Improve completions for union field patterns (#15754)

Author: kerams

src/Compiler/Service/ServiceParsedInputOps.fs

@@ -1269,20 +1269,29 @@ module ParsedInput =
                 else
                     let context = Some(PatternContext.NamedUnionCaseField(patId.idText, id.Range))
                     TryGetCompletionContextInPattern suppressIdentifierCompletions pat context pos)
-        | SynPat.LongIdent (argPats = SynArgPats.Pats pats; longDotId = id) ->
+        | SynPat.LongIdent (argPats = SynArgPats.Pats pats; longDotId = id; range = m) when rangeContainsPos m pos ->
             match pats with
-            | [ SynPat.Named _ as pat ] ->
-                TryGetCompletionContextInPattern false pat (Some(PatternContext.PositionalUnionCaseField(None, id.Range))) pos
+
+            // fun (Some v| ) ->
+            | [ SynPat.Named _ ] -> Some(CompletionContext.Pattern(PatternContext.PositionalUnionCaseField(None, id.Range)))
+
+            // fun (Case (| )) ->
+            | [ SynPat.Paren (SynPat.Const (SynConst.Unit, _), m) ] when rangeContainsPos m pos ->
+                Some(CompletionContext.Pattern(PatternContext.PositionalUnionCaseField(Some 0, id.Range)))
+
+            // fun (Case (a| , b)) ->
             | [ SynPat.Paren (SynPat.Tuple _ | SynPat.Named _ as pat, _) ] ->
                 TryGetCompletionContextInPattern false pat (Some(PatternContext.PositionalUnionCaseField(Some 0, id.Range))) pos
+                |> Option.orElseWith (fun () -> Some CompletionContext.Invalid)
+
             | _ ->
                 pats
                 |> List.tryPick (fun pat -> TryGetCompletionContextInPattern false pat None pos)
         | SynPat.Ands (pats = pats)
         | SynPat.ArrayOrList (elementPats = pats) ->
             pats
             |> List.tryPick (fun pat -> TryGetCompletionContextInPattern suppressIdentifierCompletions pat None pos)
-        | SynPat.Tuple (elementPats = pats) ->
+        | SynPat.Tuple (elementPats = pats; commaRanges = commas; range = m) ->
             pats
             |> List.indexed
             |> List.tryPick (fun (i, pat) ->
@@ -1295,6 +1304,15 @@ module ParsedInput =
                         None
 
                 TryGetCompletionContextInPattern suppressIdentifierCompletions pat context pos)
+            |> Option.orElseWith (fun () ->
+                // Last resort - check for fun (Case (a, | )) ->
+                // That is, pos is after the last comma and before the end of the tuple
+                match previousContext, List.tryLast commas with
+                | Some (PatternContext.PositionalUnionCaseField (_, caseIdRange)), Some mComma when
+                    rangeBeforePos mComma pos && rangeContainsPos m pos
+                    ->
+                    Some(CompletionContext.Pattern(PatternContext.PositionalUnionCaseField(Some(pats.Length - 1), caseIdRange)))
+                | _ -> None)
         | SynPat.Named (range = m) when rangeContainsPos m pos ->
             if suppressIdentifierCompletions then
                 Some CompletionContext.Invalid
@@ -1312,7 +1330,7 @@ module ParsedInput =
             TryGetCompletionContextInPattern suppressIdentifierCompletions pat1 None pos
             |> Option.orElseWith (fun () -> TryGetCompletionContextInPattern suppressIdentifierCompletions pat2 None pos)
         | SynPat.IsInst (_, m) when rangeContainsPos m pos -> Some CompletionContext.Type
-        | SynPat.Wild m when rangeContainsPos m pos -> Some CompletionContext.Invalid
+        | SynPat.Wild m when rangeContainsPos m pos && m.StartColumn <> m.EndColumn -> Some CompletionContext.Invalid
         | SynPat.Typed (pat = pat; targetType = synType) ->
             if rangeContainsPos pat.Range pos then
                 TryGetCompletionContextInPattern suppressIdentifierCompletions pat previousContext pos

vsintegration/tests/FSharp.Editor.Tests/CompletionProviderTests.fs

@@ -1674,6 +1674,22 @@ let x (du: Du list) =
         VerifyCompletionList(fileContents, "| [ C (first, rest); C (f", [ "option" ], [ "first" ])
         VerifyCompletionList(fileContents, "| [ C (first, rest); C (f, l", [ "list" ], [ "rest" ])
 
+    [<Fact>]
+    let ``Completion list contains relevant items for the correct union case field pattern before its identifier has been typed`` () =
+        let fileContents =
+            """
+type Du =
+    | C of first: Du * second: Result<int, string>
+
+let x du =
+    match du with
+    | C () -> ()
+    | C  (first, ) -> ()
+"""
+
+        VerifyCompletionList(fileContents, "| C (", [ "first"; "du" ], [ "second"; "result" ])
+        VerifyCompletionList(fileContents, "| C  (first, ", [ "second"; "result" ], [ "first"; "du" ])
+
     [<Fact>]
     let ``Completion list contains suggested names for union case field pattern in a let binding, lambda and member`` () =
         let fileContents =