Deprecate undocumented _u suffix

include/fmt/format.h

@@ -566,7 +566,8 @@ class FMT_DEPRECATED u8string_view : public basic_string_view<char8_t> {
 
 #if FMT_USE_USER_DEFINED_LITERALS
 inline namespace literals {
-inline basic_string_view<char8_t> operator"" _u(const char* s, std::size_t n) {
+FMT_DEPRECATED inline basic_string_view<char8_t> operator"" _u(const char* s,
+                                                               std::size_t n) {
   return {reinterpret_cast<const char8_t*>(s), n};
 }
 }  // namespace literals

test/format-test.cc

@@ -2523,18 +2523,6 @@ std::string from_u8str(const fmt::basic_string_view<char8_t>& str) {
   return std::string(str.begin(), str.end());
 }
 
-#if FMT_USE_USER_DEFINED_LITERALS
-TEST(FormatTest, U8StringViewLiteral) {
-  using namespace fmt::literals;
-  fmt::basic_string_view<char8_t> s = "ab"_u;
-  EXPECT_EQ(s.size(), 2u);
-  const char8_t* data = s.data();
-  EXPECT_EQ(char(data[0]), 'a');
-  EXPECT_EQ(char(data[1]), 'b');
-  EXPECT_EQ(from_u8str(format("{:*^5}"_u, "🤡"_u)), from_u8str("**🤡**"_u));
-}
-#endif
-
 TEST(FormatTest, EmphasisNonHeaderOnly) {
   // Ensure this compiles even if FMT_HEADER_ONLY is not defined.
   EXPECT_EQ(fmt::format(fmt::emphasis::bold, "bold error"),