private inheritance for formatter<fbstring>

Summary: And other fixes. Because Hyrum's Law comes for all.

Reviewed By: vitaut

Differential Revision: D34010783

fbshipit-source-id: 95c56f24ef4b129b2758d4ec465366cd09acac93

folly/FBString.h

@@ -2733,7 +2733,9 @@ struct IsSomeString<fbstring> : std::true_type {};
 } // namespace folly
 
 template <>
-struct fmt::formatter<folly::fbstring> : formatter<fmt::string_view> {
+struct fmt::formatter<folly::fbstring> : private formatter<fmt::string_view> {
+  using formatter<fmt::string_view>::parse;
+
   template <typename Context>
   typename Context::iterator format(
       const folly::fbstring& s, Context& ctx) const {

folly/Range.h

@@ -1554,7 +1554,7 @@ struct formatter<folly::StringPiece> : private formatter<string_view> {
   using formatter<string_view>::parse;
 
   template <typename Context>
-  auto format(folly::StringPiece s, Context& ctx) {
+  typename Context::iterator format(folly::StringPiece s, Context& ctx) const {
     return formatter<string_view>::format({s.data(), s.size()}, ctx);
   }
 };