错误：重载运算符'[]'的使用不明确（操作数类型为'ObjA_'和'char [5]'）

Question

但是，另一个模棱两可的重载运算符！尽管存在许多此类问题，但我遇到了clang++-6.0给出的另一个问题，使它们无法解决。一个类型转换运算符定义使[]重载以给出错误。对我来说没有任何意义。

#include <string>
#include <iostream>

using namespace std;
struct Obj_ {
   Obj_& operator [] (const char*) {
      cout << "const char* subscript." << endl;
      return *this;
   }
   Obj_& operator [] (const string&) {
      cout << "const string& subscript." << endl;
      return *this;
   }
   operator int () {
      cout << "int cast." << endl;
      return 0;
   }
};

int main () {
   Obj_ Obj;
   char Str1[5] = "test";
   string Str2 =  "test";

   Obj[Str1];
   Obj[Str2][Str1];

   return 0;
}

编译后吐出

/tests/test_ambiguity.cpp:26:7: error: 
      use of overloaded operator '[]' is ambiguous (with operand types 'Obj_'
      and 'char [5]')
   Obj[Str1];
   ~~~^~~~~
/tests/test_ambiguity.cpp:7:11: note: 
      candidate function
   Obj_& operator [] (const char*) {
          ^
/tests/test_ambiguity.cpp:11:11: note: 
      candidate function
   Obj_& operator [] (const string&) {
          ^
/tests/test_ambiguity.cpp:26:7: note: 
      built-in candidate operator[](int, char *)
   Obj[Str1];
      ^
/tests/test_ambiguity.cpp:26:7: note: 
      built-in candidate operator[](int, const char *)
/tests/test_ambiguity.cpp:26:7: note: 
      built-in candidate operator[](int, volatile char *)
/tests/test_ambiguity.cpp:26:7: note: 
      built-in candidate operator[](int, const volatile char *)
/tests/test_ambiguity.cpp:27:13: error: 
      use of overloaded operator '[]' is ambiguous (with operand types 'Obj_'
      and 'char [5]')
   Obj[Str2][Str1];
   ~~~~~~~~~^~~~~
/tests/test_ambiguity.cpp:7:11: note: 
      candidate function
   Obj_& operator [] (const char*) {
          ^
/tests/test_ambiguity.cpp:11:11: note: 
      candidate function
   Obj_& operator [] (const string&) {
          ^
/tests/test_ambiguity.cpp:27:13: note: 
      built-in candidate operator[](int, char *)
   Obj[Str2][Str1];
            ^
/tests/test_ambiguity.cpp:27:13: note: 
      built-in candidate operator[](int, const char *)
/tests/test_ambiguity.cpp:27:13: note: 
      built-in candidate operator[](int, volatile char *)
/tests/test_ambiguity.cpp:27:13: note: 
      built-in candidate operator[](int, const volatile char *)
2 errors generated.
CMakeFiles/test_ambiguity.dir/build.make:62: recipe for target 'CMakeFiles/test_ambiguity.dir/tests/test_ambiguity.cpp.o' failed
make[2]: *** [CMakeFiles/test_ambiguity.dir/tests/test_ambiguity.cpp.o] Error 1
CMakeFiles/Makefile2:67: recipe for target 'CMakeFiles/test_ambiguity.dir/all' failed
make[1]: *** [CMakeFiles/test_ambiguity.dir/all] Error 2
Makefile:140: recipe for target 'all' failed
make: *** [all] Error 2

gcc编译得很好，而clang编译得不好。即使没有使用类型转换运算符，也没有错误！