BUGFIX- recursive active pattern with [<return: Struct>] was not possible

src/Compiler/Checking/NameResolution.fs

@@ -92,7 +92,12 @@ let ActivePatternElemsOfValRef g (vref: ValRef) =
                 false
             else
                 let _, apReturnTy = stripFunTy g vref.TauType
-                isStructTy g apReturnTy
+                let hasStructAttribute() = 
+                    vref.Attribs
+                    |> List.exists (function 
+                        | Attrib(targetsOpt = Some(System.AttributeTargets.ReturnValue)) as a -> IsMatchingFSharpAttribute g g.attrib_StructAttribute a  
+                        | _ -> false)
+                isStructTy g apReturnTy || hasStructAttribute()
         apinfo.ActiveTags |> List.mapi (fun i _ -> APElemRef(apinfo, vref, i, isStructRetTy))
     | None -> []
 

tests/FSharp.Compiler.ComponentTests/Conformance/StructTypes/StructActivePatterns.fs

@@ -0,0 +1,71 @@
+// Copyright (c) Microsoft Corporation.  All Rights Reserved.  See License.txt in the project root for license information.
+
+module FSharp.Compiler.ComponentTests.Conformance.StructActivePatterns
+
+open Xunit
+open FSharp.Test
+open FSharp.Test.Compiler
+
+
+[<Fact>]
+let ``Struct active pattern is possible`` () =
+    Fs """
+[<return: Struct>]
+let rec (|IsOne|_|) someNumber = 
+    match someNumber with
+    | 1 -> ValueSome 1
+    | _ -> ValueNone   
+"""
+    |> withOptions ["--warnaserror+"]
+    |> typecheck
+    |> shouldSucceed
+
+[<Fact>]
+let ``Struct active pattern must not lie about its return value when using Struct attribute`` () =
+    Fs """
+[<return: Struct>]
+let rec (|IsOne|_|) someNumber = 
+    match someNumber with
+    | 1 -> Some 1
+    | _ -> None   
+"""
+    |> withOptions ["--warnaserror+"]
+    |> typecheck
+    |> shouldFail
+    |> withSingleDiagnostic (Error 1,Line 2, Col 1 , Line 3, Col 31, """This expression was expected to have type
+    ''a voption'    
+but here has type
+    'int option'    """)
+
+[<Fact>]
+let ``Voption active pattern fails if not using return:Struct attribute`` () =
+    Fs """
+let rec (|IsOne|_|) someNumber = 
+    match someNumber with
+    | 1 -> ValueSome 1
+    | _ -> ValueNone   
+"""
+    |> withOptions ["--warnaserror+"]
+    |> typecheck
+    |> shouldFail
+    |> withSingleDiagnostic (Error 1,Line 2, Col 9 , Line 2, Col 31, """This expression was expected to have type
+    ''a option'    
+but here has type
+    'int voption'    """)
+
+[<Fact>]
+let ``Rec struct active pattern is possible`` () =
+    Fs """
+[<return: Struct>]
+let rec (|HasOne|_|) xs = 
+    match xs with
+    | [] -> ValueNone
+    | h::_ when h = 1 -> ValueSome true
+    | _::tail ->
+        match tail with
+        | HasOne x -> ValueSome x
+        | _ -> ValueNone
+"""
+    |> withOptions ["--warnaserror+"]
+    |> typecheck
+    |> shouldSucceed

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

@@ -90,6 +90,7 @@
     <Compile Include="Conformance\RecordTypes\AnonymousRecords.fs" />
     <Compile Include="Conformance\RecordTypes\RecordTypes.fs" />
     <Compile Include="Conformance\StructTypes\StructTypes.fs" />
+    <Compile Include="Conformance\StructTypes\StructActivePatterns.fs" />	  
     <Compile Include="Conformance\TypesAndTypeConstraints\CheckingSyntacticTypes\CheckingSyntacticTypes.fs" />
     <Compile Include="Conformance\TypesAndTypeConstraints\LogicalPropertiesOfTypes\LogicalPropertiesOfTypes.fs" />
     <Compile Include="Conformance\TypesAndTypeConstraints\IWSAMsAndSRTPs\IWSAMsAndSRTPsTests.fs" />