Poly (Time | user class) dispatch loses the builtin Time#strftime arm: NoMethodError

[rubys]

A user class that defines strftime makes Time#strftime raise NoMethodError when the receiver is poly (Time | UserClass): the dispatch only has the user arm. A monomorphic Time receiver is fine.

Same family as #3459 / #4413 (builtin arm lost under poly dispatch), not covered for Time#strftime.

class Date
  def initialize(y); @y = y; end
  def strftime(p); "D#{@y}"; end
end
def fmt(t)
  t.strftime("%Y")
end
puts fmt(Time.at(0).utc)
puts fmt(Date.new(3))

CRuby:

1970
D3

Spinel (ab60de7):

undefined method 'strftime' for an instance of Time (NoMethodError)

Context: roundhouse PR rubys/roundhouse#423 adds a program-defined Date to the Spinel runtime; every Campfire page then 500s on Time#strftime.