Section: 23.15.1 [meta.rqmts] Status: New Submitter: Tim Song Opened: 2016-12-14 Last modified: 2017-02-02
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[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".