3352. strong_equality isn't a thing

Section: 23.2.2 [container.requirements.general] Status: C++20 Submitter: Casey Carter Opened: 2019-12-06 Last modified: 2021-02-25

Priority: 1

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Discussion:

[tab:container.req] includes the row:

Expression: i <=> j

Return Type: strong_ordering if X::iterator meets the random access iterator requirements, otherwise strong_equality.

Complexity: constant

"strong_equality" is (now) a meaningless term that appears nowhere else in the working draft. Presumably we want to make the Return Type unconditionally strong_ordering, and require this expression to be valid only when i and j are random access iterators. It's not clear to me if the best way to do so would be to add a "Constraints" to the "Assertion/note/pre-/post-condition" column of this table row, or if we should pull the row out into yet another "conditionally-supported iterator requirements" table.

[2019-12-21 Issue Prioritization]

Priority to 1 after reflector discussion.

[2020-02-10, Prague; David Olsen provides new wording based upon Tim Songs suggestion]

[2020-02 Moved to Immediate on Tuesday in Prague.]

Proposed resolution:

Table 71 — Container requirements [tab:container.req]

Expression	Return type	Operational semantics	Assertion/note pre/post-condition	Complexity
[…]
i <=> j	strong_ordering if X::iterator meets the random access iterator requirements, otherwise strong_equality		Constraints: X::iterator meets the random access iterator requirements.	constant
[…]