In cppreference.com, there is an example:

template<class T>
void f(T, T*);   // #1
template<class T>
void f(T, int*); // #2
 
void m(int* p)
{
    f(0, p); // deduction for #1: void f(T, T*) [T = int]
             // deduction for #2: void f(T, int*) [T = int]
 
    // partial ordering:
 
    // #1 from #2: void(T,T*) from void(U1,int*): P1=T, A1=U1: T=U1
    //                                            P2=T*, A2=int*: T=int: fails
 
    // #2 from #1: void(T,int*) from void(U1,U2*): P1=T A1=U1: T=U1
    //                                             P2=int* A2=U2*: fails
 
    // neither is more specialized w.r.t T, the call is ambiguous
}

I think the deduction procedure "#1 from #2" is success because "P2=T*, A2=int*: T=int". But it fails, I don't know the reason.

CodePudding user response：

For each type, non-type, and template parameter, including parameter packs, a unique fictitious type, value, or template is generated and substituted into function type of the template

U1 here is not a template type; it's a unique ficitious type and it's different from int, so the deduction fails.