Can no longer set a yielding option to a value different from the parent option's value

[nirbheek]

This was finally working correctly in 1.8.0 after the options rework, but #14776 has regressed it again.

The use-case is:

1. You have a project option tools, which is auto by default
2. You have multiple subprojects with options tools, which are also auto by default, and are all yield: true
3. You use -Dtools=disabled to disable tools for all subprojects, but you want to enable it for one subproject

This no longer works.

[bonzini]

It's not documented to do so though...

Because it's a change in 1.8, it's quite likely it was not intended and exact semantics need to be agreed upon.

Do you want only the default to change (to either the value set for the superproject, or the superproject default if none), but not an explicitly-set value?

[eli-schwartz]

This was finally working correctly in 1.8.0 after the options rework, but [14776 has regressed it again.

Since the mentioned PR is tagged for 1.8.3, this issue should be associated with the same milestone that the mentioned PR is associated with. Otherwise we may end up backporting something that changes behavior only to change it back in the next feature release...

[nirbheek]

Do you want only the default to change (to either the value set for the superproject, or the superproject default if none), but not an explicitly-set value?

If someone is explicitly setting an option, the expectation is that it should be set (in the order that it is specified on the cmdline). yield: is quite unergonomic if it makes it so that all -D options referring to it are silently ignored.

To be more precise, let's say we have option('tools', type: 'feature', value: 'auto') in a project, and option('tools', type: 'feature', value: 'auto', yield: true) in multiple subprojects.

-Dfoo1:tools=disabled should keep tools auto everywhere except in foo1, where it is disabled.

-Dfoo1:tools=disabled -Dtools=enabled should enable tools in every subproject

-Dtools=enabled -Dfoo1:tools=disabled should enable tools in every subproject except in foo1 where it is disabled

-Dauto_features=disabled -Dfoo1:tools=enabled should disable tools everywhere except in the subproject foo1 where it is enabled

Currently in all these cases -Dfoo1:tools=<value> is ignored.

[bonzini]

Fixed by #14800