Raku/doc#4513

Subtest has several variants, not all of which are tested/used in roast.

From rakudo:

multi sub subtest(Pair $what)            is export { subtest($what.value,$what.key) }
multi sub subtest($desc, &subtests)      is export { subtest(&subtests,$desc)       }
multi sub subtest(&subtests, $desc = '') is export { ... }

@JJ noted that the Pair implementation in rakudo allows you to declare the pair in either direction, and because of the multis here, it eventually dispatches to the last variant regardless and so works either way.

Assuming this is meant to be spec, all variants should be tested (though it's fine to use one or the other stylistically in roast, need to verify that this works as intended) - Or, if this is not intended to work this way, we need to open a bug in Rakudo to enforce the ordering of the Pair