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Avoid unwanted sign extensions from MSVC in is_utf8.

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Microsoft's constexpr evaluator treats the type of micro[0] and micro[1] as
plain char, and so sign extends before comparing them to ints.
The normal compiler, including the optimizer, does not fail in this way,
so this is merely a "future proof" change in case someone uses is_utf8()
in a constant expression.
This commit is contained in:
Michael Winterberg 2021-05-19 14:25:38 -07:00 committed by Victor Zverovich
parent 13e652939b
commit be48f4d657
1 changed files with 5 additions and 2 deletions
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7
include/fmt/core.h
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@ -395,8 +395,11 @@ FMT_CONSTEXPR typename std::make_unsigned<Int>::type to_unsigned(Int value) {
FMT_MSC_WARNING(suppress : 4566) constexpr unsigned char micro[] = "\u00B5";
constexpr bool is_utf8() {
return FMT_UNICODE ||
(sizeof(micro) == 3 && micro[0] == 0xC2 && micro[1] == 0xB5);
// avoid buggy sign extensions in MSVC's constant evaluation mode
// https://developercommunity.visualstudio.com/t/C-difference-in-behavior-for-unsigned/1233612
using uchar = unsigned char;
return FMT_UNICODE || (sizeof(micro) == 3 && uchar{micro[0]} == 0xC2 &&
uchar{micro[1]} == 0xB5);
}
FMT_END_DETAIL_NAMESPACE