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### 2582. §[res.on.functions]/2's prohibition against incomplete types shouldn't apply to type traits

Section: 21 [meta] Status: C++17 Submitter: Tim Song Opened: 2016-01-07 Last modified: 2017-07-30

Priority: 0

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Discussion:

16.4.5.8 [res.on.functions]/2.5 says that the behavior is undefined "if an incomplete type is used as a template argument when instantiating a template component, unless specifically allowed for that component."

This rule should not apply to type traits — a literal application would make `is_same<void, void>` undefined behavior, since nothing in 21 [meta] (or elsewhere) "specifically allows" instantiating `is_same` with incomplete types.

Traits that require complete types are already explicitly specified as such, so the proposed wording below simply negates 16.4.5.8 [res.on.functions]/2.5 for 21 [meta].

[2016-02, Issues Telecon]

P0; move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4567.

1. Insert a new paragraph after 21.3.3 [meta.type.synop]/1:

   > -?- Unless otherwise specified, an incomplete type may be used to instantiate a template in this subclause.