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### 3372. `vformat_to` should not try to deduce `Out` twice

Section: 22.14.5 [format.functions] Status: C++20 Submitter: Tim Song Opened: 2020-01-16 Last modified: 2021-02-25

Priority: 0

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Discussion:

`vformat_to` currently deduces `Out` from its first and last arguments. This requires its last argument's type to be a specialization of `basic_format_args`, which notably prevents the use of `format-arg-store` arguments directly. This is unnecessary: we should only deduce from the first argument.

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

Proposed resolution:

This wording is relative to N4842.

1. Edit 22.14.1 [format.syn], header `<format>` synopsis, as indicated:

   >     namespace std {

   >       […]

   >       template<class Out>
   >         Out vformat_to(Out out, string_view fmt, format_args_t<type_identity_t<Out>, char> args);
   >       template<class Out>
   >         Out vformat_to(Out out, wstring_view fmt, format_args_t<type_identity_t<Out>, wchar_t> args);
   >       template<class Out>
   >         Out vformat_to(Out out, const locale& loc, string_view fmt,
   >                        format_args_t<type_identity_t<Out>, char> args);
   >       template<class Out>
   >         Out vformat_to(Out out, const locale& loc, wstring_view fmt,
   >                        format_args_t<type_identity_t<Out>, wchar_t> args);

   >       […]
   >     }

2. Edit 22.14.5 [format.functions] p8 through p10 as indicated:

   >     template<class Out, class... Args>
   >       Out format_to(Out out, string_view fmt, const Args&... args);
   >     template<class Out, class... Args>
   >       Out format_to(Out out, wstring_view fmt, const Args&... args);

   > > -8- Effects: Equivalent to:

   > > >     using context = basic_format_context<Out, decltype(fmt)::value_type>;
   > > >     return vformat_to(out, fmt, {make_format_args<context>(args...)});

   >     template<class Out, class... Args>
   >       Out format_to(Out out, const locale& loc, string_view fmt, const Args&... args);
   >     template<class Out, class... Args>
   >       Out format_to(Out out, const locale& loc, wstring_view fmt, const Args&... args);

   > > -9- Effects: Equivalent to:

   > > >     using context = basic_format_context<Out, decltype(fmt)::value_type>;
   > > >     return vformat_to(out, loc, fmt, {make_format_args<context>(args...)});

   >       template<class Out>
   >         Out vformat_to(Out out, string_view fmt, format_args_t<type_identity_t<Out>, char> args);
   >       template<class Out>
   >         Out vformat_to(Out out, wstring_view fmt, format_args_t<type_identity_t<Out>, wchar_t> args);
   >       template<class Out>
   >         Out vformat_to(Out out, const locale& loc, string_view fmt,
   >                        format_args_t<type_identity_t<Out>, char> args);
   >       template<class Out>
   >         Out vformat_to(Out out, const locale& loc, wstring_view fmt,
   >                        format_args_t<type_identity_t<Out>, wchar_t> args);

   > > -10- Let `charT` be `decltype(fmt)::value_type`.

   > > […]