on incomplete relation: "combine way" deletes all ways

[malenki]

How to reproduce:

• get a relation with at least three connected ways (no difference if these are connected inner or outer ways of a multipolygon or a route. For testing purposes a route would be the easier choice)
• download two of these ways belonging to the relation
• right click the route relation in the relation window and "Select members"
• apply the Tool —> "Combine Way"

Result: all selected ways disappear, their nodes remain (JOSM reports the ways as deleted)

expected result: A warning that not all selected ways can be combined since they are not downloaded

Sure, it is a stupid thing trying to combine not-downloaded ways but since this happened to me today by chance I'd assume I am not the only one to whom this error may happen.
I tested back to r6210 for this behaviour which showed there, too.

Maybe this issue is related to #8082?

[skyper]

Right, JOSM should mot combine/merge incomplete objects.

[akks]

When we select incomplete ways there are also exceptions in L and Q, Utilsplugin actions etc.

There are 2 options:
1) Not select incomplete ways (bad solution? they are explicitly found by Search dialog with appropriate checkbox)
2) Add checks into all corresponding actions (C, L, Q, etc.)

[Don-vip]

In 12527/josm:

fix #10838 - add robustness if incomplete ways are selected