bazel: select()'s unambiguous specialization resolution fails with platform constraint_value()s

Hi, wonderful Bazel folks. Thanks for all the great work you do, including extending official support for platforms.

This is a bug report at the intersection of select() and constraint_value() (for platforms). You can definitely work around it, but it’s a bit annoying. I’m guessing it’s just an early rough edge, since platforms are still in flux.

This is easiest by example. Say you have:

config_setting(
    name="ios-aarch64",
    constraint_values = [
        "@platforms//os:ios",
        "@platforms//cpu:aarch64",
    ]
)

And you try to select() between :ios-aarch64 and @platforms//os:ios.

You’ll get an error like:

Illegal ambiguous match on configurable attribute <...>
:ios-aarch64
@platforms//os:ios
Multiple matches are not allowed unless one is unambiguously more specialized.

Despite the former being unambiguously more specialized.

This unambiguous specialization behavior works correctly for normal, non-platform config_setting()s that use the values parameter instead of constraint_values, and is documented here.

I’m seeing this on the latest macOS (12.1) with the latest Bazel rolling (6.0.0-pre.20220112.2).

Thanks so much for taking a peek! Chris (ex-Googler)