Respect named tuple arguments in type with constraints.

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## [Unreleased]
+
+### Fixed
+* Option parameter name is lost in tuple. [#2144](https://github.com/fsprojects/fantomas/issues/2144)
+
 ## [4.7.2] - 2022-03-11
 
 ### Fixed

src/Fantomas.Tests/SignatureTests.fs

@@ -1884,3 +1884,33 @@ val SampleTupledFunction:
     arg4: int ->
         int list
 """
+
+[<Test>]
+let ``optional parameter in static type member, 2144`` () =
+    formatSourceString
+        true
+        """
+// Copyright (c) Microsoft Corporation.  All Rights Reserved.  See License.txt in the project root for license information.
+
+namespace Microsoft.FSharp.Control
+
+    [<Sealed>]
+    [<CompiledName("FSharpAsync")>]
+    type Async =
+        static member AwaitEvent: event:IEvent<'Del,'T> * ?cancelAction : (unit -> unit) -> Async<'T> when 'Del : delegate<'T,unit> and 'Del :> System.Delegate 
+"""
+        config
+    |> prepend newline
+    |> should
+        equal
+        """
+// Copyright (c) Microsoft Corporation.  All Rights Reserved.  See License.txt in the project root for license information.
+
+namespace Microsoft.FSharp.Control
+
+[<Sealed>]
+[<CompiledName("FSharpAsync")>]
+type Async =
+    static member AwaitEvent: event: IEvent<'Del, 'T> * ?cancelAction: (unit -> unit) -> Async<'T>
+        when 'Del: delegate<'T, unit> and 'Del :> System.Delegate
+"""

src/Fantomas/CodePrinter.fs

@@ -4097,17 +4097,32 @@ and genConstraints astContext (t: SynType) (vi: SynValInfo) =
             match ti, vi with
             | TFuns ts, SynValInfo (curriedArgInfos, returnType) ->
                 let namedArgInfos =
-                    (List.map List.head curriedArgInfos)
-                    @ [ returnType ]
-                    |> List.map (fun (SynArgInfo (_, _, i)) -> i)
-
-                coli sepArrow ts (fun i t ->
-                    let genNamedArg =
-                        List.tryItem i namedArgInfos
-                        |> Option.bind id
-                        |> optSingle (fun (Ident s) -> !-s +> sepColon)
-
-                    genNamedArg +> genType astContext false t)
+                    [ yield! curriedArgInfos
+                      yield [ returnType ] ]
+
+                let args = List.zip namedArgInfos ts
+
+                col sepArrow args (fun (argInfo, t) ->
+                    match argInfo, t with
+                    | [], _ -> genType astContext false t
+                    | [ SynArgInfo (_, isOptional, Some (Ident s)) ], _ ->
+                        onlyIf isOptional (!- "?")
+                        +> !-s
+                        +> sepColon
+                        +> genType astContext false t
+                    | [ SynArgInfo _ ], _ -> genType astContext false t
+                    | multipleArgInfo, TTuple ts ->
+                        let combined = List.zip multipleArgInfo ts
+
+                        col sepStar combined (fun (argInfo, (_, t)) ->
+                            let genNamed =
+                                match argInfo with
+                                | SynArgInfo (_, isOptional, Some (Ident s)) ->
+                                    onlyIf isOptional (!- "?") +> !-s +> sepColon
+                                | _ -> sepNone
+
+                            genNamed +> genType astContext false t)
+                    | _ -> sepNone)
             | _ -> genType astContext false ti
 
         genType