fix: work around clang 20/21 + libc++ compile failure in TEMPLATE_PRODUCT_TEST_CASE with differing arities

[mvanhorn]

Description

Restructure the get_wrapper overload set emitted by INTERNAL_CATCH_TYPE_GEN in src/catch2/internal/catch_preprocessor.hpp so clang 20/21 no longer sees a conflicting deduction between the explicitly-specified template-template argument and the deduced parameter pack. Concretely, avoid mixing an explicitly-specified template <typename...> class argument with deduction against the same overload name: split the plain-type (TypeList<Ts...>) and template-template (TemplateTypeList<Cs...>) resolution into distinct helper names or a tag-dispatched trait, keeping the existing priority_tag ranking and identical resulting types so INTERNAL_CATCH_TEMPLATE_PRODUCT_TEST_CASE2 in src/catch2/internal/catch_template_test_registry.hpp needs at most a matching call-site rename. If a general restructuring proves too invasive, fall back to a narrowly scoped #if defined(__clang__) && __clang_major__ >= 20 && __clang_major__ < 22 alternative formulation, matching Catch2's existing compiler-workaround style.

Building Catch2 3.15.0's SelfTest suite fails under clang 20/21 with libc++ (observed on macOS and Alpine via meson wrapdb CI). The failing construct is TEMPLATE_PRODUCT_TEST_CASE("Product with differing arities", ..., std::tuple, (int, (int, double), (int, double, float))) in tests/SelfTest/UsageTests/Misc.tests.cpp:391. Clang reports "no matching function for call to 'get_wrapper'": the template-template overload of get_wrapper generated by INTERNAL_CATCH_TYPE_GEN in src/catch2/internal/catch_preprocessor.hpp (the priority_tag<1> overload returning TemplateTypeList<Cs...>) hits a "conflicting deduction 'std::tuple' against 'type-parameter-0-0'" substitution failure. The reporter notes clang 22 compiles it fine, so this is likely a clang 20/21 deduction bug, but those compiler versions ship on current macOS and Alpine toolchains, making a Catch2-side workaround valuable.

Closes #3115

GitHub Issues

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.09%. Comparing base (a15f718) to head (a0c7a39).

Additional details and impacted files

@@           Coverage Diff           @@
##            devel    #3173   +/-   ##
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  Coverage   91.09%   91.09%           
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  Files         204      204           
  Lines        8922     8922           
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  Hits         8127     8127           
  Misses        795      795           

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[rajpratham1]

Looks good to me. This is a minimal, targeted workaround for the Clang 20/21 template-template parameter issue, and it doesn't change public behavior. The added comment explains why the separate get_template_wrapper exists, which improves maintainability. One small suggestion would be to reference the relevant LLVM/Clang issue (or include the compiler error) in the comment so it's easier to remove the workaround once the upstream bug is fixed.

[mvanhorn]

Thanks for the review. Good suggestion - added the reference in a0c7a39: the workaround comment now points at llvm/llvm-project#130778 (the Clang 20 'conflicting deduction' regression; its duplicate #138078 shows the exact diagnostic) and notes get_template_wrapper can be removed once Clang 21 is no longer supported.