Proposed resolution:

This wording is relative to N4830.

1. Modify [format.arg] as indicated:

   template<class T> explicit basic_format_arg(const T& v) noexcept;
   

   -4- Constraints: The template specialization

   typename Context::template formatter_type<T>
   

   is an enabled specialization of formattermeets the Formatter requirements ([format.formatter]). The extent to which an implementation determines that the specialization is enabledmeets the Formatter requirements is unspecified, except that as a minimum the expression

   typename Context::template formatter_type<T>()
     .format(declval<const T&>(), declval<Context&>())
   

   shall be well-formed when treated as an unevaluated operand.

[ 2019-08-17 Issue Prioritization ]

Status to Tentatively Ready and priority to 0 after six positive votes on the reflector.

In P0645R10 20.?.5.1/ we find:

Constraints: typename Context::template formatter_type<T> is enabled.

… which doesn't mean anything, because that is an arbitrary type. Presumably the intent is that that type will always be a specialization of formatter, but there appear to be no constraints whatsoever on the Context template parameter, so there seems to be no requirement that that is the case.

Should basic_format_arg place some constraints on its Context template parameter? E.g., should it be required to be a specialization of basic_format_context?

Victor Zverovich:

The intent here is to allow different context types provide their own formatter extension types. The default formatter context and extension are basic_format_context and formatter respectively, but it's possible to have other. For example, in the fmt library there is a formatter context that supports printf formatting for legacy code. It cannot use the default formatter specializations because of the different syntax (%... vs {...}).

Richard Smith:

In either case, the specification here seems to be missing the rules for what is a valid Context parameter.

I'm happy to editorially change "is enabled" to "is an enabled specialization of formatter", since there's nothing else that this could mean, but we still need a wording fix for the broader issue here. Here's what I have so far for this wording:

Constraints: The template specialization typename Context::template formatter_type<T> is an enabled specialization of formatter ([formatter.requirements]).

Tim Song:

I think what we actually want here is "typename Context::template formatter_type<T> meets the Formatter requirements".