FWIW, I discovered the issue when experimenting with ways to use the class pattern in structural pattern matching.


--- Code that should work but doesn't -----------

class Warm:
    def __instancecheck__(cls, inst):
        return inst in {'red', 'orange', 'blue'}
  
match 'red':
    case Warm():              # This doesn't match but should
        print('Glowing')


--- What you have to do to get it to work -------

class MetaWarm(type):
    def __instancecheck__(cls, inst):
        return inst in {'red', 'orange', 'blue'}

class Warm(metaclass=MetaWarm):
    pass

match 'red':
    case Warm():          # This matches
        print('Hot')