No warnings for field initializer with implicit constructor

[cston]

A warning is reported for A.x but no warning is reported for B.y.

class A
{
    object x = F(null); // warning
    A() { }
    static object F(object o) => o;
}
class B
{
    object y = F(null); // no warning
    static object F(object o) => o;
}