Re: [Boost-users] puzzle with enable_if

Subject: Re: [Boost-users] puzzle with enable_if
From: John Dlugosz (JDlugosz_at_[hidden])
Date: 2010-04-20 15:52:41

> -----Original Message-----
> From: boost-users-bounces_at_[hidden] [mailto:boost-users-
> bounces_at_[hidden]] On Behalf Of Gevorg Voskanyan
> Sent: Tuesday, April 20, 2010 1:23 PM
> To: boost-users_at_[hidden]
> Subject: Re: [Boost-users] puzzle with enable_if
> 
 
> "Only invalid types and expressions in the immediate context of the
> function type and its template parameter types can result in a
> deduction failure. [ Note: The evaluation of the substituted types and


OK, so "refactoring" in general won't work.  The SFINAE must be directly caused by the template specialization mentioned in the function declaration.

Your case f (struct A contains T::value_type) is a failure of A to specialize.  My example where the function returned something derived from A, say struct A2 : A { ..., is not a failure of A2 but of a type needed by it.  T::value_type is still a dependant type, I would have thought, but I see that the error is the use of T:: that is a part of A's syntax.  The difference is a nested call to have a template instantiated.

Is that the right track?

Thanks,
--John




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