[bisected] Can no longer override subproject defaults in superproject's project() statement

[Akaricchi]

Describe the bug
In Meson 0.55.0 and below, it was possible to override a subproject's default options inside the superproject's project() statement, like so:

project('superproject', 'c',
  default_options : [
      'subproject:test_option=false'])

In current git master, this is broken. Configuring such a project for the first time results in the following (irrelevant output omitted):

$ grep test_option subprojects/subproject/meson_options.txt 
option('test_option', type : 'boolean', value : true, description : 'Test option. Superproject overrides default to "false"')

$ meson setup build

... omitted ...

WARNING: Unknown options: "subproject:test_option"
The value of new options can be set with:
meson setup <builddir> --reconfigure -Dnew_option=new_value ...

... omitted ...

|Executing subproject subproject method meson 
|
|WARNING: In subproject subproject: Unknown options: "subproject:subproject:test_option"
|The value of new options can be set with:
|meson setup <builddir> --reconfigure -Dnew_option=new_value ...

... omitted ...

Option subproject:test_option is: true [default: false]

$ meson configure build | grep test_option
  subproject:test_option     true           [true, false]       Test option. Superproject overrides default to "false"

It seems to recognize false as the new "default", however it still applies the old default value to a fresh build directory, as if -Dtest_option=true was passed explicitly. If an old build directory is --wiped, any such options are also reverted to their subproject defaults, unless they have been explicitly overriden at any point during the build dir's lifetime.

There is also a new spurious warning

|WARNING: In subproject subproject: Unknown options: "subproject:subproject:test_option"

This seems to happen for every option overriden via the project() statement.

To Reproduce
Here is a minimal test case:
meson-test-subproject-option-override-from-superproject.zip

Expected behavior
After configuring a build directory for the attached test case, subproject:test_option should be false. There should not be any warnings about missing options.

system parameters

• meson 0.55.999 (1ddd2b4)

[Akaricchi]

Bisected to 591e6e9, part of #6597. CC @dcbaker

[xclaesse]

Thanks for testing master! I didn't know we could have subprojects:foo in project(). I fixed this regression in #7574 and added a unit test for that.

[Akaricchi]

I didn't know we could have subprojects:foo in project()

I believe this isn't actually documented, and might've even been allowed by accident (correct me if I'm wrong). But this is useful, has worked for a while, and some projects depend on it, so I'd say it's worth promoting to an official feature. Pretty sure this used to be the only working way to override subproject defaults at some point; not sure whether it still is.

[dcbaker]

I'm not sure we meant to enable this. Since we have support for setting default arguments to subprojects in the subproject() call I'd prefer to deprecate this and keep the one in the subproject call.

[Akaricchi]

But what if you don't call subproject()? It's common and encouraged to just use dependency() with a fallback instead, and in 0.55.0 the fallback can even be implicit. I don't think you can specify subproject options in that way, can you? Besides, having it all in one place is just more readable and maintainable. I'd rather deprecate subproject(default_options:), especially considering that it used to be broken for a while.