Function-style cast vs. constructor

I learned that in C++,

typedef foo* mytype;

(mytype) a        // C-style cast

and

mytype(a)         // function-style cast

do the same thing.

But I notice the function-style cast share the same syntax as a constructor. Aren't there ambiguous cases, where we don't know if it is a cast or a constructor?

char s [] = "Hello";
std::string s2 = std::string(s);     // here it's a constructor but why wouldn't it be ...
std::string s3 = (std::string) s;    // ... interpreted as a function-style cast?