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### 3246. What are the constraints on the template parameter of `basic_format_arg`?

Section: 22.14.8.1 [format.arg] Status: C++20 Submitter: Richard Smith Opened: 2019-08-01 Last modified: 2021-02-25

Priority: 0

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Discussion:

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".

[2019-08-17 Issue Prioritization]

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

Proposed resolution:

This wording is relative to N4830.

1. Modify 22.14.8.1 [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 `formatter` meets the Formatter requirements (22.14.6 [format.formatter]). The extent to which an implementation determines that the specialization is enabled meets 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.