Explicit constructors and nested initializer lists

The following code successfully compiles with most modern C++11 compatible compilers (GCC >= 5.x, Clang, ICC, MSVC).

#include <string>

struct A
{
        explicit A(const char *) {}
        A(std::string) {}
};

struct B
{
        B(A) {}
        B(B &) = delete;
};

int main( void )
{
        B b1(});
}

But why does it compile in the first place, and how are the listed compilers interpreting that code?

Why is MSVC able to compile this without B(B &) = delete;, but the other 3 compilers all need it?

And why does it fail in all compilers except MSVC when I delete a different signature of the copy constructor, e.g. B(const B &) = delete;?

Are the compilers even all choosing the same constructors?

Why does Clang emit the following warning?

17 : <source>:17:16: warning: braces around scalar initializer [-Wbraced-scalar-init]
        B b1(});