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I'm glad that expression SFINAE is at least coming in an update. I like all the features of modern C++ that let you do dynamic-ish things at compile time. The syntax isn't usually pretty, but at least the functionality is there.

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Yeah, being able to determine the validity of arbitrary expressions at compile time is extremely useful for libraries. For example, it allows libraries to sense the presence of member functions in user-defined types, calling them when present and falling back to default behaviors when missing.

Like Objective C dynamic dispatch, without the runtime cost?

Yes. In fact, the usual mechanism for doing so is referred to as tag dispatch. For example, an algorithm can take an arbitrary Iterator type, and select between a correct-but-slow algorithm for weak iterators like forward iterators, and a fast algorithm for strong iterators like random-access iterators. The easiest way to achieve this is to call a helper function and pass a tag object whose type is iterator_traits<Iterator>::iterator_category, which might be forward_iterator_tag or random_access_iterator_tag. The helper function is then overloaded on those tags, achieving the effect of a compile-time if. Everything optimizes away (the compiler is very good at noticing do-nothing tag objects).

Expression SFINAE offers the ability to detect more properties of things, like is_callable or has_member_function_meow. This can then be used in tag dispatch, or other mechanisms (SFINAE can be used directly to dispatch, but it's a more lengthy explanation).

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