[REG2.111] Indexing a *cast* AA yields no lvalue anymore

[kinke]

This works with v2.110.0, but fails to compile with v2.111.0-beta.1:

shared struct Queue
{
    int[int] map;
}

void main()
{
    auto queue = Queue();
    (cast(int[int]) queue.map)[1] = 2;
    assert(queue.map[1] == 2);
}

bug.d(9): Error: cannot modify expression `(cast(int[int])queue.map)[1]` because it is not an lvalue

[kinke]

AFAICT, the problem is that in

dmd/compiler/src/dmd/expression.d

Lines 3629 to 3636 in 62a48a5

	override bool isLvalue()
	{
	//printf("e1.type = %s, to.type = %s\n", e1.type.toChars(), to.toChars());
	if (rvalue || !e1.isLvalue())
	return false;
	return (to.ty == Tsarray && (e1.type.ty == Tvector || e1.type.ty == Tsarray)) ||
	e1.type.mutableOf.unSharedOf().equals(to.mutableOf().unSharedOf());
	}

e1.type.mutableOf.unSharedOf().equals(to.mutableOf().unSharedOf()) fails for e1.type = shared(int[int]) and to.type = int[int] (e1.isLvalue() is true).

Edit: Ah, e1.type.mutableOf().unSharedOf() yields shared(int)[int].

[kinke]

#21029