MergeError if `tag` as list inherits `tag` as a string from the upper node

[smyle10]

tmt version: 1.35.0

Given a main.fmf:

test: TEST_ITEMS=${TMT_TEST_NAME##*/} bash ./runtest.sh
tag: foobar
/test_keep_iproute_routes:
    tag+:
      - customer
      - unmanaged

Running tmt test ls got:

MergeError: Key 'tag' in /networking/nmstate/anyhost/route/test_keep_iproute_routes (can only concatenate str (not "list") to str).

Though tmt hints the reason and I can change string "foobar" as a list (but consider a new tmt user may not know the inheritance feature in tmt) , as https://tmt.readthedocs.io/en/stable/spec/core.html#tag indicates that a tag can be string or list, hope such merge can be improved.

[LecrisUT]

This is an fmf issue. If you make the parent a list that would make it work. Maybe the parent could be converted to a list beforehand, but then it would make concatenation of strings impossible.

The full list of operators are here. Do you have an idea of an interface that could help here? Also related discussion here (the opposite case to yours)

[tcornell-bus]

Triage notes:
Still an issue as of tmt 1.0.75 and fmf 1.7.0. After reading through the similar issue #228 and the resulting PR #256, I do not see a reason why we shouldn't or couldn't do something similar for merging a string and a list.

I think the expectation would always be string + list = list where the string would become its own element in the list. Please look over the combinations below and tell me if the proposals are reasonable for the + operator.

parent string + child list (or vice versa)

test: 'true'
tag: foobar
/test_keep_iproute_routes:
    tag+:
      - customer

Current result: error

 ERROR  MergeError: Key 'tag' in /test_keep_iproute_routes (can only concatenate str (not "list") to str).

Proposed result: list

$ fmf show
/test_keep_iproute_routes
tag: foobar and customer
test: true

---

parent string + child string

test: 'true'
tag: foobar
/test_keep_iproute_routes:
    tag+: customer

Current result: string

$ fmf show
/test_keep_iproute_routes
tag: foobarcustomer
test: true

Proposed result: string
Stay the same

---

parent list + child list

test: 'true'
tag: 
  - foobar
/test_keep_iproute_routes:
    tag+: 
      - customer

Current result: list

$ fmf show
/test_keep_iproute_routes
tag: foobar and customer
test: true

Proposed result: list
Stay the same