From 3ffa13af61b3614bbd366c1c7abd706a1775aa74 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Muska=C5=82a?= <michal@muskala.eu>
Date: Sat, 29 Jun 2024 17:07:37 +0100
Subject: [PATCH] Fix derive with _ key

Summary:

Test Plan:
---
 lib/encoder.ex       | 11 +++++++----
 test/encode_test.exs |  8 ++++++++
 2 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/lib/encoder.ex b/lib/encoder.ex
index d693399..49f555b 100644
--- a/lib/encoder.ex
+++ b/lib/encoder.ex
@@ -77,7 +77,7 @@ end
 defimpl Jason.Encoder, for: Any do
   defmacro __deriving__(module, struct, opts) do
     fields = fields_to_encode(struct, opts)
-    kv = Enum.map(fields, &{&1, generated_var(&1, __MODULE__)})
+    kv = Enum.map(fields, &{&1, generated_var(&1)})
     escape = quote(do: escape)
     encode_map = quote(do: encode_map)
     encode_args = [escape, encode_map]
@@ -94,9 +94,12 @@ defimpl Jason.Encoder, for: Any do
     end
   end
 
-  # The same as Macro.var/2 except it sets generated: true
-  defp generated_var(name, context) do
-    {name, [generated: true], context}
+  # The same as Macro.var/2 except it sets generated: true and handles _ key
+  defp generated_var(:_) do
+    {:__, [generated: true], __MODULE__.Underscore}
+  end
+  defp generated_var(name) do
+    {name, [generated: true], __MODULE__}
   end
 
   def encode(%_{} = struct, _opts) do
diff --git a/test/encode_test.exs b/test/encode_test.exs
index f4c0ef2..20d48b9 100644
--- a/test/encode_test.exs
+++ b/test/encode_test.exs
@@ -135,6 +135,11 @@ defmodule Jason.EncoderTest do
     defstruct name: "", size: 0
   end
 
+  defmodule DerivedWeirdKey do
+    @derive Encoder
+    defstruct [:_]
+  end
+
   defmodule NonDerived do
     defstruct name: ""
   end
@@ -154,6 +159,9 @@ defmodule Jason.EncoderTest do
 
     derived_using_except = %DerivedUsingExcept{name: "derived using :except", size: 10}
     assert to_json(derived_using_except) == ~s({"size":10})
+
+    derived_weird_key = %DerivedWeirdKey{}
+    assert to_json(derived_weird_key) == ~s({"_":null})
   end
 
   test "@derive validate `except:`" do