Proposed resolution (approved by CWG 2023-11-08):

Change in 13.10.3.1 [temp.deduct.general] paragraph 7 as follows:

The deduction substitution loci are

• the function type outside of the noexcept-specifier,
• the explicit-specifier, and
• the template parameter declarations., and
• the template argument list of a partial specialization (13.7.6.1 [temp.spec.partial.general]).

The substitution occurs in all types and expressions that are used in the deduction substitution loci. ...

Additional notes (January, 2023)

The section on SFINAE (13.10.3.1 [temp.deduct.general] paragraph 8) is not specific to function templates, and 13.7.6.2 [temp.spec.partial.match] paragraph 2 hands off the "matching" determination for partial specializations to 13.10.3 [temp.deduct] in general. However, the definition of deduction substitution loci in 13.10.3.1 [temp.deduct.general] paragraph 7 does not account for the template argument list of a partial specialization.

[Accepted as a DR at the November, 2023 meeting.]

Presumably something like the following should be well-formed, where a deduction failure in a partial specialization is handled as a SFINAE case as it is with function templates and not a hard error:

  template <class T, class U> struct X   {
    typedef char member;
  };

  template<class T> struct X<T,
   typename enable_if<(sizeof(T)>sizeof(
     float)), float>::type>
  {
    typedef long long member;
  };

  int main() {
    cout << sizeof(X<double, float>::member);
  }

However, this does not appear to be described anywhere in the Standard.