Fix IndexOutOfRangeException in check for providing completion (#4138)

* Fix IndexOutOfRange in check for providing completion:

* Add test

vsintegration/src/FSharp.Editor/Completion/CompletionProvider.fs

@@ -85,10 +85,9 @@ type internal FSharpCompletionProvider
         else
             let triggerPosition = caretPosition - 1
             let triggerChar = sourceText.[triggerPosition]
-            let prevChar = sourceText.[triggerPosition - 1]
-            
+
             // do not trigger completion if it's not single dot, i.e. range expression
-            if not Settings.IntelliSense.ShowAfterCharIsTyped && prevChar = '.' then
+            if not Settings.IntelliSense.ShowAfterCharIsTyped && triggerPosition > 0 && sourceText.[triggerPosition - 1] = '.' then
                 false
             else
                 let documentId, filePath, defines = getInfo()

vsintegration/tests/unittests/CompletionProviderTests.fs

@@ -273,6 +273,18 @@ xVal**y
         let triggered = FSharpCompletionProvider.ShouldTriggerCompletionAux(SourceText.From(fileContents), caretPosition, CompletionTriggerKind.Insertion, getInfo)
         Assert.IsTrue(triggered, "Completion should trigger after typing an identifier that follows a mathematical operation")
 
+[<Test>]
+let ShouldTriggerCompletionAtStartOfFileWithInsertion =
+    let fileContents = """
+l"""
+
+    let marker = "l"
+    let caretPosition = fileContents.IndexOf(marker) + marker.Length
+    let documentId = DocumentId.CreateNewId(ProjectId.CreateNewId())
+    let getInfo() = documentId, filePath, []
+    let triggered = FSharpCompletionProvider.ShouldTriggerCompletionAux(SourceText.From(fileContents), caretPosition, CompletionTriggerKind.Insertion, getInfo)
+    Assert.IsTrue(triggered, "Completion should trigger after typing an Insertion character at the top of the file, e.g. a function definition in a new script file.")
+
 [<Test>]
 let ShouldDisplayTypeMembers() =
     let fileContents = """