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.
--- /dev/null
+// 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{});
+}