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3140. COMMON_REF is unimplementable as specified

Section: 21.3.8.7 [meta.trans.other] Status: C++20 Submitter: Casey Carter Opened: 2018-07-07 Last modified: 2021-02-25

Priority: 0

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Discussion:

21.3.8.7 [meta.trans.other]/3.2 states:

[Let] XREF(A) denote a unary class template T such that T<U> denotes the same type as U with the addition of A’s cv and reference qualifiers, for a non-reference cv-unqualified type U,
which is nonsensical: a specialization of a class template cannot possibly be a cv-qualified type or reference type. XREF(A) must be a unary alias template.

[2018-09-11; Status set to Tentatively Ready after five positive votes on the reflector]

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4762.

Change 21.3.8.7 [meta.trans.other] as indicated:

-3- Let:

(3.1) — CREF(A) be add_lvalue_reference_t<const remove_reference_t<A>>,

(3.2) — XREF(A) denote a unary classalias template T such that T<U> denotes the same type as U with the addition of A’s cv and reference qualifiers, for a non-reference cv-unqualified type U,

(3.3) — COPYCV(FROM, TO) be an alias for type TO with the addition of FROM’s top-level cv-qualifiers. [Example: COPYCV(const int, volatile short) is an alias for const volatile short. —end example]