BuildStream silently allows overriding list defined in include

[nanonyme]

When you define a list like

build-depends:
- foo.bst
- bar.bst

in include and then set same in element body, there will be silent possibly unexpected replace when user was supposed to instead append. I propose behaviour be slightly adjusted such that when you redefine list with above syntax in element, there will be a warning that can be made fatal. (implicit-list-replace or something else, I'm up to different names) To fix the warning, you would have to in element in this case state intent you want to replace existing list through

build-depends:
  (=):
  - foo.bst
  - bar.bst

If the list is not defined before, then there will be no warning with the short-hand syntax.

[nanonyme]

If this change is done, documentation at https://docs.buildstream.build/master/format_intro.html#list-overwrite should be modified as the list-overwrite functionality would no longer be redundant and only for completeness.

[juergbi]

This sounds like a good idea, assuming this is possible without causing other issues.