Fix wire encoder+decoder for type aliases with extensible records through alias chains

When a type alias like Color = ColorValue { red, green, blue, alpha }
references an extensible record alias (ColorValue compatible =
{ compatible | value : String, color : Compatible }), the encoder and
decoder's TAlias Holey branch only checked for a direct TRecord inner
type to resolve extensible records.

When the inner type is a TAlias chain (not a direct TRecord), both
encoder and decoder fell through to normalEncoder/normalDecoder, which
passed only the extension fields to the extensible record codec —
losing the base fields (value, color). This caused a type mismatch
when the codec produced a 4-field record instead of the full 6-field
record.

Fix: in both Decoder.hs and Encoder.hs, when the Holey inner type is
not a direct TRecord, use resolveTvar to resolve through alias chains.
If this produces a Filled TRecord, inline the fully merged record
encoder/decoder.

Also set decoder Filled branch to normalDecoder to match encoder.

Test: Added Wire_Union_ForeignRecordAlias.elm test fixture with
ExternalExtensibleBase/ExternalRecordViaExtensible types in External.elm.

Verified red→green with lamdera make on a project using rtfeldman/elm-css.
Runtime roundtrip verified for Css.Color and Css.Px across 12 test cases
(DirectWrap, RecordWrap, ListWrap, Mixed unions, Complex nesting, etc).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: dillonkearns

extra/Lamdera/Wire3/Decoder.hs

@@ -416,7 +416,14 @@ decoderForType ifaces cname tipe =
                      let extendedRecord = TRecord resolved Nothing & resolveTvar tvars_
                      in decoderForType ifaces cname extendedRecord
                     Nothing -> normalDecoder
-                otherTypes -> normalDecoder
+                _ ->
+                  -- Resolve extensible records through TAlias chains,
+                  -- e.g. Color = ColorValue { red, green, blue, alpha }
+                  case resolveTvar tvars_ tipe of
+                    TAlias _ _ _ (Filled (TRecord fieldMap Nothing)) ->
+                      let fields = fieldMap & fieldsToList & List.sortOn (\(name, field) -> name)
+                      in decodeRecord ifaces cname fields
+                    _ -> normalDecoder
             Filled tipe ->
               case tipe of
                 TRecord fieldMap extensibleName ->

extra/Lamdera/Wire3/Encoder.hs

@@ -365,7 +365,14 @@ inlineIfRecordOrCall depth ifaces cname tipe tvars aType =
               in deepEncoderForType depth ifaces cname extendedRecord
             Nothing -> normalEncoder
 
-        otherTypes -> normalEncoder
+        _ ->
+          -- Resolve extensible records through TAlias chains,
+          -- e.g. Color = ColorValue { red, green, blue, alpha }
+          case resolveTvar tvars tipe of
+            TAlias _ _ _ (Filled (TRecord fieldMap Nothing)) ->
+              let extendedRecord = TRecord fieldMap Nothing
+              in deepEncoderForType depth ifaces cname extendedRecord
+            _ -> normalEncoder
     Filled _ -> normalEncoder
 
 {-| Called for encoding tvar type values, i.e.

test/Test/Wire.hs

@@ -130,6 +130,7 @@ wire = do
             , "src/Test/Wire_Tvar_Recursive_Reference.elm"
             , "src/Test/Wire_Unsupported.elm"
             , "src/Test/Wire_Unconstructable.elm"
+            , "src/Test/Wire_Union_ForeignRecordAlias.elm"
             ]
 
       let

test/scenario-alltypes/src/Test/External.elm

@@ -30,6 +30,14 @@ type alias SubSubRecordAlias threadedTvar =
     }
 
 
+type alias ExternalExtensibleBase compatible =
+    { compatible | base : String }
+
+
+type alias ExternalRecordViaExtensible =
+    ExternalExtensibleBase { red : Int, green : Int }
+
+
 expected_w3_encode_ExternalRecordBasic : ExternalRecordBasic -> Lamdera.Wire3.Encoder
 expected_w3_encode_ExternalRecordBasic =
     \w3_rec_var0 -> Lamdera.Wire3.encodeSequenceWithoutLength [ Lamdera.Wire3.encodeInt w3_rec_var0.int ]
@@ -113,3 +121,30 @@ expected_w3_decode_ExternalCustomThreaded w3_x_c_threadedTvar w3_x_c_threadedTva
                     _ ->
                         Lamdera.Wire3.failDecode
             )
+
+
+expected_w3_encode_ExternalExtensibleBase : ({ compatible | base : String.String } -> Lamdera.Wire3.Encoder) -> ExternalExtensibleBase compatible -> Lamdera.Wire3.Encoder
+expected_w3_encode_ExternalExtensibleBase w3_x_c_compatible =
+    w3_x_c_compatible
+
+
+expected_w3_decode_ExternalExtensibleBase w3_x_c_compatible =
+    w3_x_c_compatible
+
+
+expected_w3_encode_ExternalRecordViaExtensible : ExternalRecordViaExtensible -> Lamdera.Wire3.Encoder
+expected_w3_encode_ExternalRecordViaExtensible =
+    \w3_rec_var0 ->
+        Lamdera.Wire3.encodeSequenceWithoutLength
+            [ Lamdera.Wire3.encodeString w3_rec_var0.base
+            , Lamdera.Wire3.encodeInt w3_rec_var0.green
+            , Lamdera.Wire3.encodeInt w3_rec_var0.red
+            ]
+
+
+expected_w3_decode_ExternalRecordViaExtensible =
+    Lamdera.Wire3.succeedDecode
+        (\base0 green0 red0 -> { base = base0, green = green0, red = red0 })
+        |> Lamdera.Wire3.andMapDecode Lamdera.Wire3.decodeString
+        |> Lamdera.Wire3.andMapDecode Lamdera.Wire3.decodeInt
+        |> Lamdera.Wire3.andMapDecode Lamdera.Wire3.decodeInt

test/scenario-alltypes/src/Test/Wire_Union_ForeignRecordAlias.elm

@@ -0,0 +1,85 @@
+module Test.Wire_Union_ForeignRecordAlias exposing (..)
+
+import Bytes.Decode
+import Bytes.Encode
+import Lamdera.Wire3
+import Test.External exposing (ExternalRecordBasic, ExternalRecordViaExtensible)
+
+
+{-| Regression test: extensible record aliases through alias chains.
+See: <https://github.com/elm-explorations/test/pull/249#issuecomment-4076937757>
+-}
+type WrapsBasicRecord
+    = WrapsBasicRecord ExternalRecordBasic
+
+
+type WrapsExtensibleRecord
+    = WrapsExtensibleRecord ExternalRecordViaExtensible
+
+
+type WrapsInRecord
+    = WrapsInRecord { field : ExternalRecordViaExtensible }
+
+
+expected_w3_encode_WrapsBasicRecord : WrapsBasicRecord -> Lamdera.Wire3.Encoder
+expected_w3_encode_WrapsBasicRecord w3v =
+    case w3v of
+        WrapsBasicRecord v0 ->
+            Lamdera.Wire3.encodeSequenceWithoutLength [ Bytes.Encode.unsignedInt8 0, Test.External.w3_encode_ExternalRecordBasic v0 ]
+
+
+expected_w3_decode_WrapsBasicRecord =
+    Bytes.Decode.unsignedInt8
+        |> Lamdera.Wire3.andThenDecode
+            (\w3v ->
+                case w3v of
+                    0 ->
+                        Lamdera.Wire3.succeedDecode WrapsBasicRecord |> Lamdera.Wire3.andMapDecode Test.External.w3_decode_ExternalRecordBasic
+
+                    _ ->
+                        Lamdera.Wire3.failDecode
+            )
+
+
+expected_w3_encode_WrapsExtensibleRecord : WrapsExtensibleRecord -> Lamdera.Wire3.Encoder
+expected_w3_encode_WrapsExtensibleRecord w3v =
+    case w3v of
+        WrapsExtensibleRecord v0 ->
+            Lamdera.Wire3.encodeSequenceWithoutLength [ Bytes.Encode.unsignedInt8 0, Test.External.w3_encode_ExternalRecordViaExtensible v0 ]
+
+
+expected_w3_decode_WrapsExtensibleRecord =
+    Bytes.Decode.unsignedInt8
+        |> Lamdera.Wire3.andThenDecode
+            (\w3v ->
+                case w3v of
+                    0 ->
+                        Lamdera.Wire3.succeedDecode WrapsExtensibleRecord |> Lamdera.Wire3.andMapDecode Test.External.w3_decode_ExternalRecordViaExtensible
+
+                    _ ->
+                        Lamdera.Wire3.failDecode
+            )
+
+
+expected_w3_encode_WrapsInRecord : WrapsInRecord -> Lamdera.Wire3.Encoder
+expected_w3_encode_WrapsInRecord w3v =
+    case w3v of
+        WrapsInRecord v0 ->
+            Lamdera.Wire3.encodeSequenceWithoutLength [ Bytes.Encode.unsignedInt8 0, Test.External.w3_encode_ExternalRecordViaExtensible v0.field ]
+
+
+expected_w3_decode_WrapsInRecord =
+    Bytes.Decode.unsignedInt8
+        |> Lamdera.Wire3.andThenDecode
+            (\w3v ->
+                case w3v of
+                    0 ->
+                        Lamdera.Wire3.succeedDecode WrapsInRecord
+                            |> Lamdera.Wire3.andMapDecode
+                                (Lamdera.Wire3.succeedDecode (\field0 -> { field = field0 })
+                                    |> Lamdera.Wire3.andMapDecode Test.External.w3_decode_ExternalRecordViaExtensible
+                                )
+
+                    _ ->
+                        Lamdera.Wire3.failDecode
+            )