Re: [boost] "Simple C++11 metaprogramming"

On 6/5/2015 5:42 AM, Peter Dimov wrote:

Eric Niebler wrote:

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(This implements the specification in the latest C++17 draft.)

I'm not so sure. If a user defines a specialization for common_type, it won't get used when computing common_type.

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 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 Boost.org http://www.boost.org