Allow usage of [<TailCall>] with older FSharp.Core package versions

docs/release-notes/FSharp.Compiler.Service/8.0.200.md

@@ -3,4 +3,5 @@
 - Parens analysis: fix some parenthesization corner-cases in record expressions - https://github.com/dotnet/fsharp/pull/16370
 - Parens analysis: keep parens in method calls in dot-lambdas - https://github.com/dotnet/fsharp/pull/16395
 - Fixes #16359 - correctly handle imports with 0 length public key tokens - https://github.com/dotnet/fsharp/pull/16363
-- Raise a new error when interfaces with auto properties are implemented on constructor-less types - https://github.com/dotnet/fsharp/pull/16352
+- Raise a new error when interfaces with auto properties are implemented on constructor-less types - https://github.com/dotnet/fsharp/pull/16352
+- Allow usage of `[<TailCall>]` with older `FSharp.Core` package versions - https://github.com/dotnet/fsharp/pull/16373

src/Compiler/Checking/TailCallChecks.fs

@@ -738,10 +738,7 @@ let CheckModuleBinding cenv (isRec: bool) (TBind _ as bind) =
             | Some info -> info.HasNoArgs
             | _ -> false
 
-        if
-            (not isRec || isNotAFunction)
-            && HasFSharpAttribute cenv.g cenv.g.attrib_TailCallAttribute bind.Var.Attribs
-        then
+        if (not isRec || isNotAFunction) && cenv.g.HasTailCallAttrib bind.Var.Attribs then
             warning (Error(FSComp.SR.chkTailCallAttrOnNonRec (), bind.Var.Range))
 
     // Check if a let binding to the result of a rec expression is not inside the rec expression
@@ -807,7 +804,7 @@ and CheckDefnInModule cenv mdef =
                 let mustTailCall =
                     Seq.fold
                         (fun mustTailCall (v: Val) ->
-                            if HasFSharpAttribute cenv.g cenv.g.attrib_TailCallAttribute v.Attribs then
+                            if cenv.g.HasTailCallAttrib v.Attribs then
                                 let newSet = Zset.add v mustTailCall
                                 newSet
                             else

src/Compiler/TypedTree/TcGlobals.fs

@@ -1532,7 +1532,6 @@ type TcGlobals(
   member val attrib_CompilerFeatureRequiredAttribute       = findSysAttrib "System.Runtime.CompilerServices.CompilerFeatureRequiredAttribute"
   member val attrib_SetsRequiredMembersAttribute           = findSysAttrib "System.Diagnostics.CodeAnalysis.SetsRequiredMembersAttribute"
   member val attrib_RequiredMemberAttribute                = findSysAttrib "System.Runtime.CompilerServices.RequiredMemberAttribute"
-  member val attrib_TailCallAttribute                      = mk_MFCore_attrib "TailCallAttribute"
 
   member g.improveType tcref tinst = improveTy tcref tinst
 
@@ -1870,6 +1869,10 @@ type TcGlobals(
 
   member _.DebuggerNonUserCodeAttribute = debuggerNonUserCodeAttribute
 
+  member _.HasTailCallAttrib (attribs: Attribs) =
+    attribs
+    |> List.exists (fun a -> a.TyconRef.CompiledRepresentationForNamedType.FullName = "Microsoft.FSharp.Core.TailCallAttribute")
+
   member _.MakeInternalsVisibleToAttribute(simpleAssemName) =
       mkILCustomAttribute (tref_InternalsVisibleToAttribute, [ilg.typ_String], [ILAttribElem.String (Some simpleAssemName)], [])
 

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

@@ -1486,3 +1486,29 @@ namespace N
               Message =
                            "The TailCall attribute should only be applied to recursive functions." }
         ]
+
+    [<FSharp.Test.FactForNETCOREAPP>]
+    let ``Warn about self-defined attribute`` () = // is the analysis available for users of older FSharp.Core versions
+        """
+module Microsoft.FSharp.Core
+
+    open System
+
+    [<AttributeUsage(AttributeTargets.Method)>]
+    type TailCallAttribute() = inherit Attribute()
+
+    [<TailCall>]
+    let rec f x = 1 + f x
+        """
+        |> FSharp
+        |> compile
+        |> shouldFail
+        |> withResults [
+            { Error = Warning 3569
+              Range = { StartLine = 10
+                        StartColumn = 23
+                        EndLine = 10
+                        EndColumn = 26 }
+              Message =
+                "The member or function 'f' has the 'TailCallAttribute' attribute, but is not being used in a tail recursive way." }
+        ]

tests/projects/misc/SelfDefinedTailCallAttribute/Attribute.fs

@@ -0,0 +1,6 @@
+module Microsoft.FSharp.Core
+
+    open System
+
+    [<AttributeUsage(AttributeTargets.Method)>]
+    type TailCallAttribute() = inherit Attribute()

tests/projects/misc/SelfDefinedTailCallAttribute/Program.fs

@@ -0,0 +1,13 @@
+namespace N
+
+    module M =
+
+        open Microsoft.FSharp.Core
+
+        [<TailCall>]
+        let rec f x = 1 + f x
+
+        [<EntryPoint>]
+        let main argv =
+            printfn "Hello from F#"
+            0

tests/projects/misc/SelfDefinedTailCallAttribute/tailcallaltattr.fsproj

@@ -0,0 +1,20 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net6.0</TargetFramework>
+    <DisableImplicitFSharpCoreReference>true</DisableImplicitFSharpCoreReference>
+    <LangVersion>preview</LangVersion>
+  </PropertyGroup>
+
+  <PropertyGroup>
+    <DotnetFscCompilerPath>$(MSBuildThisFileDirectory)../../../../artifacts/bin/fsc/Debug/net8.0/fsc.dll</DotnetFscCompilerPath>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <Compile Include="Attribute.fs" />
+    <Compile Include="Program.fs" />
+  </ItemGroup>
+  <ItemGroup>
+    <PackageReference Include="FSharp.Core" Version="6.0.7" />
+  </ItemGroup>
+</Project>