I cannot understand why you argue the new behavior is a 'mesfeature'.

Const/immutable variable type and specifying variable initializer should not interfere with each other.

Under the old behavior, following code never work.

struct S {

    immutable int x = 1;

    this(int x) { this.x = x; }

}

void main() {

    S s1;    // default construction

    assert(s1.x == 1);

    S s2 = S(2);    // 

    assert(s2.x == 2);

}

I think this is necessary to make non-mutable object field more usable.

Kenji Hara