Smooth node behaves as cusp

[LucaDC]

See the attached drawing with explanations inside.
Basically, I created a shape as a union of two circles: its nodes are expected to be all smooth but the two internal. Actually the topmost "smooth" node snaps as a cusp or even as a line midpoint (is it a zero length segment?) but has its handles and using the node tool I can't see any difference with a normal smooth node. It's even possible to convert it to a cusp and then back to smooth with its peculiarity remaining.

[su_v (suv-lp)]

> (is it a zero length segment?)

AFAICT yes - the new path after the union does start/end at this smooth node: select the path, switch to the node tool and use <TAB> to select the first (start) node (of the first sub-path) of the path (if a curved path is closed, there has to be a zero-length closing 'z' segment which connects start and end node).

[su_v (suv-lp)]

Reproduced with stable Inkscape 0.48.2 and 0.48+devel r10582 on Mac OS X 10.5.8 (i386)
(AFAICT not related to the latest refactoring of the snapping code)

[LucaDC (lucadc)]

I agree that it's not related to the latest snapping code refactoring, but it breaks the ability of selecting the types of nodes to snap.
Given that the node tool correctly handles this particular kind of zero-length-path-closing-segment nodes while the snapping tooltip clearly indicates that the snapping code doesn't, I think that to solve this problem a special case should be added in the snapping code.