Do not write a | if it's a single case DU with a visibility modifier.

src/Fantomas.Tests/UnionTests.fs

@@ -192,7 +192,7 @@ type CustomerId =
    """ config
    |> prepend newline
    |> should equal """
-type CustomerId = private | CustomerId of int
+type CustomerId = private CustomerId of int
 """
 
 [<Test>]
@@ -306,6 +306,13 @@ let ``single case DU with fields should not have a pipe after formatting`` () =
 type DU = Record of string
 """
 
+[<Test>]
+let ``single case DU with private fields should not have a pipe after formatting`` () =
+    formatSourceString false """type String50 = private String50 of string"""  config
+    |> prepend newline
+    |> should equal """
+type String50 = private String50 of string
+"""
 
 [<Test>]
 let ``single case DU, no UnionCaseFields in signature file`` () =

src/Fantomas/CodePrinter.fs

@@ -2066,32 +2066,41 @@ and genTypeDefn astContext (TypeDef(ats, px, ao, tds, tcs, tdr, ms, s, preferPos
             +> unindent)
 
     | Simple(TDSRUnion(ao', xs) as unionNode) ->
+        let hasLeadingTrivia (t : TriviaNode) = 
+            t.Range = unionNode.Range && not (List.isEmpty t.ContentBefore)
+
         let sepNlnBasedOnTrivia =
             fun (ctx: Context) ->
                 let trivia =
-                    ctx.Trivia
-                    |> List.tryFind (fun t -> t.Range = unionNode.Range && not (List.isEmpty t.ContentBefore))
+                    ctx.Trivia |> List.tryFind hasLeadingTrivia
 
                 match trivia with
                 | Some _ -> sepNln
                 | None -> sepNone
                 <| ctx
 
-        let unionCases =
+        let unionCases ctx =
             match xs with
-            | [] -> id
-            | [UnionCase(attrs, _,_,_,(UnionCaseType fs)) as x] when List.isEmpty ms ->
-                let hasVerticalBar = Option.isSome ao' || not (List.isEmpty attrs) || List.isEmpty fs
+            | [] -> ctx
+            | [UnionCase(attrs, _,_,_,(UnionCaseType fields)) as x] when List.isEmpty ms ->
+
+                let hasVerticalBar = 
+                    (Option.isSome ao' && List.length fields <> 1) || 
+                    ctx.Trivia |> List.exists hasLeadingTrivia || 
+                    not (List.isEmpty attrs) || 
+                    List.isEmpty fields
 
                 indent +> sepSpace +> sepNlnBasedOnTrivia
                 +> genTrivia tdr.Range
                     (opt sepSpace ao' genAccess
                     +> genUnionCase { astContext with HasVerticalBar = hasVerticalBar } x)
+                <| ctx
             | xs ->
                 indent +> sepNln
                 +> genTrivia tdr.Range
                     (opt sepNln ao' genAccess
                     +> col sepNln xs (genUnionCase { astContext with HasVerticalBar = true }))
+                <| ctx
 
         typeName +> sepEq
         +> unionCases