cmake -E capabilities output doesn't contain supportedPlatforms
The 3.21 release note for the cmake -E capabilities
feature says that each generator's entry will contain a supportedPlatforms
item. On Linux and macOS, I see that none of the generators have this field. It appears that the field will only be present if the generator has details for what platforms are supported. From what I can tell, that is currently only the Visual Studio generators at the moment. Note too that the GHS generator claims to support platforms, but doesn't have supportedPlatforms
populated either.
The relevant change appears to be !6102 (merged), but it does seem to have been implemented under the assumption that the supported platforms would always be populated where relevant, which is not the case.
This is ultimately probably a documentation issue, as I suspect it won't be possible to reliably determine the supported platforms in all cases. For example, I have heard of cases where the Intel compiler is used with Xcode by setting the generator platform. I don't know how we'd reliably detect that or even whether it is a formally supported way of setting things up.