fix o{(p!)}q.r = s

An unlikely edge case, but nevertheless one for which there was a bug.
Found while investigating unrelated issues.

The fix is to consider only the last tail of a chain to be in assignment
position if the entire chain is assigned to.

Author: rhendric

lib/ast.js

@@ -932,8 +932,9 @@ class exports.Chain extends Node
     rewrite-shorthand: (o, assign) ->
         return that.rewrite-shorthand o, assign or that if @unfold-soak o
         @head = that if @head.rewrite-shorthand o
+        last-i = @tails.length - 1
         for item, i in @tails
-            @tails[i] = that if item.rewrite-shorthand o, assign
+            @tails[i] = that if item.rewrite-shorthand(o, assign && i is last-i)
         @expand-slice o, assign
         @unwrap!
 

src/ast.ls

@@ -291,3 +291,9 @@ eq true delete! a?{x}[\x]y
 eq void b.x.y
 a = [4 3]
 eq '4,3,2' "#{a?[0 1] <<< {2}}"
+
+# Assigning through computed properties in slices
+p = -> \q
+o = q: r: 1
+o{(p!)}q.r = 1
+eq 1 o.q.r