Clang 19 -Wc++20-extensions warning

[ChrisThrasher]

Describe the bug
Clang reports a -Wc++20-extensions warning when expanding the TEMPLATE_TEST_CASE macro with the following message

/Users/thrasher/Projects/Catch2/tests/SelfTest/IntrospectiveTests/FloatingPoint.tests.cpp:27:1: error: passing no argument for the '...' parameter of a variadic macro is a C++20 extension [-Werror,-Wc++20-extensions]
   27 | TEMPLATE_TEST_CASE("type-shared ulpDistance tests", "[floating-point][ulp][approvals]", float, double) {
      | ^
/Users/thrasher/Projects/Catch2/src/catch2/../catch2/catch_template_test_macros.hpp:76:39: note: expanded from macro 'TEMPLATE_TEST_CASE'
   76 |     #define TEMPLATE_TEST_CASE( ... ) INTERNAL_CATCH_TEMPLATE_TEST_CASE( __VA_ARGS__ )
      |                                       ^
/Users/thrasher/Projects/Catch2/src/catch2/../catch2/internal/catch_template_test_registry.hpp:104:9: note: expanded from macro 'INTERNAL_CATCH_TEMPLATE_TEST_CASE'
  104 |         INTERNAL_CATCH_TEMPLATE_TEST_CASE_2( INTERNAL_CATCH_UNIQUE_NAME( CATCH2_INTERNAL_TEMPLATE_TEST_ ), INTERNAL_CATCH_UNIQUE_NAME( CATCH2_INTERNAL_TEMPLATE_TEST_ ), Name, Tags, typename TestType, __VA_ARGS__ )
      |         ^
/Users/thrasher/Projects/Catch2/src/catch2/../catch2/internal/catch_template_test_registry.hpp:82:13: note: expanded from macro 'INTERNAL_CATCH_TEMPLATE_TEST_CASE_2'
   82 |             INTERNAL_CATCH_NTTP_GEN(INTERNAL_CATCH_REMOVE_PARENS(Signature))\
      |             ^
/Users/thrasher/Projects/Catch2/src/catch2/../catch2/internal/catch_preprocessor.hpp:217:465: note: expanded from macro 'INTERNAL_CATCH_NTTP_GEN'
  217 | #define INTERNAL_CATCH_NTTP_GEN(...) INTERNAL_CATCH_VA_NARGS_IMPL(__VA_ARGS__, INTERNAL_CATCH_NTTP_1(__VA_ARGS__), INTERNAL_CATCH_NTTP_1(__VA_ARGS__), INTERNAL_CATCH_NTTP_1(__VA_ARGS__), INTERNAL_CATCH_NTTP_1(__VA_ARGS__), INTERNAL_CATCH_NTTP_1(__VA_ARGS__), INTERNAL_CATCH_NTTP_1( __VA_ARGS__), INTERNAL_CATCH_NTTP_1( __VA_ARGS__), INTERNAL_CATCH_NTTP_1( __VA_ARGS__), INTERNAL_CATCH_NTTP_1( __VA_ARGS__),INTERNAL_CATCH_NTTP_1( __VA_ARGS__), INTERNAL_CATCH_NTTP_0)
      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 ^
/Users/thrasher/Projects/Catch2/src/catch2/../catch2/internal/catch_preprocessor.hpp:100:9: note: macro 'INTERNAL_CATCH_VA_NARGS_IMPL' defined here
  100 | #define INTERNAL_CATCH_VA_NARGS_IMPL(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, N, ...) N
      |         ^

Reproduction steps
I'm using the all-tests preset with Clang 19 as provided by Homebrew. I also encounter this issue when consuming Catch2 via FetchContent for use in various other projects.

Platform information:

• OS: macOS 15
• Compiler+version: Clang 19.1.0
• Catch version: v3.7.1 fa43b77429ba76c462b1898d6cd2f2d7a9416b14

[toughengineer]

This problem is still not fully fixed in the new 3.8.1 version when using Clang 19.

When using macros

SUCCEED();
FAIL();

with no arguments compilation fails with the following messages:

example.cpp(21,24): error GBB18C139: passing no argument for the '...' parameter of a variadic macro is a C++20 extension [-Werror,-Wc++20-extensions]
     21 |                        SUCCEED();
        |                        ^
  Catch2\src\catch2\catch_test_macros.hpp:153:136: note: expanded from macro 'SUCCEED'
    153 |   #define SUCCEED( ... ) INTERNAL_CATCH_MSG( "SUCCEED", Catch::ResultWas::Ok, Catch::ResultDisposition::ContinueOnFailure, __VA_ARGS__ )
        |                                                                                                                                        ^
  Catch2\src\catch2\catch_message.hpp:93:9: note: macro 'INTERNAL_CATCH_MSG' defined here
     93 | #define INTERNAL_CATCH_MSG( macroName, messageType, resultDisposition, ... ) \
  
        |         ^

and

example.cpp(22,25): error GBB18C139: passing no argument for the '...' parameter of a variadic macro is a C++20 extension [-Werror,-Wc++20-extensions]
     22 |                         FAIL();
        |                         ^
  Catch2\src\catch2\catch_test_macros.hpp:151:132: note: expanded from macro 'FAIL'
    151 |   #define FAIL( ... ) INTERNAL_CATCH_MSG( "FAIL", Catch::ResultWas::ExplicitFailure, Catch::ResultDisposition::Normal, __VA_ARGS__ )
        |                                                                                                                                    ^
  Catch2\src\catch2\catch_message.hpp:93:9: note: macro 'INTERNAL_CATCH_MSG' defined here
     93 | #define INTERNAL_CATCH_MSG( macroName, messageType, resultDisposition, ... ) \
        |         ^

Ugly workaround: put empty strings inside parentheses like this

SUCCEED("");
FAIL("");

[ChrisThrasher]

Thanks for letting me know. I think what we need is a CI job that can recreate this so we can more robustly find all of the APIs that trigger this warning.