template<class Array, class Less> 
void Sort(Array& arr, const Less& less)

bool Cmp(const SortType& a, const SortType& b)
{
    return a.Key < b.Key;
}
. . .
Sort(array, Cmp);

#include <iostream>
#include <vector>
using namespace std;

template<class Array, class Less> 
void Sort(Array& arr, const Less& less) {}
 
struct SortType {
   int Key;
};

bool Cmp(const SortType& a, const SortType& b)
{
    return a.Key < b.Key;
}

int main()
    {
 vector<int> array;
 Sort(array, Cmp);

    }

If P is a reference type, the type referred to by P is used for type deduction.

If P is a cv-qualified type, the top level cv-qualifiers of P’s type are ignored for type deduction.

MS>template<class Array, class Less> 
MS>void Sort(Array& arr, const Less& less)
MS>

MS>bool Cmp(const SortType& a, const SortType& b)
MS>{
MS>    return a.Key < b.Key;
MS>}
MS>. . .
MS>Sort(array, Cmp);
MS>

 Sort(array, &Cmp);

A cv-qualifier-seq shall only be part of the function type for a nonstatic member function, the function type to which a pointer to member refers, or the top-level function type of a function typedef declaration. The effect of a cv-qualifier-seq in a function declarator is not the same as adding cv-qualification on top of the function type, i.e., it does not create a cv-qualified function type. In fact, if at any time in the determination of a type a cv-qualified function type is formed, the program is ill-formed.

Type deduction may fail for the following reasons:
[...]
— Attempting to create a cv-qualified function type

A function template can be overloaded either by (non-template) functions of its name or by (other) function templates of the same name. When a call to that name is written (explicitly, or implicitly using the operator notation), template argument deduction (14.8.2) and checking of any explicit template arguments (14.3) are performed for each function template to find the template argument values (if any) that can be used with that function template to instantiate a function template specialization that can be invoked with the call arguments. For each function template, if the argument deduction and checking succeeds, the template-arguments (deduced and/or explicit) are used to instantiate a single function template specialization which is added to the candidate functions set to be used in overload resolution. If, for a given function template, argument deduction fails, no such function is added to the set of candidate functions for that template. The complete set of candidate functions includes all the function templates instantiated in this way and all of the non-template overloaded functions of the same name. The function template specializations are treated like any other functions in the remainder of overload resolution, except as explicitly noted in 13.3.3.

A cv-qualifier-seq shall only be part of the function type for a non-static member function, the function type to which a pointer to member refers, or the top-level function type of a function typedef declaration. [ Note: a function type that has a cv-qualifier-seq is not a cv-qualified type; there are no cv-qualified function types. —end note ] The effect of a cv-qualifier-seq in a function declarator is not the same as adding cv-qualification on top of the function type. In the latter case, the cv-qualifiers are ignored.