[Adopted at the February, 2016 meeting.]
The rationale for the restriction in 13.7.6.1 [temp.spec.partial.general] bullet 9.1 is not clear:
A partially specialized non-type argument expression shall not involve a template parameter of the partial specialization except when the argument expression is a simple identifier. [Example:
template <int I, int J> struct A {};
template <int I> struct A<I+5, I*2> {}; // error
template <int I, int J> struct B {};
template <int I> struct B<I, I> {}; // OK
—end example]
In the example, it's clear that I is non-deducible, but this rule prevents plausible uses like:
template <int I, int J> struct A {};
template <int I> struct A<I, I*2> {};