From 5852b4e987ab8241bdd291e457175ef402f453a8 Mon Sep 17 00:00:00 2001
From: Mangel Maxime <me@mangelmaxime.fr>
Date: Fri, 18 Sep 2026 17:12:41 +0200
Subject: [PATCH] fix(python): keep the payload type when reading an erased
 union field

---
 src/Fable.Transforms/FSharp2Fable.fs |  7 +++++--
 tests/Beam/InteropTests.fs           | 20 ++++++++++++++++++++
 tests/Js/Main/UnionTypeTests.fs      | 15 +++++++++++++++
 tests/Python/TestUnionType.fs        | 16 ++++++++++++++++
 4 files changed, 56 insertions(+), 2 deletions(-)

diff --git a/src/Fable.Transforms/FSharp2Fable.fs b/src/Fable.Transforms/FSharp2Fable.fs
index ddbabedd80..db18d095fb 100644
--- a/src/Fable.Transforms/FSharp2Fable.fs
+++ b/src/Fable.Transforms/FSharp2Fable.fs
@@ -1316,10 +1316,13 @@ let private transformExpr (com: IFableCompiler) (ctx: Context) appliedGenArgs fs
             | ErasedUnionCase ->
                 let index = unionCase.Fields |> Seq.findIndex (fun x -> x.Name = field.Name)
 
-                return Fable.Get(unionExpr, Fable.TupleIndex(index), makeType ctx.GenericArgs unionType, r)
+                return Fable.Get(unionExpr, Fable.TupleIndex(index), fieldType, r)
             | ErasedUnion _ ->
                 if unionCase.Fields.Count = 1 then
-                    return unionExpr
+                    if unionExpr.Type = fieldType then
+                        return unionExpr
+                    else
+                        return Fable.TypeCast(unionExpr, fieldType)
                 else
                     let index = unionCase.Fields |> Seq.findIndex (fun x -> x.Name = field.Name)
 
diff --git a/tests/Beam/InteropTests.fs b/tests/Beam/InteropTests.fs
index 7c4ac25c19..1c0d4282bf 100644
--- a/tests/Beam/InteropTests.fs
+++ b/tests/Beam/InteropTests.fs
@@ -104,6 +104,13 @@ type ErasedUnion =
 type ErasedUnionWithMultipleFields =
     | ErasedUnionWithMultipleFields of string * int
 
+type ErasedPayload = { Description: string; Membership: int }
+
+[<Erase>]
+type ErasedWrapper =
+    | ErasedSuccess of ErasedPayload
+    | ErasedFailure of string
+
 // ============================================================
 // StringEnum tests
 // ============================================================
@@ -408,6 +415,19 @@ let ``test Erased types can have members`` () =
     ()
 #endif
 
+[<Fact>]
+let ``test Erased union payload fields can be read`` () =
+#if FABLE_COMPILER
+    let payload = { Description = "a"; Membership = 2 }
+
+    match ErasedSuccess payload with
+    | ErasedSuccess details -> details.Membership
+    | ErasedFailure _ -> -1
+    |> equal 2
+#else
+    ()
+#endif
+
 [<Fact>]
 let ``test Erased unions with multiple fields work`` () =
 #if FABLE_COMPILER
diff --git a/tests/Js/Main/UnionTypeTests.fs b/tests/Js/Main/UnionTypeTests.fs
index 3d714753b4..3c6a311449 100644
--- a/tests/Js/Main/UnionTypeTests.fs
+++ b/tests/Js/Main/UnionTypeTests.fs
@@ -81,6 +81,13 @@ open Fable.Core.JsInterop
 #endif
 type DU = Int of int | Str of string
 
+type ErasedPayload = { Description: string; Membership: int }
+
+[<Fable.Core.Erase>]
+type ErasedWrapper =
+    | ErasedSuccess of ErasedPayload
+    | ErasedFailure of string
+
 type T1 = T1
 type T2 = T2
 type T3 = T3
@@ -213,6 +220,14 @@ let tests =
         "HELLO" |> Wrapper |> U3.Case3 |> toString |> equal "OLLEH"
     #endif
 
+    testCase "Erased union payload fields can be read" <| fun () ->
+        let payload = { Description = "a"; Membership = 2 }
+
+        match ErasedSuccess payload with
+        | ErasedSuccess details -> details.Membership
+        | ErasedFailure _ -> -1
+        |> equal 2
+
     testCase "Equality works in filter" <| fun () ->
         let original = [| { Name = "1"; Case = MyUnion.Case1 } ; { Name = "2"; Case = MyUnion.Case1 }; { Name = "3"; Case = MyUnion.Case2 }; { Name = "4"; Case = MyUnion.Case3 } |]
         original
diff --git a/tests/Python/TestUnionType.fs b/tests/Python/TestUnionType.fs
index f9b3d3f7bc..a9775b681c 100644
--- a/tests/Python/TestUnionType.fs
+++ b/tests/Python/TestUnionType.fs
@@ -82,6 +82,13 @@ open Fable.Core
 #endif
 type DU = Int of int | Str of string
 
+type ErasedPayload = { Description: string; Membership: int }
+
+[<Fable.Core.Erase>]
+type ErasedWrapper =
+    | ErasedSuccess of ErasedPayload
+    | ErasedFailure of string
+
 [<Fact>]
 let ``test Union cases matches with no arguments can be generated`` () =
     let x = Male
@@ -200,6 +207,15 @@ let ``test Erased union type testing works`` () =
     "HELLO" |> Wrapper |> U3.Case3 |> toString |> equal "OLLEH"
 #endif
 
+[<Fact>]
+let ``test Erased union payload fields can be read`` () =
+    let payload = { Description = "a"; Membership = 2 }
+
+    match ErasedSuccess payload with
+    | ErasedSuccess details -> details.Membership
+    | ErasedFailure _ -> -1
+    |> equal 2
+
 [<Fact>]
 let ``test Equality works in filter`` () =
     let original = [| { Name = "1"; Case = MyUnion3.Case1 } ; { Name = "2"; Case = MyUnion3.Case1 }; { Name = "3"; Case = MyUnion3.Case2 }; { Name = "4"; Case = MyUnion3.Case3 } |]