Error when using label difference as immediate for MOV on aarch64

[N00byEdge]

To reproduce:
clang -c test.S -target aarch64
test.S:

.section .text
_start:
    MOV X0, L2 - L1 // test.S:3:13: error: expected compatible register or logical immediate
    MOV X1, #0 // Works just fine

L1:
L2:

The amount of data between the labels doesn't seem to matter. Just inserting the calculated values in here works fine.

The code compiles with gas and the difference between the labels is used as the immediate value, the expected behavior.

Is there a different syntax for using this as an immediate value, or is it just not implemented yet? I've tried sticking a breakpoint on ::encodeLogicalImmediate() but it looks like it's not being hit, so I suspect that it would probably be the latter.

[efriedma-quic]

Compiles successfully on main.