Avoid a space in the UDL definition (fmtlib#3610)

* Avoid a space in the UDL definition except on GCC before 4.9

Clang 18 has grown a warning about the space being deprecated which
is enabled by default in their nightly binaries. However GCC before 4.9
will reject the UDL definition unless there is a space there, so we need
to keep the space conditionally for it.

* Remove UDLs on GCC before 4.9 to simplify things

GCC before 4.9 rejects the syntax that is now
rejected on more modern compilers.

* Disable compile-error-test on GCC < 4.9

This avoids the UDL tests failing as GCC < 4.9 can not parse UDLs
without a space, but the space is malformed in modern compilers.

include/fmt/format.h

@@ -153,7 +153,10 @@ FMT_END_NAMESPACE
 
 #ifndef FMT_USE_USER_DEFINED_LITERALS
 // EDG based compilers (Intel, NVIDIA, Elbrus, etc), GCC and MSVC support UDLs.
-#  if (FMT_HAS_FEATURE(cxx_user_literals) || FMT_GCC_VERSION >= 407 || \
+//
+// GCC before 4.9 requires a space in `operator"" _a` which is invalid in later
+// compiler versions.
+#  if (FMT_HAS_FEATURE(cxx_user_literals) || FMT_GCC_VERSION >= 409 || \
        FMT_MSC_VERSION >= 1900) &&                                     \
       (!defined(__EDG_VERSION__) || __EDG_VERSION__ >= /* UDL feature */ 480)
 #    define FMT_USE_USER_DEFINED_LITERALS 1
@@ -4459,7 +4462,7 @@ template <detail_exported::fixed_string Str> constexpr auto operator""_a() {
   return detail::udl_arg<char_t, sizeof(Str.data) / sizeof(char_t), Str>();
 }
 #  else
-constexpr auto operator"" _a(const char* s, size_t) -> detail::udl_arg<char> {
+constexpr auto operator""_a(const char* s, size_t) -> detail::udl_arg<char> {
   return {s};
 }
 #  endif

test/CMakeLists.txt

@@ -175,7 +175,12 @@ if (FMT_PEDANTIC)
 endif ()
 
 # These tests are disabled on Windows because they take too long.
-if (FMT_PEDANTIC AND NOT WIN32)
+# They are disabled on GCC < 4.9 because it can not parse UDLs without
+# a space after `operator""` but that is an incorrect syntax for any more
+# modern compiler.
+if (FMT_PEDANTIC AND NOT WIN32 AND NOT (
+    CMAKE_CXX_COMPILER_ID MATCHES "GNU" AND
+    CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.9))
   # Test if incorrect API usages produce compilation error.
   add_test(compile-error-test ${CMAKE_CTEST_COMMAND}
     --build-and-test