Workaround a compilation error on gcc 9.4

include/fmt/format.h

@@ -227,7 +227,9 @@ FMT_CONSTEXPR inline void abort_fuzzing_if(bool condition) {
 #if defined(FMT_USE_STRING_VIEW)
 template <typename Char> using std_string_view = std::basic_string_view<Char>;
 #else
-template <typename T> struct std_string_view {};
+template <typename Char> struct std_string_view {
+  operator basic_string_view<Char>() const;
+};
 #endif
 
 template <typename Char, Char... C> struct string_literal {