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Subject: Re: [boost] "Simple C++11 metaprogramming"
From: Eric Niebler (eniebler_at_[hidden])
Date: 2015-06-05 11:20:34

On 6/5/2015 5:42 AM, Peter Dimov wrote:
> Eric Niebler wrote:
>>> (This implements the specification in the latest C++17 draft.)
>> I'm not so sure. If a user defines a specialization for
>> common_type<MyInt, int>, it won't get used when computing
>> common_type<MyInt, int, int>.
> As it turns out, users are not allowed to specialize common_type,
> which is why the C++17 specification does not concern itself with
> such specializations.

It's in Table 57 - Other transformations, where common_type is described:

> template <class... T> struct common_type;
> The member typedef type shall be defined as set out below. All
> types in the parameter pack T shall be complete or (possibly cv)
> void. A program may specialize this trait if at least one template
> parameter in the specialization is a user-defined type. [ Note:
> Such specializations are needed when only explicit conversions are
> desired among the template arguments. --end note ]

> As a curiosity, decaying the arguments before the conditional
> operator is also subtly wrong.

Hrm, interesting. I think there's a library issue about that.

Eric Niebler

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