From 9cddb79893449d37a8d9655ae280a49ebd7d5698 Mon Sep 17 00:00:00 2001
From: Dag Brattli <dag@brattli.net>
Date: Wed, 30 Sep 2026 18:58:23 +0200
Subject: [PATCH] fix(python): avoid name collisions in nested tuple matches

---
 src/Fable.Transforms/FSharp2Fable.Util.fs |  7 ++++
 tests/Python/TestPatternMatch.fs          | 50 +++++++++++++++++++++++
 2 files changed, 57 insertions(+)

diff --git a/src/Fable.Transforms/FSharp2Fable.Util.fs b/src/Fable.Transforms/FSharp2Fable.Util.fs
index 61eb69055e..13d68c3871 100644
--- a/src/Fable.Transforms/FSharp2Fable.Util.fs
+++ b/src/Fable.Transforms/FSharp2Fable.Util.fs
@@ -810,6 +810,13 @@ module Helpers =
         || ctx.UsedNamesInDeclarationScope.Contains name
 
     let getIdentUniqueName (com: Compiler) (ctx: Context) name =
+        // Reserve the emitted Python name so tuple-match temporaries cannot shadow
+        // existing bindings when camelCase is converted to snake_case.
+        let name =
+            match com.Options.Language with
+            | Python -> Fable.Py.Naming.toPythonNaming name
+            | _ -> name
+
         let sanitizeIdent =
             match com.Options.Language with
             | Python -> Fable.Py.Naming.sanitizeIdent
diff --git a/tests/Python/TestPatternMatch.fs b/tests/Python/TestPatternMatch.fs
index 932d0eb987..0095333d90 100644
--- a/tests/Python/TestPatternMatch.fs
+++ b/tests/Python/TestPatternMatch.fs
@@ -311,6 +311,56 @@ let ``test nested match complex`` () =
     nestedComplex 2 2 |> equal "x=2, y=other"
     nestedComplex 3 1 |> equal "x=other"
 
+// https://github.com/fable-compiler/Fable/issues/5026
+module NestedMatchNames =
+    type Shape =
+        | Circle of radius: int
+        | Square
+
+    let describe (shapes: Shape list) n =
+        match List.head shapes with
+        | Circle radius ->
+            match n + 1, radius with
+            | 2, 1 -> "unit circle"
+            | _ -> "other"
+        | Square -> "square"
+
+    let describeInClosure (shapes: Shape list) =
+        match List.head shapes with
+        | Circle radius ->
+            fun xs ->
+                match List.tryHead xs, radius with
+                | Some 2, 1 -> "unit circle"
+                | _ -> "other"
+        | Square -> fun _ -> "square"
+
+    let describeWithUserBinding (match_value: string) n =
+        match n + 1, n + 2 with
+        | 2, 3 -> match_value
+        | _ -> "other"
+
+[<Fact>]
+let ``test nested tuple match preserves outer union fields`` () =
+    NestedMatchNames.describe [ NestedMatchNames.Circle 1 ] 1 |> equal "unit circle"
+    NestedMatchNames.describe [ NestedMatchNames.Circle 2 ] 1 |> equal "other"
+    NestedMatchNames.describe [ NestedMatchNames.Circle 1 ] 2 |> equal "other"
+    NestedMatchNames.describe [ NestedMatchNames.Square ] 1 |> equal "square"
+
+[<Fact>]
+let ``test tuple match in closure preserves captured union fields`` () =
+    let describe = NestedMatchNames.describeInClosure [ NestedMatchNames.Circle 1 ]
+    describe [ 2 ] |> equal "unit circle"
+    describe [ 3 ] |> equal "other"
+    describe [] |> equal "other"
+    describe [ 2 ] |> equal "unit circle"
+    NestedMatchNames.describeInClosure [ NestedMatchNames.Circle 2 ] [ 2 ] |> equal "other"
+    NestedMatchNames.describeInClosure [ NestedMatchNames.Square ] [ 2 ] |> equal "square"
+
+[<Fact>]
+let ``test tuple match temporaries preserve user bindings`` () =
+    NestedMatchNames.describeWithUserBinding "matched" 1 |> equal "matched"
+    NestedMatchNames.describeWithUserBinding "matched" 2 |> equal "other"
+
 // ----------------------------------------------------------------------------
 // 7. Option Matching
 // ----------------------------------------------------------------------------