[Bug c++/12672] Evals template defaults args that it should not

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=12672

--- Comment #12 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Template argument deduction requires substituting the template arguments where
they appear. If that is in the return type, it gets substituted into the return
type. If that causes a substitution error then deduction fails (that is how
using enable_if on the return type works, it *must* consider the return type
during argument deduction for that  to work!). If it causes an error outside
the immediate context you get an error, not deduction failure (14.8.2 p8).