Return None when Trivia is missing for Ident in SynLongIdent.

src/Compiler/SyntaxTree/SyntaxTree.fs

@@ -47,10 +47,14 @@ type SynLongIdent =
     member this.IdentsWithTrivia =
         let (SynLongIdent (lid, _, trivia)) = this
 
-        if lid.Length <> trivia.Length then
-            failwith "difference between idents and trivia"
-        else
+        if lid.Length = trivia.Length then
+            List.zip lid trivia |> List.map SynIdent
+        elif lid.Length > trivia.Length then
+            let delta = lid.Length - trivia.Length
+            let trivia = [ yield! trivia; yield! List.replicate delta None ]
             List.zip lid trivia |> List.map SynIdent
+        else
+            failwith "difference between idents and trivia"
 
     member this.ThereIsAnExtraDotAtTheEnd =
         match this with

src/Compiler/SyntaxTree/SyntaxTreeOps.fs

@@ -468,7 +468,7 @@ let mkSynDotMissing mDot m l =
     | SynExpr.LongIdent (isOpt, SynLongIdent (lid, dots, trivia), None, _) ->
         // REVIEW: MEMORY PERFORMANCE: This list operation is memory intensive (we create a lot of these list nodes)
         SynExpr.LongIdent(isOpt, SynLongIdent(lid, dots @ [ mDot ], trivia), None, m)
-    | SynExpr.Ident id -> SynExpr.LongIdent(false, SynLongIdent([ id ], [ mDot ], []), None, m)
+    | SynExpr.Ident id -> SynExpr.LongIdent(false, SynLongIdent([ id ], [ mDot ], [ None ]), None, m)
     | SynExpr.DotGet (e, dm, SynLongIdent (lid, dots, trivia), _) -> SynExpr.DotGet(e, dm, SynLongIdent(lid, dots @ [ mDot ], trivia), m) // REVIEW: MEMORY PERFORMANCE: This is memory intensive (we create a lot of these list nodes)
     | expr -> SynExpr.DiscardAfterMissingQualificationAfterDot(expr, m)
 

tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.Tests.fsproj

@@ -113,6 +113,9 @@
     <Compile Include="..\service\SyntaxTreeTests\OperatorNameTests.fs">
       <Link>SyntaxTree\OperatorNameTests.fs</Link>
     </Compile>
+    <Compile Include="..\service\SyntaxTreeTests\SynIdentTests.fs">
+      <Link>SyntaxTree\SynIdentTests.fs</Link>
+    </Compile>
     <Compile Include="..\service\FileSystemTests.fs">
       <Link>FileSystemTests.fs</Link>
     </Compile>

tests/service/SyntaxTreeTests/SynIdentTests.fs

@@ -0,0 +1,31 @@
+module FSharp.Compiler.Service.Tests.SyntaxTreeTests.SynIdentTests
+
+open FSharp.Compiler.Service.Tests.Common
+open FSharp.Compiler.Syntax
+open FSharp.Compiler.Text
+open NUnit.Framework
+
+[<Test>]
+let ``Incomplete long ident`` () =
+    let ast =
+        """
+module Module
+
+A.
+"""
+        |> getParseResults
+
+    match ast with
+    | ParsedInput.ImplFile (ParsedImplFileInput(modules = [ SynModuleOrNamespace.SynModuleOrNamespace(decls =
+        [ SynModuleDecl.Expr(expr = SynExpr.LongIdent (longDotId = lid)) ]) ])) ->
+        Assert.AreEqual(1, lid.IdentsWithTrivia.Length)
+    | _ -> Assert.Fail $"Could not get valid AST, got {ast}"
+
+[<Test>]
+let ``IdentsWithTrivia with unbalance collection should not throw`` () =
+    let synLongIdent =
+        SynLongIdent([ Ident("A", Range.Zero); Ident("B", Range.Zero) ], [ Range.Zero ], [ None ])
+
+    match synLongIdent.IdentsWithTrivia with
+    | [ SynIdent (_, None); SynIdent (_, None) ] -> Assert.Pass()
+    | identsWithTrivia -> Assert.Fail $"Unexpected identsWithTrivia, got {identsWithTrivia}"