Fix missing [<TailCall>] warning in list comprehension (#16652)

* Add test for recursive call in list comprehension * Change the default to TailCall.No when checking the args of a Coerce * add release-notes entry * add PR number to release notes
dotnet · Feb 7, 2024 · 2e021f5 · 2e021f5
1 parent 0de949c
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diff --git a/docs/release-notes/.FSharp.Compiler.Service/8.0.300.md b/docs/release-notes/.FSharp.Compiler.Service/8.0.300.md
@@ -1,5 +1,6 @@
 ### Fixed
 
+* Fix missing warning for recursive calls in list comprehensions. ([PR #16652](https://github.com/dotnet/fsharp/pull/16652))
 * Code generated files with > 64K methods and generated symbols crash when loaded. Use infered sequence points for debugging. ([Issue #16399](https://github.com/dotnet/fsharp/issues/16399), [#PR 16514](https://github.com/dotnet/fsharp/pull/16514))
 * `nameof Module` expressions and patterns are processed to link files in `--test:GraphBasedChecking`. ([PR #16550](https://github.com/dotnet/fsharp/pull/16550))
 * Graph Based Checking doesn't throw on invalid parsed input so it can be used for IDE scenarios ([PR #16575](https://github.com/dotnet/fsharp/pull/16575), [PR #16588](https://github.com/dotnet/fsharp/pull/16588), [PR #16643](https://github.com/dotnet/fsharp/pull/16643))

diff --git a/src/Compiler/Checking/TailCallChecks.fs b/src/Compiler/Checking/TailCallChecks.fs
@@ -539,12 +539,10 @@ and CheckExprOp cenv (op, tyargs, args, m) ctxt : unit =
     | TOp.ValFieldSet _rf, _, [ _arg1; _arg2 ] -> ()
 
     | TOp.Coerce, [ tgtTy; srcTy ], [ x ] ->
-        let tailCall = TailCall.YesFromExpr cenv.g x
-
         if TypeDefinitelySubsumesTypeNoCoercion 0 g cenv.amap m tgtTy srcTy then
-            CheckExpr cenv x ctxt tailCall
+            CheckExpr cenv x ctxt TailCall.No
         else
-            CheckExprNoByrefs cenv tailCall x
+            CheckExprNoByrefs cenv TailCall.No x
 
     | TOp.Reraise, [ _ty1 ], [] -> ()
 

diff --git a/tests/FSharp.Compiler.ComponentTests/ErrorMessages/TailCallAttribute.fs b/tests/FSharp.Compiler.ComponentTests/ErrorMessages/TailCallAttribute.fs
@@ -1512,3 +1512,29 @@ module Microsoft.FSharp.Core
               Message =
                 "The member or function 'f' has the 'TailCallAttribute' attribute, but is not being used in a tail recursive way." }
         ]
+
+    [<FSharp.Test.FactForNETCOREAPP>]
+    let ``Warn for recursive call in list comprehension`` () =
+        """
+namespace N
+
+    module M =
+
+        [<TailCall>]
+        let rec reverse (input: list<'t>) =
+            match input with
+            | head :: tail -> [ yield! reverse tail; head ]
+            | [] -> []
+        """
+        |> FSharp
+        |> compile
+        |> shouldFail
+        |> withResults [
+            { Error = Warning 3569
+              Range = { StartLine = 9
+                        StartColumn = 40
+                        EndLine = 9
+                        EndColumn = 52 }
+              Message =
+                "The member or function 'reverse' has the 'TailCallAttribute' attribute, but is not being used in a tail recursive way." }
+        ]