[cfe-dev] Why doesn't this compile? clang implicit conversions & overload resolution

[Ryan Ericson]

Code:

struct Base
{
    operator bool() const { return false; }
};

struct foo
{
    foo(const char* c) {}
};

struct Something : public Base
{
    void operator[](const foo& f) {}
};

int main()
{
    Something d;
    d["32"]
    return 0;
}

clang complains:
test4.cpp:22:6: error: use of overloaded operator '[]' is ambiguous
(with operand types 'Something' and 'const char [3]')
    d["32"]
    ~^~~~~
test4.cpp:16:10: note: candidate function
    void operator[](const foo& f) {}
         ^
test4.cpp:22:6: note: built-in candidate operator[](long, const char *)
    d["32"]
     ^
test4.cpp:22:6: note: built-in candidate operator[](long, const restrict char *)
test4.cpp:22:6: note: built-in candidate operator[](long, const volatile char *)
test4.cpp:22:6: note: built-in candidate operator[](long, const
volatile restrict char *)

It looks like it is considering between:
1. d["32"] -> which will call Something::operator[] after 1 user
defined conversion through constructor
2. "32"[d] -> which will call built in operator[] of const char* after
1 user defined conversion through operator double AND 1 standard
conversion (promotion?) from bool to long

My question is, why doesn't the compiler pick 1 when it seems to be a
better conversion sequence (cheaper according to the standard?).

I'm absolutely not an expert in overload resolution nor implicit
conversions, and may have said some incorrect things. Would appreciate
to get some explanation for this =).

Thanks,
Ryan