Why implicit conversion of bool to string isn't an error?

Question

I goggled it & tried to find similar question on SO also but didn\'t find anything useful. So, posting my question here.

Consider this program:

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Answer 1

What happens is that an std::string is implicitly constructed from false, using the const CharT* overload and converting false to a null pointer. According to the documentation for that constructor :

The behavior is undefined if s [the pointer] does not point at an array of at least Traits::length(s)+1 elements of CharT.

Hence the malfunctioning (in the form of a friendly exception, but don't rely on it).

Now, is it correct ? According to [conv.ptr] :

A null pointer constant is an integer literal (2.13.2) with value zero or a prvalue of type std::nullptr_t.

false has indeed a value of zero, but is not an integer literal (it's a boolean literal). The implicit conversion to the CharT* that std::string's constructor takes is thus non-standard.

And indeed, while GCC emits a warning, Clang refuses to compile it.