Unexpected overloading error

[pschachte]

This code

?lst = [2]

if { [?hd | ?tl] = lst :: !println "nonempty"
   | else              :: !println "empty"
}

fails to compile with this error:

Error detected during type checking of module(s) tst
tst.wybe:1:10: Ambiguous overloading: call could refer to:
    wybe.string.[]<1> (partial)
    wybe.string.[]<0> (partial)
    wybe.range.[]<0> (partial)
    wybe.list.[]<1>
    wybe.list.[]<0> (partial)
    wybe.c_string.[]<0> (partial)
    wybe.array.[]<1> (partial)
    wybe.array.[]<0> (partial)

Oddly, leaving out the if statement, or even just removing the !println calls, fixes it, so there's something in the println statements that causes a bogus overloading error in the first line.