instead of using a potentially wrong config for argument nodes, check…

… if the node.Expr is an AppSingleParenArg and just be always atomic in that case when writing the FunctionExpr

src/Fantomas.Core.Tests/DotLambdaTests.fs

@@ -104,3 +104,30 @@ let ``idempotency problem when _.property shorthand quoted, 3050`` () =
         """
 "ABC" |> _.``to Lower``()
 """
+
+[<Test>]
+let ``idempotency problem when _.Property shorthand with app arg, 3050`` () =
+    formatSourceString
+        """
+let Meh () = 1
+
+type Bar() =
+    member this.Foo(v:int):int = v + 1
+
+let b = Bar()
+b |> _.Foo(Meh ())
+"""
+        { config with
+            SpaceBeforeUppercaseInvocation = true }
+    |> prepend newline
+    |> should
+        equal
+        """
+let Meh () = 1
+
+type Bar() =
+    member this.Foo(v: int) : int = v + 1
+
+let b = Bar ()
+b |> _.Foo(Meh ())
+"""

src/Fantomas.Core/CodePrinter.fs

@@ -1554,19 +1554,9 @@ let genExpr (e: Expr) =
     | Expr.Typar node -> genSingleTextNode node
     | Expr.DotLambda node ->
         let genDotLambdaExpr expr =
-            fun ctx ->
-                let currentConfig = ctx.Config
-
-                let dotLambdaConfig =
-                    { currentConfig with
-                        // override the SpaceBefore[Upper|Lower]caseInvocation to avoid generating breaking code.
-                        // see unit tests for issue 3050``
-                        SpaceBeforeUppercaseInvocation = false
-                        SpaceBeforeLowercaseInvocation = false }
-
-                genExpr expr { ctx with Config = dotLambdaConfig }
-                // Restore the existing configuration after printing the DotLambda expression
-                |> fun ctx -> { ctx with Config = currentConfig }
+            match expr with
+            | Expr.AppSingleParenArg p -> genExpr p.FunctionExpr +> genExpr p.ArgExpr // be always atomic, see 3050
+            | _ -> genExpr expr
 
         genSingleTextNode node.Underscore
         +> genSingleTextNode node.Dot