[Bug c++/71879] Error in unevaluated context breaks SFINAE

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71879

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Taking the address of Foo::foo<Caller> instantiates it, which results in an
error outside the immediate context, so you get an error not a substitution
failure.

http://stackoverflow.com/a/15261234/981959