Use underlying type for enums in is_signed/is_unsigned. #199
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With clang >= 21, using is_signed/is_unsigned with "small" enums (having an underlying type smaller than an int) results in strong error, after being reported as warnings for years (see boostorg#171). Since boost::is_signed/is_unsigned differs explicitly from the std one, and can return "true" for enums (contrary to the std one), keep this boost behavior and implement a special check for enums. This allows in particular boost::lexical_cast to keep working when converting non-scoped enums from int to strings with clang >= 21, see boostorg/lexical_cast#87).
This was referenced Sep 5, 2025
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The problem you have here, is that |
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With clang >= 21, using is_signed/is_unsigned with "small" enums (having an underlying type smaller than an int) results in strong error, after being reported as warnings for years (see #171).
Since boost::is_signed/is_unsigned differs explicitly from the std one, and can return "true" for enums (contrary to the std one), keep this boost behavior and implement a special check for enums.
This allows in particular boost::lexical_cast to keep working when converting non-scoped enums from int to strings with clang >= 21, see boostorg/lexical_cast#87).