immutable postblit unusable?

[Cauterite]

https://dpaste.dzfl.pl/80d2a208519c

struct A {
	this(this) immutable {}
}

struct B {
	A a;
}

// Error: immutable method f18.A.__postblit is not callable using 
a mutable object

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I honestly don't know what to make of this.
I guess the auto-generated postblit of B is throwing this?

Does it even make sense to have an immutable postblit?
Should such code be explicitly rejected?