Don't use ostream for types convertible to string_view

include/fmt/core.h

@@ -9,7 +9,7 @@
 #define FMT_CORE_H_
 
 #include <cstdio>   // std::FILE
-#include <cstring>
+#include <cstring>  // std::strlen
 #include <iterator>
 #include <limits>
 #include <string>

include/fmt/ostream.h

@@ -45,7 +45,7 @@ struct is_streamable<
     enable_if_t<
         std::is_arithmetic<T>::value || std::is_array<T>::value ||
         std::is_pointer<T>::value || std::is_same<T, char8_type>::value ||
-        std::is_same<T, std::basic_string<Char>>::value ||
+        std::is_convertible<T, fmt::basic_string_view<Char>>::value ||
         std::is_same<T, std_string_view<Char>>::value ||
         (std::is_convertible<T, int>::value && !std::is_enum<T>::value)>>
     : std::false_type {};