Add a test showing that order of patterns matters

src/Fantomas.Tests/PipingTests.fs

@@ -45,4 +45,21 @@ let m =
     | y -> ErrorMessage msg
     | _ -> LogMessage(msg, true)
     |> console.Write
+"""
+
+[<Test>]
+let ``should break new lines on piping``() =
+    formatSourceString false """
+let runAll() =
+    urlList
+    |> Seq.map fetchAsync |> Async.Parallel 
+    |> Async.RunSynchronously |> ignore""" config
+    |> prepend newline
+    |> should equal """
+let runAll() = 
+    urlList
+    |> Seq.map fetchAsync
+    |> Async.Parallel
+    |> Async.RunSynchronously
+    |> ignore
 """

src/Fantomas/CodePrinter.fs

@@ -12,7 +12,8 @@ module List =
         | [] | [_] -> true
         | _ -> false
 
-/// Check whether an expression should be broken into multiple lines
+/// Check whether an expression should be broken into multiple lines. 
+/// Notice that order of patterns matter due to non-disjoint property
 let rec multiline = function
     | ConstExpr _
     | NullExpr
@@ -52,6 +53,12 @@ let rec multiline = function
 
     | Tuple es ->
         List.exists multiline es
+
+    // An infix app is multiline if it contains at least two new line infix ops
+    | InfixApps(e, es) ->
+        multiline e
+        || not (List.atmostOne (List.filter (fst >> NewLineInfixOps.Contains) es))
+        || List.exists (snd >> multiline) es
 
     | App(e1, es) ->
         multiline e1 || List.exists multiline es
@@ -75,12 +82,6 @@ let rec multiline = function
         let fields = xs |> List.choose ((|RecordFieldName|) >> snd) 
         not (List.atmostOne fields) || List.exists multiline fields
 
-    // An infix app is multiline if it contains at least two new line infix ops
-    | InfixApps(e, es) ->
-        multiline e
-        || not (List.atmostOne (List.filter (fst >> NewLineInfixOps.Contains) es))
-        || List.exists (snd >> multiline) es
-
     // Default mode is single-line
     | _ -> false