The following code fails with an InexactError():

X = [1,2,3]
Y = [4 5]
broadcast(atan2, X, Y)

whereas

[atan2(x,y) for x in X, y in Y ]

(albeit producing an array of type Any), while

atan2([1,2,3],[4,5,6])

produces an array of Float64.

Can we improve the type inference so that all three cases can generate Float64 arrays? Note that this is needed for #4363 (for @vectorize_2arg to use broadcast).