c++ - Template function argument deduction with an implicit conversion -


i understand template function argument deduction not take implicit conversions account.

so code doesn't compile:

#include <iostream>  template<class t> struct {}; struct b : public a<int> {}; struct c {   operator b() { return {}; } };  template<class x> void test(a<x> arg1, a<x> arg2) {   std::cout << "ok1"; }  int main() {   b b;   c c;   test(b, c);  // error: no matching function call 'test' }

what don't understand how adding level of indirection identity typedef somehow makes work:

#include <iostream>  template<class t> struct {}; struct b : public a<int> {}; struct c {   operator b() { return {}; } };  template<typename u> struct magic { typedef u type; };  template<class t> using magic_t = typename magic<t>::type;  template<class x> void test(a<x> arg1, a<x> arg2) {   std::cout << "ok1"; }  template<class x> void test(a<x> arg3, magic_t<a<x>> arg4) {   std::cout << "ok2"; }  int main() {   b b;   c c;   test(b, c);  // prints "ok2" }

live demo on godbolt

how magic_t<a<x>> end matching c?

the second parameter becomes non-deduced context , doesn't participate in template argument deduction. x deduced first argument.


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