type_checker.rb

module TypeChecker
  Error = Class.new(StandardError)

  class << self
    def register(this, meth, klass)
      this.module_eval(<<~RUBY, __FILE__, __LINE__ + 1)
        def #{meth}
          ret = super
          unless ret.kind_of?(#{klass})
            raise Error, "no implicit conversion of \#{self.class} into #{klass} got \#{ret.class}"
          end
        end
      RUBY
    end
  end
end

module TypeChecker
  module Standard
    {
      to_i:    Integer,
      to_int:  Integer,
      to_f:    Float,
      to_s:    String,
      to_str:  String,
      to_sym:  Symbol,
      to_h:    Hash,
      to_hash: Hash,
      to_a:    Array,
      to_io:   IO,
    }.each do |m, c|
      TypeChecker.register(self, m, c)
    end
  end
end

class A
  prepend TypeChecker::Standard

  def to_h
    []
  end
end

begin
  A.new.to_h
rescue TypeChecker::Error
else
  raise
end

begin
  A.new.to_a
rescue NoMethodError
else
  raise
end