[temp.spec.partial.match]

(1.1)
If exactly one matching partial specialization is found, the instantiation is generated from that partial specialization.
(1.2)
If more than one matching partial specialization is found, the partial order rules ([temp.spec.partial.order]) are used to determine whether one of the partial specializations is more specialized than the others.

If such a partial specialization exists, the instantiation is generated from that partial specialization; otherwise, the use of the template is ambiguous and the program is ill-formed.
(1.3)
If no matches are found, the instantiation is generated from the primary template.

A partial specialization matches a given actual template argument list if the template arguments of the partial specialization can be deduced from the actual template argument list, and the deduced template arguments satisfy the associated constraints of the partial specialization, if any.

[Example 1: template<class T1, class T2, int I> class A { }; / #1 template<class T, int I> class A<T, T*, I> { }; / #2 template<class T1, class T2, int I> class A<T1*, T2, I> { }; / #3 template<class T> class A<int, T*, 5> { }; / #4 template<class T1, class T2, int I> class A<T1, T2*, I> { }; / #5 A<int, int, 1> a1; / uses #1 A<int, int*, 1> a2; / uses #2, T is int, I is 1 A<int, char*, 5> a3; / uses #4, T is char A<int, char*, 1> a4; / uses #5, T1 is int, T2 is char, I is 1 A<int*, int*, 2> a5; / ambiguous: matches #3 and #5 — end example]

[Example 2: template<typename T> concept C = requires (T t) { t.f(); }; template<typename T> struct S { }; / #1 template<C T> struct S<T> { }; / #2 struct Arg { void f(); }; S<int> s1; / uses #1; the constraints of #2 are not satisfied S<Arg> s2; / uses #2; both constraints are satisfied but #2 is more specialized — end example]

If the template arguments of a partial specialization cannot be deduced because of the structure of its template-parameter-list and the template-id, the program is ill-formed.

[Example 3: template <int I, int J> struct A {}; template <int I> struct A<I+5, I*2> {}; / error template <int I> struct 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]

In a name that refers to a specialization of a class or variable template (e.g., A<int, int, 1>), the argument list shall match the template parameter list of the primary template.

The template arguments of a partial specialization are deduced from the arguments of the primary template.