Fix C++20 concept detection #4653
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vitaut
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Jan 15, 2026
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Thanks for the PR but I think we should only support C++20 concepts.
vitaut
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Jan 16, 2026
include/fmt/base.h
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| using is_formattable = bool_constant<!std::is_same< | ||
| detail::mapped_t<conditional_t<std::is_void<T>::value, int*, T>, Char>, | ||
| void>::value>; | ||
| #ifdef __cpp_concepts |
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To elaborate: we should only check __cpp_concepts >= 201907L here.
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9ab39ba Updated again to fix -Wundef warning.
vitaut
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Jan 16, 2026
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Merged, thanks for the fix! |
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Fix compilation on GCC < 8.4 with
-fconcepts.From GCC 8.4 onwards, ISO-19217 conceptsEdit: Irrelevant-fconceptsare compatible with C++20 concepts thanks to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88075.Repro of what I'm trying to fix: https://godbolt.org/z/vv179vWGe