Fix false negative [<TailCall>] warning (#18399)

Author: dawedawe

docs/release-notes/.FSharp.Compiler.Service/9.0.300.md

@@ -1,4 +1,5 @@
 ### Fixed
+* Fix missing TailCall warning in TOp.IntegerForLoop ([PR #18399](https://github.com/dotnet/fsharp/pull/18399))
 * Fix classification of `nameof` in `nameof<'T>`, `match … with nameof ident -> …`. ([Issue #10026](https://github.com/dotnet/fsharp/issues/10026), [PR #18300](https://github.com/dotnet/fsharp/pull/18300))
 * Fix Realsig+ generates nested closures with incorrect Generic  ([Issue #17797](https://github.com/dotnet/fsharp/issues/17797), [PR #17877](https://github.com/dotnet/fsharp/pull/17877))
 * Fix optimizer internal error for records with static fields  ([Issue #18165](https://github.com/dotnet/fsharp/issues/18165), [PR #18280](https://github.com/dotnet/fsharp/pull/18280))

src/Compiler/Checking/TailCallChecks.fs

@@ -792,7 +792,12 @@ let CheckModuleBinding cenv (isRec: bool) (TBind _ as bind) =
                     // warn for recursive calls in TryWith/TryFinally operations
                     exprs |> Seq.iter (checkTailCall true)
                 | Expr.Op(args = exprs) -> exprs |> Seq.iter (checkTailCall insideSubBindingOrTry)
-                | Expr.Sequential(expr2 = expr2) -> checkTailCall insideSubBindingOrTry expr2
+                | Expr.Sequential(expr1 = expr1; expr2 = expr2) ->
+                    match expr1 with
+                    | Expr.Op(args = exprs; op = TOp.IntegerForLoop _) -> checkTailCall insideSubBindingOrTry expr1
+                    | _ -> ()
+
+                    checkTailCall insideSubBindingOrTry expr2
                 | _ -> ()
 
             checkTailCall false bodyExpr

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

@@ -773,20 +773,20 @@ namespace N
         |> compile
         |> shouldFail
         |> withResults [
-            { Error = Warning 3569
-              Range = { StartLine = 21
-                        StartColumn = 27
-                        EndLine = 21
-                        EndColumn = 35 }
-              Message =
-                "The member or function 'instType' has the 'TailCallAttribute' attribute, but is not being used in a tail recursive way." }
             { Error = Warning 3569
               Range = { StartLine = 17
                         StartColumn = 32
                         EndLine = 17
                         EndColumn = 77 }
               Message =
                 "The member or function 'instType' has the 'TailCallAttribute' attribute, but is not being used in a tail recursive way." }
+            { Error = Warning 3569
+              Range = { StartLine = 21
+                        StartColumn = 27
+                        EndLine = 21
+                        EndColumn = 35 }
+              Message =
+                "The member or function 'instType' has the 'TailCallAttribute' attribute, but is not being used in a tail recursive way." }
         ]
 
     [<FSharp.Test.FactForNETCOREAPP>]
@@ -1769,3 +1769,72 @@ module M =
         |> withLangVersion80
         |> compile
         |> shouldSucceed
+
+    [<FSharp.Test.FactForNETCOREAPP>]
+    let ``Warn successfully in Array-mapped recursive call`` () =
+        """
+namespace N
+
+module M =
+
+    type Value =
+        { Code: string }
+
+    [<TailCall>]
+    let rec fooArray (values: Value[], code: string) =
+        match values with
+        | [||] -> seq { code }
+        | values ->
+            let replicatedValues =
+                values
+                |> Array.map (fun value -> fooArray (values, value.Code))
+            replicatedValues |> Seq.concat
+        """
+        |> FSharp
+        |> withLangVersion80
+        |> compile
+        |> shouldFail
+        |> withResults [
+            { Error = Warning 3569
+              Range = { StartLine = 16
+                        StartColumn = 20
+                        EndLine = 16
+                        EndColumn = 74 }
+              Message =
+               "The member or function 'fooArray' has the 'TailCallAttribute' attribute, but is not being used in a tail recursive way." }
+        ]
+        
+    [<FSharp.Test.FactForNETCOREAPP>]
+    let ``Warn successfully in Seq-mapped recursive call`` () =
+        """
+namespace N
+
+module M =
+
+    type Value =
+        { Code: string }
+
+    [<TailCall>]
+    let rec fooSeq (values: Value[], code: string) =
+        match values with
+        | [||] -> seq { code }
+        | values ->
+            let replicatedValues =
+                values
+                |> Seq.map (fun value -> fooSeq (values, value.Code))
+                |> Seq.toArray
+            replicatedValues |> Seq.concat
+        """
+        |> FSharp
+        |> withLangVersion80
+        |> compile
+        |> shouldFail
+        |> withResults [
+            { Error = Warning 3569
+              Range = { StartLine = 16
+                        StartColumn = 42
+                        EndLine = 16
+                        EndColumn = 48 }
+              Message =
+               "The member or function 'fooSeq' has the 'TailCallAttribute' attribute, but is not being used in a tail recursive way." }
+        ]