Workaround a gcc 9.1 bug (#2334)

include/fmt/format-inl.h

@@ -148,8 +148,7 @@ template <> FMT_FUNC int count_digits<4>(detail::fallback_uintptr n) {
 }
 
 #if __cplusplus < 201703L
-template <typename T>
-constexpr const typename basic_data<T>::digit_pair basic_data<T>::digits[];
+template <typename T> constexpr const char basic_data<T>::digits[][2];
 template <typename T> constexpr const char basic_data<T>::hex_digits[];
 template <typename T> constexpr const char basic_data<T>::signs[];
 template <typename T> constexpr const unsigned basic_data<T>::prefixes[];

include/fmt/format.h

@@ -875,8 +875,7 @@ template <typename T = void> struct basic_data {
   static const uint64_t log10_2_significand = 0x4d104d427de7fbcc;
 
   // GCC generates slightly better code for pairs than chars.
-  using digit_pair = char[2];
-  FMT_API static constexpr const digit_pair digits[] = {
+  FMT_API static constexpr const char digits[][2] = {
       {'0', '0'}, {'0', '1'}, {'0', '2'}, {'0', '3'}, {'0', '4'}, {'0', '5'},
       {'0', '6'}, {'0', '7'}, {'0', '8'}, {'0', '9'}, {'1', '0'}, {'1', '1'},
       {'1', '2'}, {'1', '3'}, {'1', '4'}, {'1', '5'}, {'1', '6'}, {'1', '7'},