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Subject: LWG issue: Time formatters should not be locale sensitive by default
From: Corentin (corentin.jabot_at_[hidden])
Date: 2021-04-27 02:29:00


In [time.format], it is specified

> Some of the conversion specifiers depend on the locale that is passed to
the formatting function if the latter takes one, or the global locale
otherwise

This is not consistent with the format design after the adoption of P1892
In [format.string.std] we say:

> When the L option is used, the form used for the conversion is called the
locale-specific form. The L option is only valid for arithmetic types, and
its effect depends upon the type

This has 2 issues:
First, it is inconsistent.

format("{}, 0.0"); // locale independent
format("{:L}", 0.0); // use locale
format("{:%r}, some_time); // use globale locale
format("{:%rL}, some_time); // error

And second it perpetuates the issues P1892 intended to solve.
It is likely that this inconsistency resulted from both papers being in
flight around the same time

Proposed resolution

The L option should be used and consistent with floating point. We suggest
using the C locale which is the non-locale locale. (
https://pubs.opengroup.org/onlinepubs/009604499/basedefs/xbd_chap07.html)

Suggested wording

Modify [time.format]/1

*chrono-format-spec: *
* fill-and-align(opt) width(opt) precision(opt) chrono-specs(opt)L(opt)*

Modify [time.format]/2

Each conversion specifier conversion-spec is replaced by appropriate
characters as described in Table 101; the formats specified in ISO
8601:2004 shall be used where so described. Some of the conversion
specifiers depend on <del>the locale that is passed to the formatting
function if the latter takes one, or the global locale otherwise</del>
*<ins>a locale. If the L option is used, that locale is the locale that is
passed to the formatting function if the later takes one, or the global
locale otherwise. If the L option is not used, that locale is the "C"
locale</ins>*.If the formatted object does not contain the information the
conversion specifier refers to, an exception of type format_error is thrown.



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