[question] More details about overriding upstream options from downstream recipes

[SpaceIm]

• I've read the CONTRIBUTING guide.

I still don't completely understand how and when upstream options are overridden, and which recipe takes precedence if multiple downstream recipes override options of a common upstream recipe.

https://docs.conan.io/en/latest/mastering/conditional.html claims that upstream recipes options should be overridden in configure method when it is absolutely necessary.

• If multiple recipes override the same upstream option, which recipe wins?
  Let's say for these two cases:

  • two direct parents override the same option of their common child.
  • one direct parent and root override the same option.

• some recipes like https://github.com/conan-community/conan-opencv conditionally override several upstream options in requirements method instead of configure method. Can we expect the same behaviour as if they would have been overridden in configure method (in case of conflicts with other recipes, does overriding this option in requirements take precedence over overriding inconfigure method, without respect of rules in first point)? Is is the same to override after self.requires("upstream") or before, in requirements method? Is it bad practice to override in requirements method?

[jgsogo]

💯 I also find it difficult, and usually need to run a test to confirm how it works.

Conan 2.0 should be much more strict regarding attributes visibility and constness.

• The plan is to add/remove attributes dynamically when entering/exiting functions, so only the attributes that make sense in that context are available inside the function: for example, no settings or options inside the source() method. We are already adding some checks if using CONAN_V2_MODE.
• Probably we will need wrappers with getters/setters to forbid modifying values: settings are const, options depending on the context cannot be modified,...

Both should improve usability.

I'll move this issue to the docs and assign it to the next release, at least we will improve the documentation in this iteration, it is also somehow related to conan-io/conan#7591 which is assigned to 1.30.