Allow calling method with both Optional and ParamArray

docs/release-notes/.FSharp.Compiler.Service/8.0.300.md

@@ -9,6 +9,7 @@
 * Fix crash in DOTNET_SYSTEM_GLOBALIZATION_INVARIANT mode ([PR #16471](https://github.com/dotnet/fsharp/pull/16471))
 * `[<CliEvent>]` member should not produce property symbol. ([Issue #16640](https://github.com/dotnet/fsharp/issues/16640), [PR #16658](https://github.com/dotnet/fsharp/pull/16658))
 * Fix discriminated union initialization. ([#PR 16661](https://github.com/dotnet/fsharp/pull/16661))
+* Allow calling method with both Optional and ParamArray. ([#PR 16688](https://github.com/dotnet/fsharp/pull/16688), [suggestions #1120](https://github.com/fsharp/fslang-suggestions/issues/1120))
 
 ### Added
 

src/Compiler/Checking/MethodCalls.fs

@@ -556,6 +556,12 @@ type CalledMeth<'T>
                 let nUnnamedCalledArgs = unnamedCalledArgs.Length
                 if allowOutAndOptArgs && nUnnamedCallerArgs < nUnnamedCalledArgs then
                     let unnamedCalledArgsTrimmed, unnamedCalledOptOrOutArgs = List.splitAt nUnnamedCallerArgs unnamedCalledArgs
+
+                    // take the last ParamArray arg out, make it not break the optional/out params check
+                    let unnamedCalledArgsTrimmed, unnamedCalledOptOrOutArgs =
+                        match List.rev unnamedCalledOptOrOutArgs with
+                        | h :: t when h.IsParamArray -> unnamedCalledArgsTrimmed @ [h], List.rev t
+                        | _ -> unnamedCalledArgsTrimmed, unnamedCalledOptOrOutArgs
 
                     let isOpt x = x.OptArgInfo.IsOptional
                     let isOut x = x.IsOutArg && isByrefTy g x.CalledArgumentType

tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/MethodResolution.fs

@@ -262,3 +262,34 @@ let _, _ = Thing.Do()
             (Error 501, Line 6, Col 12, Line 6, Col 22, "The member or object constructor 'Do' takes 1 argument(s) but is here given 0. The required signature is 'static member Thing.Do: [<Optional>] i: outref<bool> -> bool'.")
         ]
 
+    [<Fact>]
+    let ``optional and ParamArray parameter resolves correctly `` () =
+        Fsx """
+open System.Runtime.InteropServices
+
+type Thing =
+    static member Do(
+        [<Optional; DefaultParameterValue "">] something: string, 
+        [<System.ParamArray>] args: obj[]) = something, args
+    static member Do2(
+        [<Optional; DefaultParameterValue "">] something: string, 
+        outvar: outref<int>,
+        [<System.ParamArray>] args: obj[]) = 
+
+        outvar <- 1
+        something, args
+let _, _ = Thing.Do()
+let _, _ = Thing.Do("123")
+let _, _ = Thing.Do("123", 1, 2, 3, 4)
+
+let _, _ = Thing.Do2()
+let _, _ = Thing.Do2("123")
+let _ =
+    let mutable x = 0
+    Thing.Do2("123", &x)
+let _ =
+    let mutable x = 0
+    Thing.Do2("123", &x, 1, 2, 3, 4)
+    """
+        |> typecheck
+        |> shouldSucceed