Fix 15958: Missing warning FS3365 for SynLongIdent (dotless indexing vs application) (#16224)

* Add tests for FS3365

* improve tests

* add missing warning for 3365

* format

* refactor

Author: dawedawe

src/Compiler/Checking/CheckExpressions.fs

@@ -3950,6 +3950,16 @@ let GetInstanceMemberThisVariable (vspec: Val, expr) =
     else
         None
 
+/// c.atomicLeftMethExpr[idx] and atomicLeftExpr[idx] as applications give warnings
+let checkHighPrecedenceFunctionApplicationToList (g: TcGlobals) args atomicFlag exprRange =
+    match args, atomicFlag with
+    | ([SynExpr.ArrayOrList (false, _, _)] | [SynExpr.ArrayOrListComputed (false, _, _)]), ExprAtomicFlag.Atomic ->
+        if g.langVersion.SupportsFeature LanguageFeature.IndexerNotationWithoutDot then
+            informationalWarning(Error(FSComp.SR.tcHighPrecedenceFunctionApplicationToListDeprecated(), exprRange))
+        elif not (g.langVersion.IsExplicitlySpecifiedAs50OrBefore()) then
+            informationalWarning(Error(FSComp.SR.tcHighPrecedenceFunctionApplicationToListReserved(), exprRange))
+    | _ -> ()
+
 /// Indicates whether a syntactic type is allowed to include new type variables
 /// not declared anywhere, e.g. `let f (x: 'T option) = x.Value`
 type ImplicitlyBoundTyparsAllowed =
@@ -8202,13 +8212,7 @@ and TcApplicationThen (cenv: cenv) (overallTy: OverallTy) env tpenv mExprAndArg
 
         // atomicLeftExpr[idx] unifying as application gives a warning 
         if not isSugar then
-            match synArg, atomicFlag with
-            | (SynExpr.ArrayOrList (false, _, _) | SynExpr.ArrayOrListComputed (false, _, _)), ExprAtomicFlag.Atomic ->
-                if g.langVersion.SupportsFeature LanguageFeature.IndexerNotationWithoutDot then
-                    informationalWarning(Error(FSComp.SR.tcHighPrecedenceFunctionApplicationToListDeprecated(), mExprAndArg))
-                elif not (g.langVersion.IsExplicitlySpecifiedAs50OrBefore()) then
-                    informationalWarning(Error(FSComp.SR.tcHighPrecedenceFunctionApplicationToListReserved(), mExprAndArg))
-            | _ -> ()
+            checkHighPrecedenceFunctionApplicationToList g [synArg] atomicFlag mExprAndArg
 
         match leftExpr with
         | ApplicableExpr(expr=NameOfExpr g _) when g.langVersion.SupportsFeature LanguageFeature.NameOf ->
@@ -9366,6 +9370,9 @@ and TcMethodApplicationThen
     // Nb. args is always of List.length <= 1 except for indexed setters, when it is 2
     let mWholeExpr = (m, args) ||> List.fold (fun m arg -> unionRanges m arg.Range)
 
+    // c.atomicLeftMethExpr[idx] as application gives a warning
+    checkHighPrecedenceFunctionApplicationToList g args atomicFlag mWholeExpr
+
     // Work out if we know anything about the return type of the overall expression. If there are any delayed
     // lookups then we don't know anything.
     let exprTy = if isNil delayed then overallTy else MustEqual (NewInferenceType g)

tests/FSharp.Compiler.ComponentTests/ErrorMessages/IndexingSyntax.fs

@@ -0,0 +1,74 @@
+namespace FSharp.Compiler.ComponentTests.ErrorMessages
+
+open FSharp.Test.Compiler
+open FSharp.Test.Compiler.Assertions.StructuredResultsAsserts
+
+module ``Indexing Syntax`` =
+
+    [<FSharp.Test.FactForNETCOREAPP>]
+    let ``Warn successfully for SynExpr.Ident app`` () =
+        """
+namespace N
+
+    module M =
+
+        let f (a: int list) = a
+        
+        let g () = f [1] // should not warn
+        
+        let h () = f[1] // should warn
+        """
+        |> FSharp
+        |> withLangVersion70
+        |> compile
+        |> shouldFail
+        |> withResults
+            [
+                {
+                    Error = Information 3365
+                    Range =
+                        {
+                            StartLine = 10
+                            StartColumn = 20
+                            EndLine = 10
+                            EndColumn = 24
+                        }
+                    Message =
+                        "The syntax 'expr1[expr2]' is used for indexing. Consider adding a type annotation to enable indexing, or if calling a function add a space, e.g. 'expr1 [expr2]'."
+                }
+            ]
+
+    [<FSharp.Test.FactForNETCOREAPP>]
+    let ``Warn successfully for SynExpr.LongIdent app`` () =
+        """
+namespace N
+
+    module N =
+
+        type C () =
+            member _.MyFunc (inputList: int list) = inputList
+
+        let g () =
+            let c = C()
+            let _ = c.MyFunc [23]   // should not warn
+            c.MyFunc[42]    // should warn
+        """
+        |> FSharp
+        |> withLangVersion70
+        |> compile
+        |> shouldFail
+        |> withResults
+            [
+                {
+                    Error = Information 3365
+                    Range =
+                        {
+                            StartLine = 12
+                            StartColumn = 13
+                            EndLine = 12
+                            EndColumn = 25
+                        }
+                    Message =
+                        "The syntax 'expr1[expr2]' is used for indexing. Consider adding a type annotation to enable indexing, or if calling a function add a space, e.g. 'expr1 [expr2]'."
+                }
+            ]

tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj

@@ -167,6 +167,7 @@
     <Compile Include="EmittedIL\FixedBindings\FixedBindings.fs" />
     <Compile Include="ErrorMessages\UnsupportedAttributes.fs" />
     <Compile Include="ErrorMessages\TailCallAttribute.fs" />
+    <Compile Include="ErrorMessages\IndexingSyntax.fs" />
     <Compile Include="ErrorMessages\TypeEqualsMissingTests.fs" />
     <Compile Include="ErrorMessages\AccessOfTypeAbbreviationTests.fs" />
     <Compile Include="ErrorMessages\AssignmentErrorTests.fs" />