Double evaluation in SET due to lowering

[shawwn]

I ran across an interesting one. It's not immediately clear how to solve it:

> (set x (set (get (or (print 'foo) (obj)) 'a) 42))
foo
foo

The problem is that "foo" should only be printed once, but it's being printed twice here.

You can get a clearer idea of the issue by looking at the compiled output:

> (print (compile (expand '(set x (set (get (or (print 'foo) (obj)) 'a) 42)))))
(print("foo") or {}).a = 42
x = (print("foo") or {}).a

I made a couple attempts at solving this by modifying lower-set, but I haven't been able to come up with a solution at all, let alone a solution that's simple.

I think it's necessary to check whether the left-hand side expression starts with (get ...), since I'm not sure there's another situation where this can arise. But it feels odd to special case it like that.

It's possible this might not be a bug at all -- emacs mentions in their gv macro documentation that they can be expanded multiple times. But I'm not sure.