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### 3371. `visit_format_arg` and `make_format_args` are not hidden friends

Section: 20.20.7.1 [format.arg] Status: C++20 Submitter: Tim Song Opened: 2020-01-16 Last modified: 2021-02-25

Priority: 0

View all other issues in [format.arg].

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Discussion:

After P1965R0, friend function and function template declarations always introduce hidden friends under the new blanket wording in 16.4.6.6 [hidden.friends]. However, 20.20.7.1 [format.arg] contains "exposition only" friend declarations of `visit_format_arg` and `make_format_args`, and those are not intended to be hidden. The only reason to have these declarations in the first place is because these function templates are specified using the exposition-only private data members of `basic_format_arg`, but that's unnecessary — for example, `shared_ptr`'s constructors are not exposition-only friends of `enable_shared_from_this`, even though the former are shown as assigning to the latter's exposition-only `weak_this` private data member (see 20.11.3.2 [util.smartptr.shared.const]p1).

[2020-02-01 Status set to Tentatively Ready after five positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4842.

1. Edit 20.20.7.1 [format.arg], class template `basic_format_arg` synopsis, as indicated:

   >     namespace std {
   >       template<class Context>
   >       class basic_format_arg {

   >         […]

   >         template<class Visitor, class Ctx>
   >           friend auto visit_format_arg(Visitor&& vis,
   >                                        basic_format_arg<Ctx> arg);                  // exposition only

   >         template<class Ctx, class... Args>
   >           friend format-arg-store<Ctx, Args...>
   >             make_format_args(const Args&... args);                                  // exposition only

   >         […]
   >       };
   >     }