Disable UDL templates on GCC 9 by default (#1148)

include/fmt/format.h

@@ -148,15 +148,18 @@ FMT_END_NAMESPACE
 #  endif
 #endif
 
-// EDG C++ Front End based compilers (icc, nvcc) do not currently support UDL
-// templates.
-#if FMT_USE_USER_DEFINED_LITERALS && FMT_ICC_VERSION == 0 && \
-    FMT_CUDA_VERSION == 0 &&                                 \
-    ((FMT_GCC_VERSION >= 600 && __cplusplus >= 201402L) ||   \
-     (defined(FMT_CLANG_VERSION) && FMT_CLANG_VERSION >= 304))
-#  define FMT_UDL_TEMPLATE 1
-#else
-#  define FMT_UDL_TEMPLATE 0
+#ifndef FMT_USE_UDL_TEMPLATE
+// EDG front end based compilers (icc, nvcc) do not support UDL templates yet
+// and GCC 9 warns about them.
+#  if FMT_USE_USER_DEFINED_LITERALS && FMT_ICC_VERSION == 0 && \
+      FMT_CUDA_VERSION == 0 &&                                 \
+      ((FMT_GCC_VERSION >= 600 && FMT_GCC_VERSION <= 900 &&    \
+        __cplusplus >= 201402L) ||                             \
+       (defined(FMT_CLANG_VERSION) && FMT_CLANG_VERSION >= 304))
+#    define FMT_USE_UDL_TEMPLATE 1
+#  else
+#    define FMT_USE_UDL_TEMPLATE 0
+#  endif
 #endif
 
 #if FMT_HAS_GXX_CXX11 || FMT_HAS_FEATURE(cxx_trailing_return) || \
@@ -3570,7 +3573,7 @@ inline std::size_t formatted_size(string_view format_str, const Args&... args) {
 #if FMT_USE_USER_DEFINED_LITERALS
 namespace internal {
 
-#  if FMT_UDL_TEMPLATE
+#  if FMT_USE_UDL_TEMPLATE
 template <typename Char, Char... CHARS> class udl_formatter {
  public:
   template <typename... Args>
@@ -3593,7 +3596,7 @@ template <typename Char> struct udl_formatter {
     return format(str, std::forward<Args>(args)...);
   }
 };
-#  endif  // FMT_UDL_TEMPLATE
+#  endif  // FMT_USE_UDL_TEMPLATE
 
 template <typename Char> struct udl_arg {
   const Char* str;
@@ -3606,7 +3609,7 @@ template <typename Char> struct udl_arg {
 }  // namespace internal
 
 inline namespace literals {
-#  if FMT_UDL_TEMPLATE
+#  if FMT_USE_UDL_TEMPLATE
 template <typename Char, Char... CHARS>
 FMT_CONSTEXPR internal::udl_formatter<Char, CHARS...> operator""_format() {
   return {};
@@ -3630,7 +3633,7 @@ inline internal::udl_formatter<wchar_t> operator"" _format(const wchar_t* s,
                                                            std::size_t) {
   return {s};
 }
-#  endif  // FMT_UDL_TEMPLATE
+#  endif  // FMT_USE_UDL_TEMPLATE
 
 /**
   \rst