quiz.rb

class A
  def to_s
    'A1'
  end

  def to_str
    'A2'
  end
end

class B
  def to_str
    'B'
  end
end

class C
  def to_s
    'C'
  end
end

class D
end

def check
  yield
rescue => e
  e
end

check { "hello, #{A.new}" } # ?
check { "hello, " + A.new } # ?
check { "hello, " << A.new } # ?

check { "hello, #{B.new}" } # ?
check { "hello, " + B.new } # ?
check { "hello, " << B.new } # ?

check { "hello, #{C.new}" } # ?
check { "hello, " + C.new } # ?
check { "hello, " << C.new } # ?

check { "hello, #{D.new}" } # ?
check { "hello, " + D.new } # ?
check { "hello, " << D.new } # ?