fix idempotency for single case union w/o pipe (#3116)

* fix idempotency for single case union w/o pipe

* update changelog

* Avoid indexed list access.

* Add new version

---------

Co-authored-by: nojaf <florian.verdonck@outlook.com>

Author: joprice

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## 6.3.13 - 2024-09-10
+
+### Fixed
+* Idempotency problem when adding members to a single case union without a pipe [#3102](https://github.com/fsprojects/fantomas/issues/3102)
+
 ## 6.3.12 - 2024-09-05
 
 ### Fixed

src/Fantomas.Core.Tests/TypeDeclarationTests.fs

@@ -3643,3 +3643,42 @@ type FSharpChecker with
         : Async<(FSharpParseFileResults * ParsedInput * FSharpCheckFileResults) option> =
         ()
 """
+
+[<Test>]
+let ``single member union extensions without pipe, 3102`` () =
+    formatSourceString
+        """
+type X = X
+    with
+        static member x = 1
+    """
+        config
+    |> prepend newline
+    |> should
+        equal
+        """
+type X = X
+    with
+
+        static member x = 1
+"""
+
+[<Test>]
+let ``single member union extensions without pipe idempotent, 3102`` () =
+    formatSourceString
+        """
+type X = X
+    with
+
+        static member x = 1
+    """
+        config
+    |> prepend newline
+    |> should
+        equal
+        """
+type X = X
+    with
+
+        static member x = 1
+"""

src/Fantomas.Core/CodePrinter.fs

@@ -3585,16 +3585,16 @@ let genTypeDefn (td: TypeDefn) =
              | _ ->
                  header
                  +> sepSpaceOrIndentAndNlnIfExpressionExceedsPageWidth (genType node.Type)
-                 +> onlyIf
-                     hasMembers
-                     (optSingle
-                         (fun withNode ->
-                             indentSepNlnUnindent (
-                                 genSingleTextNode withNode
-                                 +> onlyIfCtx (fun ctx -> ctx.Config.NewlineBetweenTypeDefinitionAndMembers) sepNln
-                                 +> indentSepNlnUnindent (genMemberDefnList members)
-                             ))
-                         typeName.WithKeyword)
+                 +> (match List.tryHead members, typeName.WithKeyword with
+                     | Some firstMember, Some withNode ->
+                         indentSepNlnUnindent (
+                             genSingleTextNode withNode
+                             +> onlyIfCtx
+                                 (fun ctx -> ctx.Config.NewlineBetweenTypeDefinitionAndMembers)
+                                 (sepNlnUnlessContentBefore (MemberDefn.Node firstMember))
+                             +> indentSepNlnUnindent (genMemberDefnList members)
+                         )
+                     | _ -> sepNone)
                  |> genNode node)
                 ctx
     | TypeDefn.Explicit node ->