diff --git a/issue4067.html b/issue4067.html new file mode 100644 index 0000000000..5ad163d5f2 --- /dev/null +++ b/issue4067.html @@ -0,0 +1,221 @@ + + + + +Issue 4067: Inconsistency and potential infinity meta-recursion in std::chrono::zoned_time's constructors + + + + + + + + + +
+

This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of New status.

+

4067. Inconsistency and potential infinity meta-recursion in std::chrono::zoned_time's constructors

+

Section: 29.11.7.2 [time.zone.zonedtime.ctor] Status: New + Submitter: Jiang An Opened: 2024-04-14 Last modified: 2024-04-14

+

Priority: Not Prioritized +

+

View all other issues in [time.zone.zonedtime.ctor].

+

View all issues with New status.

+

Discussion:

+

+Currently, there are no requirements on a program-defined std::chrono::zoned_traits specialization. +So a zoned_traits<UserDefinedTzPtr>::locate_zone possibly returns a string_view, +which leads to infinity meta-recursion when checking the constraints of zoned_time's constructors +that take a string_view. +

+Also, the constructor of zoned_time taking only one string_view is inconsistent with others +and even self-inconsistent. For other constructors taking string_view, it is expected that the return +value of locate_zone is only implicitly converted to a TimeZonePtr. But the return value is +permitted to be convertible to zoned_time or sys_time<Duration> in this constructor. +And given the locate_name function may be an overload set that distinguishes lvalues and rvalues of +string_view, the use of locate_time(string_view{}) in the constraints doesn't always reflect +locate_time(name) in the effects as the return types can be different. +

+ + +

Proposed resolution:

+

+This wording is relative to N4971. +

+ +
    +

  1. Modify 29.11.7.2 [time.zone.zonedtime.ctor] as indicated:

    + +
    +

    +[Drafting note: Add the following two paragraphs to the very beginning of subclause +29.11.7.2 [time.zone.zonedtime.ctor]] +

    +
    + +
    +

    +-?- Let locatable-as-tzptr be true if given an lvalue sv of type +string_view, traits::locate_zone(sv) is well-formed and TimeZonePtr is implicitly +convertible from its return type, and false otherwise. Only the validity of the immediate context +of the invocation and conversion is considered. +

    +-?- In every constructor that takes a string_view parameter name, let +converted-tzptr be a variable of type TimeZonePtr that is copy-initialized +from traits::locate_zone(name). +

    +[…] +

    +
    +explicit zoned_time(string_view name);
+
    +
    +

    +-7- Constraints: traits::locate_zone(string_view{}) is a well-formed expression and +zoned_time is constructible from the return type of traits::locate_zone(string_view{}) +locatable-as-tzptr is true. +

    +-8- Effects: Initializes zone_ with traits::locate_zone(name) and default +constructs tp_. +

    +
    +

    +[…] +

    +
    +zoned_time(string_view name, const sys_time<Duration>& st);
+
    +
    +

    +-13- Constraints: zoned_time is constructible from the return type of +traits::locate_zone(name) and stlocatable-as-tzptr is +true. +

    +-14- Effects: Equivalent to construction with +{traits::locate_zone(name)std::move(converted-tzptr), st}. +

    +
    +

    +[…] +

    +
    +zoned_time(string_view name, const local_time<Duration>& tp);
+
    +
    +

    +-18- Constraints: locatable-as-tzptr is true and +zoned_time is constructible from the return type of +traits::locate_zone(name)TimeZonePtr and tp. +

    +-19- Effects: Equivalent to construction with +{traits::locate_zone(name)std::move(converted-tzptr), tp}. +

    +
    +

    +[…] +

    +
    +zoned_time(string_view name, const local_time<Duration>& tp, choose c);
+
    +
    +

    +-23- Constraints: locatable-as-tzptr is true and +zoned_time is constructible from the return type of +traits::locate_zone(name)TimeZonePtr, +local_time<Duration>, and choose. +

    +-24- Effects: Equivalent to construction with +{traits::locate_zone(name)std::move(converted-tzptr), tp, c}. +

    +
    +

    +[…] +

    +
    +template<class Duration2, class TimeZonePtr2>
+  zoned_time(string_view name, const zoned_time<Duration2, TimeZonePtr2>& y);
+
    +
    +

    +-32- Constraints: locatable-as-tzptr is true and +zoned_time is constructible from the return type of +traits::locate_zone(name)TimeZonePtr +and the type zoned_time<Duration2, TimeZonePtr2>. +

    +-33- Effects: Equivalent to construction with +{traits::locate_zone(name)std::move(converted-tzptr), y}. +

    +
    +
    +template<class Duration2, class TimeZonePtr2>
+  zoned_time(string_view name, const zoned_time<Duration2, TimeZonePtr2>& y, choose c);
+
    +
    +

    +-34- Constraints: locatable-as-tzptr is true and +zoned_time is constructible from the return type of +traits::locate_zone(name)TimeZonePtr, the type +zoned_time<Duration2, TimeZonePtr2>, and the type choose. +

    +-35- Effects: Equivalent to construction with +{traits::locate_zone(name)std::move(converted-tzptr), y, c}. +

    +
    +
    +
    2. +
+ + + + + + + diff --git a/lwg-active.html b/lwg-active.html index 46a185f305..921d779e47 100644 --- a/lwg-active.html +++ b/lwg-active.html @@ -79,7 +79,7 @@

C++ Standard Library Active Issues List (Revision D125)

-

Revised 2024-04-14 at 08:50:03 UTC +

Revised 2024-04-14 at 13:22:55 UTC

Reference ISO/IEC IS 14882:2020(E)

Also see:

@@ -199,17 +199,17 @@

Revision History