Completion: fix completion in empty dot lambda prefix (#16829)

Author: auduchinok

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

@@ -25,6 +25,7 @@
 * Enforce AttributeTargets on enums ([PR #16887](https://github.com/dotnet/fsharp/pull/16887))
 * Completion: fix for unfinished record field decl ([PR #16893](https://github.com/dotnet/fsharp/pull/16893))
 * Enforce AttributeTargets on delegates ([PR #16891](https://github.com/dotnet/fsharp/pull/16891))
+* Completion: fix completion in empty dot lambda prefix ([#16829](https://github.com/dotnet/fsharp/pull/16829))
 * Fix StackOverflow when checking non-recursive bindings in module or namespace in `fscAnyCpu`/`fsiAnyCpu`. ([PR #16908](https://github.com/dotnet/fsharp/pull/16908))
 
 ### Added

src/Compiler/SyntaxTree/SyntaxTreeOps.fs

@@ -100,6 +100,8 @@ let rec pushUnaryArg expr arg =
     | SynExpr.TypeApp(innerExpr, mLess, tyargs, mCommas, mGreater, mTypars, m) ->
         let innerExpr = pushUnaryArg innerExpr arg
         SynExpr.TypeApp(innerExpr, mLess, tyargs, mCommas, mGreater, mTypars, m)
+    | SynExpr.ArbitraryAfterError(_, m) when m.Start = m.End ->
+        SynExpr.DiscardAfterMissingQualificationAfterDot(SynExpr.Ident arg, m.StartRange, unionRanges arg.idRange m)
     | _ ->
         errorR (Error(FSComp.SR.tcDotLambdaAtNotSupportedExpression (), expr.Range))
         expr

tests/service/CompletionTests.fs

@@ -82,6 +82,14 @@ let myFancyFunc (x:string) =
     |> _.ToL"""
     assertHasItemWithNames ["ToLower"] info
 
+[<Test>]
+let ``Underscore dot lambda - No prefix`` () =
+    let info = getCompletionInfo "[s] |> List.map _. " (3, 18) """
+let s = ""
+[s] |> List.map _. 
+"""
+    assertHasItemWithNames ["Length"] info
+
 [<Test>]
 let ``Type decl - Record - Field type 01`` () =
     let info = getCompletionInfo "type Record = { Field:  }" (2, 23)  """