@nogc inconsistent for array comparison depending on mutability of elements

[rcorre via Digitalmars-d-learn]

@nogc unittest {
    int[2] a = [1, 2];
    assert(a == [1, 2]); // OK
    immutable(int)[2] b = [1, 2];
    assert(b == [1, 2]); // fail: array literal may cause 
allocation
}

Is there any logic behind allowing the comparison with `a` but 
not `b`, or is this a compiler bug?