module T::Private::Methods::SignatureValidation

def self.validate(signature)
  # Constructors in any language are always a bit weird: they're called in a
  # static context, but their bodies are implemented by instance methods. So
  # a mix of the rules that apply to instance methods and class methods
  # apply.
  #
  # In languages like Java and Scala, static methods/companion object methods
  # are never inherited. (In Java it almost looks like you can inherit them,
  # because `Child.static_parent_method` works, but this method is simply
  # resolved statically to `Parent.static_parent_method`). Even though most
  # instance methods overrides have variance checking done, constructors are
  # not treated like this, because static methods are never
  # inherited/overridden, and the constructor can only ever be called
  # indirectly by way of the static method. (Note: this is only a mental
  # model--there's not actually a static method for the constructor in Java,
  # there's an `invokespecial` JVM instruction that handles this).
  #
  # But Ruby is not like Java: singleton class methods in Ruby *are*
  # inherited, unlike static methods in Java. In fact, this is similar to how
  # JavaScript works. TypeScript simply then sidesteps the issue with
  # structural typing: `typeof Parent` is not compatible with `typeof Child`
  # if their constructors are different. (In a nominal type system, simply
  # having Child descend from Parent should be the only factor in determining
  # whether those types are compatible).
  #
  # Flow has nominal subtyping for classes. When overriding (static and
  # instance) methods in a child class, the overrides must satisfy variance
  # constraints. But it still carves out an exception for constructors,
  # because then literally every class would have to have the same
  # constructor. This is simply unsound. Hack does a similar thing--static
  # method overrides are checked, but not constructors. Though what Hack
  # *does* have is a way to opt into override checking for constructors with
  # a special annotation.
  #
  # It turns out, Sorbet already has this special annotation: either
  # `abstract` or `overridable`. At time of writing, *no* static override
  # checking happens unless marked with these keywords (though at runtime, it
  # always happens). Getting the static system to parity with the runtime by
  # always checking overrides would be a great place to get to one day, but
  # for now we can take advantage of it by only doing override checks for
  # constructors if they've opted in.
  #
  # (When we get around to more widely checking overrides statically, we will
  # need to build a matching special case for constructors statically.)
  #
  # Note that this breaks with tradition: normally, constructors are not
  # allowed to be abstract. But that's kind of a side-effect of everything
  # above: in Java/Scala, singleton class methods are never abstract because
  # they're not inherited, and this extends to constructors. TypeScript
  # simply rejects `new klass()` entirely if `klass` is
  # `typeof AbstractClass`, requiring instead that you write
  # `{ new(): AbstractClass }`. We may want to consider building some
  # analogue to `T.class_of` in the future that works like this `{new():
  # ...}` type.
  if signature.method_name == :initialize && signature.method.owner.is_a?(Class)
    if signature.mode == Modes.standard
      return
    end
  end
  super_method = signature.method.super_method
  if super_method && super_method.owner != signature.method.owner
    Methods.maybe_run_sig_block_for_method(super_method)
    super_signature = Methods.signature_for_method(super_method)
    # If the super_method has any kwargs we can't build a
    # Signature for it, so we'll just skip validation in that case.
    if !super_signature && !super_method.parameters.select {|kind, _| kind == :rest || kind == :kwrest}.empty?
      nil
    else
      # super_signature can be nil when we're overriding a method (perhaps a builtin) that didn't use
      # one of the method signature helpers. Use an untyped signature so we can still validate
      # everything but types.
      #
      # We treat these signatures as overridable, that way people can use `.override` with
      # overrides of builtins. In the future we could try to distinguish when the method is a
      # builtin and treat non-builtins as non-overridable (so you'd be forced to declare them with
      # `.overridable`).
      #
      super_signature ||= Methods::Signature.new_untyped(method: super_method)
      validate_override_mode(signature, super_signature)
      validate_override_shape(signature, super_signature)
      validate_override_types(signature, super_signature)
    end
  else
    validate_non_override_mode(signature)
  end
end