Proposed resolution:

This wording is relative to N4950.

1. Modify [time.parse] as indicated:

   -17- If the from_stream overload fails to parse everything specified by the format string, or if insufficient information is parsed to specify a complete duration, time point, or calendrical data structure, setstate(ios_base::failbit) is called on the basic_istream.

   -?- A format string can contain multiple flags that correspond to the same information, (e.g., "%Y %C%y" or "%R %H:%M"). If such a format string is successfully parsed but the extracted dates or times are not consistent, the value assigned to the tp argument is unspecified.

   [Example:

   chrono::year y;
   stringstream("1905 1915") >> parse("%Y %C%y", y);
   

   The value of y is unspecified; y.ok() can be false. — end example]

[ 2023-11-02; Reflector poll ]

Set priority to 3 after reflector poll. "Should allow setting failbit if an implementation does detect inconsistent values." "Yes, that's what MSVC does, so the wording needs something like: it is unspecified whether setstate(ios_base::failbit) is called on the basic_istream or whether an unspecified value is assigned to the p argument."

A call to chrono::parse can encounter redundant or contradictory data, e.g.

stringstream("2023 2022") >> chrono::parse("%Y %C%y", yr);

stringstream("2023 2022") >> chrono::parse("%Y %Y", yr);