Fix wrong [<TailCall>] warning with unit-returning match expression (#…

…17637)

* add unit test for issue 17604

* check if the expr type is unit for match expressions, too.

* add release notes entry

---------

Co-authored-by: Petr <psfinaki@users.noreply.github.com>

docs/release-notes/.FSharp.Compiler.Service/9.0.100.md

@@ -1,5 +1,6 @@
 ### Fixed
 
+* Fix wrong TailCall warning ([Issue #17604](https://github.com/dotnet/fsharp/issues/17604), [PR #17637](https://github.com/dotnet/fsharp/pull/17637))
 * Compiler hangs when compiling inline recursive invocation ([Issue #17376](https://github.com/dotnet/fsharp/issues/17376), [PR #17394](https://github.com/dotnet/fsharp/pull/17394))
 * Fix reporting IsFromComputationExpression only for CE builder type constructors and let bindings. ([PR #17375](https://github.com/dotnet/fsharp/pull/17375))
 * Optimize simple mappings in comprehensions when the body of the mapping has `let`-bindings and/or sequential expressions before a single yield. ([PR #17419](https://github.com/dotnet/fsharp/pull/17419))

src/Compiler/Checking/TailCallChecks.fs

@@ -47,8 +47,16 @@ type TailCall =
     static member YesFromVal (g: TcGlobals) (v: Val) = TailCall.Yes(TailCall.IsVoidRet g v)
 
     static member YesFromExpr (g: TcGlobals) (expr: Expr) =
+        let yesFromTType (t: TType) =
+            if isUnitTy g t then
+                TailCall.Yes TailCallReturnType.MustReturnVoid
+            else
+                TailCall.Yes TailCallReturnType.NonVoid
+
         match expr with
         | ValUseAtApp(valRef, _) -> TailCall.Yes(TailCall.IsVoidRet g valRef.Deref)
+        | Expr.Const(constType = constType) -> yesFromTType constType
+        | Expr.Match(exprType = exprType) -> yesFromTType exprType
         | _ -> TailCall.Yes TailCallReturnType.NonVoid
 
     member x.AtExprLambda =

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

@@ -1722,3 +1722,22 @@ module M =
         |> withLangVersion80
         |> compile
         |> shouldSucceed
+
+    [<FSharp.Test.FactForNETCOREAPP>]
+    let ``Don't warn for tail rec call returning unit`` () =
+        """
+namespace N
+
+module M =
+
+    [<TailCall>]
+    let rec go (args: string list) =
+        match args with
+        | [] -> ()
+        | "--" :: _ -> ()
+        | arg :: args -> go args
+        """
+        |> FSharp
+        |> withLangVersion80
+        |> compile
+        |> shouldSucceed