/v1/__ostream/basic_ostream.h:542:62: note: resolves to this declaration
  542 | _LIBCPP_HIDE_FROM_ABI inline basic_ostream<_CharT, _Traits>& endl(basic_ostream<_CharT, _Traits>& __os) {
      |                                                              ^
/Users/sgilles/Codes/Sandbox/hello.cpp:6:36: warning: do not use 'std::endl' with streams; use '\n' instead [performance-avoid-endl]
    6 |     std::cout << "Hello world!" << std::endl;
      |                                    ^~~~~~~~~
      |                                    '\n'
/Users/sgilles/Codes/Sandbox/hello.cpp:8:12: warning: no header providing "EXIT_SUCCESS" is directly included [misc-include-cleaner]
    2 | #include <iostream>
    3 | 
    4 | int main([[maybe_unused]] int argc, [[maybe_unused]] char** argv)
    5 | {
    6 |     std::cout << "Hello world!" << std::endl;
    7 | 
    8 |     return EXIT_SUCCESS;
      |            ^
Suppressed 127402 warnings (127400 in non-user code, 2 NOLINT).
Use -header-filter=.* to display errors from all non-system headers. Use -system-headers to display errors from system headers as well.

~/Codes/Sandbox took 4.5s 
➜ clang-tidy --checks="-*,modernize-*" hello.cpp -- -std=c++20 -x c++ 
13296 warnings and 10 errors generated.
Error while processing /Users/sgilles/Codes/Sandbox/hello.cpp.
error: missing '(' after '__has_feature' [clang-diagnostic-error]
/Users/sgilles/Codes/Sandbox/hello.cpp:4:5: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
    4 | int main([[maybe_unused]] int argc, [[maybe_unused]] char** argv)
      | ~~~ ^                                                            
      | auto                                                              -> int
Suppressed 13297 warnings (13295 in non-user code, 2 NOLINT).
Use -header-filter=.* to display errors from all non-system headers. Use -system-headers to display errors from system headers as well.
Found compiler error(s).

[LegalizeAdulthood]

Can you reproduce it on compiler explorer? https://godbolt.org

[vbvictor]

Can you reproduce it on compiler explorer? https://godbolt.org

I bet it's not reproductive on godbolt because It runs only linux and I couldn't reproduce locally on linux.

[vbvictor]

For now, marked as macos specific because of #168360 (comment).