Proposed resolution (September, 2015):

Change 13.7.6.1 [temp.spec.partial.general] bullet 9.1 as follows:

• 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. Each template-parameter shall appear at least once in the template-id outside a non-deduced context. [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 A<I, I> {};     // OK

  template <int I, int J, int K> struct B {};
  template <int I> struct B<I, I*2, 2> {};  // OK

—end example]

[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> {};

(See also issues 1647, 2033, and 2127.)