ac-library: fix tests on platforms where char is unsigned

The type_traits_test asserts `to_unsigned_t<char>` resolves to
`unsigned char`, which only holds on platforms where `char` is signed
(e.g. x86_64-linux). On platforms where `char` is unsigned by default
(e.g. aarch64-linux), `to_unsigned_t<char>` resolves to `char`, and
the static_assert fails to compile.

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

Also drop `-Wno-character-conversion` from NIX_CFLAGS_COMPILE since
GCC 15 does not recognize it (the build log shows it being reported
as an unrecognized command-line option).

Reported upstream: https://github.com/atcoder/ac-library/issues/191

ZHF: #516381

pkgs/by-name/ac/ac-library/fix-char-signedness-tests.patch

@@ -0,0 +1,24 @@
+diff --git i/test/unittest/type_traits_test.cpp w/test/unittest/type_traits_test.cpp
+index 164cc79..123b3a2 100644
+--- i/test/unittest/type_traits_test.cpp
++++ w/test/unittest/type_traits_test.cpp
+@@ -1,5 +1,6 @@
+ #include "atcoder/internal_type_traits"
+ 
++#include <climits>
+ #include <type_traits>
+ 
+ #include <gtest/gtest.h>
+@@ -44,7 +45,12 @@ static_assert(internal::is_unsigned_int<unsigned long long>::value, "");
+ static_assert(!internal::is_signed_int<A>::value, "");
+ static_assert(!internal::is_unsigned_int<A>::value, "");
+ 
++#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
++
+ static_assert(
+     is_same<unsigned char, internal::to_unsigned_t<signed char>>::value,
+     "");

pkgs/by-name/ac/ac-library/package.nix

@@ -18,7 +18,12 @@ stdenv.mkDerivation (finalAttrs: {
     fetchSubmodules = true;
     hash = "sha256-1wwzN/JPS6daj1vDFuEN5z20tMdLfMvEKti0sxCVlHA=";
   };
-
+  patches = [
+    # Fix type_traits_test assumptions about char signedness on platforms
+    # where char is unsigned by default (e.g. aarch64-linux).
+    # Reported upstream: https://github.com/atcoder/ac-library/issues/191
+    ./fix-char-signedness-tests.patch
+  ];
   outputs = [
     "dev"
     "out"
@@ -60,7 +65,6 @@ stdenv.mkDerivation (finalAttrs: {
   env = {
     NIX_CFLAGS_COMPILE = toString [
       "-Wno-error=array-bounds"
-      "-Wno-character-conversion"
     ];
   };