Thanks for digging this out!
So if I summarize what this document says, the feature has been considered a while ago, and left out because the version of concepts that would itself be later abandoned should have solved the problem.
This suggests that there is nothing wrong in allowing function template partial specialization, but we don’t know if it will be in the language one day.
Thanks for sharing this, it’s helpful.

You’re absolutely right, I fixed this. This lets the function be used in static asserts for example. Thanks for pointing this out!

> Does anyone know the reason for restricting on function template specialization?

I think that this document shows a reason.
n2173 #229: partial specialization of function templates: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2173.html