"this" member for structs with methods

[simendsjo]

When a struct contains methods, __traits(allMembers reports a 
member called "this". What is "this"?

void main() {
    struct S { int i; }
    struct A { int i; void f() {} }
    pragma(msg, __traits(allMembers, S)); // i
    pragma(msg, __traits(allMembers, A)); // i, f, this
}