Fix chrono_test.locale

UTF-8: https://datatracker.ietf.org/doc/html/rfc3629

include/fmt/chrono.h

@@ -298,8 +298,15 @@ inline auto do_write(const std::tm& time, const std::locale& loc, char format,
   if (end.failed()) FMT_THROW(format_error("failed to format time"));
   auto str = os.str();
   if (!detail::is_utf8() || loc == std::locale::classic()) return str;
-  // char16_t and char32_t codecvts are broken in MSVC (linkage errors).
-  using code_unit = conditional_t<FMT_MSC_VER != 0, wchar_t, char16_t>;
+    // char16_t and char32_t codecvts are broken in MSVC (linkage errors) and
+    // gcc-4.
+#if FMT_MSC_VER != 0 || \
+    (defined(__GLIBCXX__) && !defined(_GLIBCXX_USE_DUAL_ABI))
+  // macro _GLIBCXX_USE_DUAL_ABI is always defined in libstdc++ from gcc-5+
+  using code_unit = wchar_t;
+#else
+  using code_unit = char32_t;
+#endif
   auto& f = std::use_facet<std::codecvt<code_unit, char, std::mbstate_t>>(loc);
   auto mb = std::mbstate_t();
   const char* from_next = nullptr;
@@ -312,12 +319,30 @@ inline auto do_write(const std::tm& time, const std::locale& loc, char format,
     FMT_THROW(format_error("failed to format time"));
   str.clear();
   for (code_unit* p = buf; p != to_next; ++p) {
-    auto c = *p;
+    uint32_t c = static_cast<uint32_t>(*p);
+    if (sizeof(code_unit) == 2 && c >= 0xd800 && c <= 0xdfff) {
+      // surrogate pair
+      ++p;
+      if (p == to_next || (c & 0xfffffc00) != 0xd800 ||
+          (static_cast<uint32_t>(*p) & 0xfffffc00) != 0xdc00) {
+        FMT_THROW(format_error("failed to format time"));
+      }
+      c = (c << 10) + static_cast<uint32_t>(*p) - 0x35fdc00;
+    }
     if (c < 0x80) {
       str.push_back(static_cast<char>(c));
     } else if (c < 0x800) {
       str.push_back(static_cast<char>(0xc0 | (c >> 6)));
       str.push_back(static_cast<char>(0x80 | (c & 0x3f)));
+    } else if ((c >= 0x800 && c <= 0xd7ff) || (c >= 0xe000 && c <= 0xffff)) {
+      str.push_back(static_cast<char>(0xe0 | (c >> 12)));
+      str.push_back(static_cast<char>(0x80 | ((c & 0xfff) >> 6)));
+      str.push_back(static_cast<char>(0x80 | (c & 0x3f)));
+    } else if (c >= 0x10000 && c <= 0x10ffff) {
+      str.push_back(static_cast<char>(0xf0 | (c >> 18)));
+      str.push_back(static_cast<char>(0x80 | ((c & 0x3ffff) >> 12)));
+      str.push_back(static_cast<char>(0x80 | ((c & 0xfff) >> 6)));
+      str.push_back(static_cast<char>(0x80 | (c & 0x3f)));
     } else {
       FMT_THROW(format_error("failed to format time"));
     }