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### 2845. `enable_if`, `result_of`, `common_type` and `aligned_storage` do not meet the definition of `TransformationTrait`

Section: 20.15.2 [meta.rqmts] Status: New Submitter: Tim Song Opened: 2016-12-14 Last modified: 2017-02-02

Priority: 3

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Discussion:

[meta.rqmts]/3 defines `TransformationTrait` as follows:

> A `TransformationTrait` modifies a property of a type. It shall be a class template that takes one template type argument and, optionally, additional arguments that help define the modification. It shall define a publicly accessible nested type named `type`, which shall be a synonym for the modified type.

`enable_if`, `result_of` and `common_type` do not necessarily "define a publicly accessible nested type named `type`". `aligned_storage` takes no template type argument (it only has two non-type parameters). Yet [meta.trans]/2 says that they are all `TransformationTraits`.

Incidentally, with the exception of `decay`, it's not clear that any of the traits in [meta.trans.other] could really be described as "modify[ing] a property of a type".

[2017-01-27 Telecon]

Priority 3

Proposed resolution: