Fix decoding of maps to match conformance tests

lib/protobuf/decoder.ex

@@ -233,8 +233,17 @@ defmodule Protobuf.Decoder do
     %FieldProps{type: type, map?: map?, oneof: oneof, name_atom: name_atom, repeated?: repeated?} =
       prop
 
-    embedded_msg = decode(bin, type)
-    val = if map?, do: %{embedded_msg.key => embedded_msg.value}, else: embedded_msg
+    embed_msg = decode(bin, type)
+
+    val =
+      if map? do
+        key = if is_nil(embed_msg.key), do: map_default(prop, :key), else: embed_msg.key
+        value = if is_nil(embed_msg.value), do: map_default(prop, :value), else: embed_msg.value
+        %{key => value}
+      else
+        embed_msg
+      end
+
     val = if oneof, do: {name_atom, val}, else: val
 
     cond do
@@ -255,6 +264,15 @@ defmodule Protobuf.Decoder do
     end
   end
 
+  defp map_default(prop, key_or_value) do
+    prop.type.__message_props__().field_props
+    |> Enum.find(fn {_key, field_props} -> field_props.name_atom == key_or_value end)
+    |> then(fn {_key, field_props} ->
+      # Conformance only works when we use proto3 defaults here, even for proto2...
+      Protobuf.DSL.field_default(:proto3, field_props)
+    end)
+  end
+
   defp deep_merge(_oneof1 = {tag1, val1}, oneof2 = {tag2, val2}, props) do
     if tag1 == tag2 do
       # If the field is a oneof, we merge its value and keep the tag.

test/protobuf/conformance_regressions_test.exs

@@ -83,6 +83,12 @@ defmodule Protobuf.ConformanceRegressionsTest do
 
     @describetag message_type: "protobuf_test_messages.proto3.TestAllTypesProto3"
 
+    test "Required.Proto2.ProtobufInput.ValidDataMap.INT32.INT32.MissingDefault.JsonOutput" do
+      mod = ProtobufTestMessages.Proto2.TestAllTypesProto2
+      problematic_payload = <<194, 3, 0>>
+      assert %{map_int32_int32: %{0 => 0}} = mod.decode(problematic_payload)
+    end
+
     test "Required.Proto3.JsonInput.Int32FieldQuotedExponentialValue.JsonOutput" do
       mod = ProtobufTestMessages.Proto3.TestAllTypesProto3
       problematic_payload = ~S({"optionalInt32": "1e5"})