[Bug c++/59759] internal compiler error: in unify, using std::enable_if on classes

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59759

--- Comment #16 from Martin Sebor <msebor at gcc dot gnu.org> ---
Kai's patch with the small from comment #15 test case posted for review:
https://gcc.gnu.org/ml/gcc-patches/2016-01/msg01446.html