tmp/tmpky34y9c1/{from.md → to.md}
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#### Partial ordering of class template specializations <a id="temp.class.order">[[temp.class.order]]</a>
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For two class template partial specializations, the first is *more
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specialized* than the second if, given the following rewrite to two
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function templates, the first function template is more specialized than
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the second according to the ordering rules for function templates
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[[temp.func.order]]:
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- Each of the two function templates has the same template parameters
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and associated constraints [[temp.constr.decl]] as the corresponding
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partial specialization.
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- Each function template has a single function parameter whose type is a
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class template specialization where the template arguments are the
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corresponding template parameters from the function template for each
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template argument in the *template-argument-list* of the
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*simple-template-id* of the partial specialization.
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[*Example 1*:
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``` cpp
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template<int I, int J, class T> class X { };
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template<int I, int J> class X<I, J, int> { }; // #1
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template<int I> class X<I, I, int> { }; // #2
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template<int I0, int J0> void f(X<I0, J0, int>); // A
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template<int I0> void f(X<I0, I0, int>); // B
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template <auto v> class Y { };
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template <auto* p> class Y<p> { }; // #3
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template <auto** pp> class Y<pp> { }; // #4
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template <auto* p0> void g(Y<p0>); // C
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template <auto** pp0> void g(Y<pp0>); // D
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```
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According to the ordering rules for function templates, the function
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template *B* is more specialized than the function template *A* and the
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function template *D* is more specialized than the function template
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*C*. Therefore, the partial specialization \#2 is more specialized than
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the partial specialization \#1 and the partial specialization \#4 is
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more specialized than the partial specialization \#3.
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— *end example*]
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[*Example 2*:
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``` cpp
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template<typename T> concept C = requires (T t) { t.f(); };
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template<typename T> concept D = C<T> && requires (T t) { t.f(); };
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template<typename T> class S { };
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template<C T> class S<T> { }; // #1
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template<D T> class S<T> { }; // #2
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template<C T> void f(S<T>); // A
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template<D T> void f(S<T>); // B
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```
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The partial specialization \#2 is more specialized than \#1 because `B`
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is more specialized than `A`.
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— *end example*]
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