c++: Add fixed test [PR81952]

This was fixed by r258755:
PR c++/81311 - wrong C++17 overload resolution.

	PR c++/81952

gcc/testsuite/ChangeLog:

	* g++.dg/overload/conv-op4.C: New test.

gcc/testsuite/g++.dg/overload/conv-op4.C

@@ -0,0 +1,22 @@
+// PR c++/81952
+// { dg-do run { target c++11 } }
+
+template <class T>
+struct opt {
+    opt() { }
+    opt(opt const& ) { __builtin_abort (); }
+    opt(opt&& ) { __builtin_abort (); }
+
+    template <class U>
+    opt(U&& ) { }
+};
+
+struct foo 
+{
+    explicit operator opt<int>() { __builtin_abort (); return {}; }
+};
+
+int main()
+{
+    opt<int> o(foo{});
+}