Fix encoding specs

Author: josevalim

lib/elixir/lib/json.ex

@@ -199,6 +199,8 @@ defmodule JSON do
 
  @moduledoc since: "1.18.0"
 
+ @type encoder :: (term(), encoder() -> iodata())
+
  @type decode_error_reason ::
  {:unexpected_end, non_neg_integer()}
  | {:invalid_byte, non_neg_integer(), byte()}
@@ -332,7 +334,7 @@ defmodule JSON do
  "[123,\"string\",{\"key\":\"value\"}]"
 
  """
- @spec encode!(a, (a -> iodata())) :: binary() when a: var
+ @spec encode!(term(), encoder()) :: binary()
  def encode!(term, encoder \\ &protocol_encode/2) do
  IO.iodata_to_binary(encoder.(term, encoder))
  end
@@ -353,7 +355,7 @@ defmodule JSON do
  "[123,\"string\",{\"key\":\"value\"}]"
 
  """
- @spec encode_to_iodata!(a, (a -> iodata())) :: iodata() when a: var
+ @spec encode_to_iodata!(term(), encoder()) :: iodata()
  def encode_to_iodata!(term, encoder \\ &protocol_encode/2) do
  encoder.(term, encoder)
  end
@@ -365,7 +367,7 @@ defmodule JSON do
  `encode!/2` and `encode_to_iodata!/2`. The default implementation
  is an optimized dispatch to the `JSON.Encoder` protocol.
  """
- @spec protocol_encode(a, (a -> iodata())) :: iodata() when a: var
+ @spec protocol_encode(term(), encoder()) :: iodata()
  def protocol_encode(value, encoder) when is_atom(value) do
  case value do
  nil -> "null"