Re: [Boost-users] puzzle with enable_if

Subject: Re: [Boost-users] puzzle with enable_if
From: John Dlugosz (JDlugosz_at_[hidden])
Date: 2010-04-19 18:13:45

> The difference is whether the template parameter substitution causes an
> invalid template argument or an invalid resulting function type. Let's
> consider the three cases one by one.
> 

Hmm, I still think that the template expansion of cases 1 and 2 would "choke" in the same way.  But the difference, I think, must be one of indirection:  Case 1, the template instantiation of enable_if fails while directly seeing if it forms a usable return type.  In Case 2, it fails as a nested invocation, a type needed by the (possible) instantiation of the return type of the function.

As for the difference between 2 and 3, "once the class is known, do individual members including nested types make sense"? seems to be more arbitrary, like the compiler takes some things but not others.  I don't see how that difference is explained in the standard.

Thanks for your detailed analysis.

--John




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