On Wed, Mar 28, 2012 at 6:09 PM, Jeremiah Willcock <jewillco@osl.iu.edu>wrote:
On Wed, 28 Mar 2012, Robert Jones wrote:
On Wed, Mar 28, 2012 at 4:24 PM, Jeremiah Willcock <jewillco@osl.iu.edu>
wrote:
It looks like the trick is to put the enable_if outside the BOOST_CONCEPT_REQUIRES, as in:
template <typename T> typename enable_if< condition, BOOST_CONCEPT_REQUIRES(**concept, (void))>::type
Paragraph 4 of [dcl.fct] (in the latest draft) seems to say that the SFINAE error you are getting is required: only the specific type void, written in a non-dependent way, counts as a valid function parameter type.
Well, I can't deny that it works! Seems to be reading quite a bit into the standard tho', assuming you're looking at
If I'm reading it right it is actually talking about function parameter lists, rather than function return types, and stating that void is not a valid parameter type except for f(void) to indicate an empty parameter list.
The internals of BOOST_CONCEPT_REQUIRES take the type that you give as the return type and use it as a function parameter type (that is why you need the parentheses around the outside). That is where those rules in the standard get involved.
Ah, thank you, clears it up nicely. I can sleep peacefully! - Rob.