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3154. Common and CommonReference have a common defect

Section: 18.4.6 [concept.common] Status: C++20 Submitter: Casey Carter Opened: 2018-08-10 Last modified: 2021-02-25

Priority: 0

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Discussion:

The semantic requirements of both Common (18.4.6 [concept.common]):

-2- Let C be common_­type_­t<T, U>. Let t be a function whose return type is T, and let u be a function whose return type is U. Common<T, U> is satisfied only if:

(2.1) — C(t()) equals C(t()) if and only if t() is an equality-preserving expression (18.2 [concepts.equality]).

(2.2) — C(u()) equals C(u()) if and only if u() is an equality-preserving expression (18.2 [concepts.equality]).

and similarly CommonReference ( [concept.commonreference]):

-2- Let C be common_­reference_­t<T, U>. Let t be a function whose return type is T, and let u be a function whose return type is U. CommonReference<T, U> is satisfied only if:

(2.1) — C(t()) equals C(t()) if and only if t() is an equality-preserving expression (18.2 [concepts.equality]).

(2.2) — C(u()) equals C(u()) if and only if u() is an equality-preserving expression.

don't properly reflect the intended design that conversions to the common type / common reference type are identity-preserving: in other words, that converting two values to the common type produces equal results if and only if the values were initially equal. The phrasing "C(E) equals C(E) if and only if E is an equality-preserving expression" is also clearly defective regardless of the intended design: the assertion "E is not equality-preserving" does not imply that every evaluation of E produces different results.

[2018-08 Batavia Monday issue prioritization]

Priority set to 0, status to 'Tentatively Ready'

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4762.

1. Modify 18.4.5 [concept.commonref] p2 as follows:

   -2- Let C be common_­reference_­t<T, U>. Let t be a function whose return type is t1 and t2 be equality-preserving expressions (18.2 [concepts.equality]) such that decltype((t1)) and decltype((t2)) are each T, and let u be a function whose return type is u1 and u2 be equality-preserving expressions such that decltype((u1)) and decltype((u2)) are each U. T and U model CommonReference<T, U> is satisfied only if:

   (2.1) — C(t1()) equals C(t2()) if and only if t1() equals t2, and is an equality-preserving expression (18.2 [concepts.equality]).

   (2.2) — C(u1()) equals C(u2()) if and only if u1() equals u2 is an equality-preserving expression.

2. Modify 18.4.6 [concept.common] p2 similarly:

   -2- Let C be common_­type_­t<T, U>. Let t be a function whose return type is t1 and t2 be equality-preserving expressions (18.2 [concepts.equality]) such that decltype((t1)) and decltype((t2)) are each T, and let u be a function whose return type is u1 and u2 be equality-preserving expressions such that decltype((u1)) and decltype((u2)) are each U. T and U model Common<T, U> is satisfied only if:

   (2.1) — C(t1()) equals C(t2()) if and only if t1() equals t2, and is an equality-preserving expression (18.2 [concepts.equality]).

   (2.2) — C(u1()) equals C(u2()) if and only if u1() equals u2 is an equality-preserving expression (18.2 [concepts.equality]).