type_traits_test fails to compile on platforms where char is unsigned

[singhc7]

Summary

test/unittest/type_traits_test.cpp contains a static_assert that assumes char is signed. This fails to compile on platforms where char is unsigned by default, such as aarch64-linux, ppc64, and several embedded targets.

Affected line

https://github.com/atcoder/ac-library/blob/v1.6/test/unittest/type_traits_test.cpp#L47

static_assert(is_same<unsigned char, internal::to_unsigned_t<char>>::value, "");

This holds when char is signed (e.g. x86_64-linux), because to_unsigned_t<char> resolves to unsigned char. On platforms where char is unsigned, internal::is_signed_int<char> is false, so to_unsigned_t<char> resolves to char itself — not unsigned char — and the assertion fails.

Reproduction

Build the unit tests on any platform where char is unsigned, or force it on x86_64:

g++ -funsigned-char -std=c++14 -I. test/unittest/type_traits_test.cpp -c

Output:

type_traits_test.cpp:47:70: error: static assertion failed
   47 | static_assert(is_same<unsigned char, internal::to_unsigned_t<char>>::value, "");
      |               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~

Suggested fix

Guard the assertion with CHAR_MIN < 0 so the correct relationship is checked on each platform:

#if CHAR_MIN < 0
static_assert(is_same<unsigned char, internal::to_unsigned_t<char>>::value, "");
#else
static_assert(is_same<char, internal::to_unsigned_t<char>>::value, "");
#endif

(<climits> needs to be included.)

The library itself works correctly on these platforms — only this one test assertion is incorrect. PR coming.

Environment

• Compiler: GCC 15.2.0
• Platform: aarch64-linux
• Standard: C++14 / C++17 (both reproduce)