No errors for non-virtual members overrides (#14263)

* No errors for non virtual members overrides

* Check for isFSharpObjModelTy as we are interested on csharp virtual methods

* More testing

* Add a feature lang preview

* More testing

* Fix PR comment

src/Compiler/Checking/CheckExpressions.fs

@@ -10950,7 +10950,7 @@ and ComputeIsComplete enclosingDeclaredTypars declaredTypars ty =
 /// Determine if a uniquely-identified-abstract-slot exists for an override member (or interface member implementation) based on the information available
 /// at the syntactic definition of the member (i.e. prior to type inference). If so, we know the expected signature of the override, and the full slotsig
 /// it implements. Apply the inferred slotsig.
-and ApplyAbstractSlotInference (cenv: cenv) (envinner: TcEnv) (argsAndRetTy, m, synTyparDecls, declaredTypars, memberId, tcrefObjTy, renaming, _objTy, intfSlotTyOpt, valSynData, memberFlags: SynMemberFlags, attribs) =
+and ApplyAbstractSlotInference (cenv: cenv) (envinner: TcEnv) (baseValOpt: Val option) (argsAndRetTy, m, synTyparDecls, declaredTypars, memberId, tcrefObjTy, renaming, intfSlotTyOpt, valSynData, memberFlags: SynMemberFlags, attribs) =
 
     let g = cenv.g
     let ad = envinner.eAccessRights
@@ -10997,7 +10997,21 @@ and ApplyAbstractSlotInference (cenv: cenv) (envinner: TcEnv) (argsAndRetTy, m,
                      | _ -> [] // check that method to override is sealed is located at CheckOverridesAreAllUsedOnce (typrelns.fs)
                       // We hit this case when it is ambiguous which abstract method is being implemented.
 
-
+             if g.langVersion.SupportsFeature(LanguageFeature.ErrorForNonVirtualMembersOverrides) then
+                 // Checks if the declaring type inherits from a base class and is not FSharpObjModelTy
+                 // Raises an error if we try to override an non virtual member with the same name in both
+                 match baseValOpt with
+                 | Some ttype when not(isFSharpObjModelTy g ttype.Type) ->
+                    match stripTyEqns g ttype.Type with
+                    | TType_app(tyconRef, _, _) ->
+                        let ilMethods = tyconRef.ILTyconRawMetadata.Methods.AsList()
+                        let nameOpt = ilMethods |> List.tryFind(fun id -> id.Name = memberId.idText)
+                        match nameOpt with
+                        | Some name when not name.IsVirtual ->
+                            errorR(Error(FSComp.SR.tcNoMemberFoundForOverride(), memberId.idRange))
+                        | _ -> ()
+                    | _ -> ()
+                 | _ -> ()
 
              // If we determined a unique member then utilize the type information from the slotsig
              let declaredTypars =
@@ -11159,14 +11173,14 @@ and AnalyzeRecursiveStaticMemberOrValDecl
            CheckForNonAbstractInterface declKind tcref memberFlags id.idRange
 
            let isExtrinsic = (declKind = ExtrinsicExtensionBinding)
-           let tcrefObjTy, enclosingDeclaredTypars, renaming, objTy, _ = FreshenObjectArgType cenv mBinding TyparRigidity.WillBeRigid tcref isExtrinsic declaredTyconTypars
+           let tcrefObjTy, enclosingDeclaredTypars, renaming, _, _ = FreshenObjectArgType cenv mBinding TyparRigidity.WillBeRigid tcref isExtrinsic declaredTyconTypars
            let envinner = AddDeclaredTypars CheckForDuplicateTypars enclosingDeclaredTypars envinner
            let envinner = MakeInnerEnvForTyconRef envinner tcref isExtrinsic
 
            let (ExplicitTyparInfo(_, declaredTypars, infer)) = explicitTyparInfo
 
            let optInferredImplSlotTys, declaredTypars =
-               ApplyAbstractSlotInference cenv envinner (ty, mBinding, synTyparDecls, declaredTypars, id, tcrefObjTy, renaming, objTy, intfSlotTyOpt, valSynInfo, memberFlags, bindingAttribs)
+               ApplyAbstractSlotInference cenv envinner None (ty, mBinding, synTyparDecls, declaredTypars, id, tcrefObjTy, renaming, intfSlotTyOpt, valSynInfo, memberFlags, bindingAttribs)
 
            let explicitTyparInfo = ExplicitTyparInfo(declaredTypars, declaredTypars, infer)
 
@@ -11231,7 +11245,6 @@ and AnalyzeRecursiveStaticMemberOrValDecl
     | _ ->
         envinner, tpenv, id, None, None, vis, vis2, None, [], None, explicitTyparInfo, bindingRhs, declaredTypars
 
-
 and AnalyzeRecursiveInstanceMemberDecl
        (cenv: cenv,
         envinner: TcEnv,
@@ -11290,7 +11303,7 @@ and AnalyzeRecursiveInstanceMemberDecl
          // at the member signature. If so, we know the type of this member, and the full slotsig
          // it implements. Apply the inferred slotsig.
          let optInferredImplSlotTys, declaredTypars =
-             ApplyAbstractSlotInference cenv envinner (argsAndRetTy, mBinding, synTyparDecls, declaredTypars, memberId, tcrefObjTy, renaming, objTy, intfSlotTyOpt, valSynInfo, memberFlags, bindingAttribs)
+             ApplyAbstractSlotInference cenv envinner baseValOpt (argsAndRetTy, mBinding, synTyparDecls, declaredTypars, memberId, tcrefObjTy, renaming, intfSlotTyOpt, valSynInfo, memberFlags, bindingAttribs)
 
          // Update the ExplicitTyparInfo to reflect the declaredTypars inferred from the abstract slot
          let explicitTyparInfo = ExplicitTyparInfo(declaredTypars, declaredTypars, infer)

src/Compiler/FSComp.txt

@@ -1561,6 +1561,7 @@ featureInitProperties,"support for consuming init properties"
 featureLowercaseDUWhenRequireQualifiedAccess,"Allow lowercase DU when RequireQualifiedAccess attribute"
 featureMatchNotAllowedForUnionCaseWithNoData,"Pattern match discard is not allowed for union case that takes no data."
 featureCSharpExtensionAttributeNotRequired,"Allow implicit Extension attribute on declaring types, modules"
+featureErrorForNonVirtualMembersOverrides,"Raises errors for non-virtual members overrides"
 3353,fsiInvalidDirective,"Invalid directive '#%s %s'"
 3354,tcNotAFunctionButIndexerNamedIndexingNotYetEnabled,"This value supports indexing, e.g. '%s.[index]'. The syntax '%s[index]' requires /langversion:preview. See https://aka.ms/fsharp-index-notation."
 3354,tcNotAFunctionButIndexerIndexingNotYetEnabled,"This expression supports indexing, e.g. 'expr.[index]'. The syntax 'expr[index]' requires /langversion:preview. See https://aka.ms/fsharp-index-notation."

src/Compiler/Facilities/LanguageFeatures.fs

@@ -56,6 +56,7 @@ type LanguageFeature =
     | SelfTypeConstraints
     | MatchNotAllowedForUnionCaseWithNoData
     | CSharpExtensionAttributeNotRequired
+    | ErrorForNonVirtualMembersOverrides
 
 /// LanguageVersion management
 type LanguageVersion(versionText) =
@@ -128,6 +129,7 @@ type LanguageVersion(versionText) =
                 LanguageFeature.FromEndSlicing, previewVersion
                 LanguageFeature.MatchNotAllowedForUnionCaseWithNoData, previewVersion
                 LanguageFeature.CSharpExtensionAttributeNotRequired, previewVersion
+                LanguageFeature.ErrorForNonVirtualMembersOverrides, previewVersion
 
             ]
 
@@ -237,6 +239,7 @@ type LanguageVersion(versionText) =
         | LanguageFeature.SelfTypeConstraints -> FSComp.SR.featureSelfTypeConstraints ()
         | LanguageFeature.MatchNotAllowedForUnionCaseWithNoData -> FSComp.SR.featureMatchNotAllowedForUnionCaseWithNoData ()
         | LanguageFeature.CSharpExtensionAttributeNotRequired -> FSComp.SR.featureCSharpExtensionAttributeNotRequired  ()
+        | LanguageFeature.ErrorForNonVirtualMembersOverrides -> FSComp.SR.featureErrorForNonVirtualMembersOverrides  ()
 
     /// Get a version string associated with the given feature.
     static member GetFeatureVersionString feature =

src/Compiler/Facilities/LanguageFeatures.fsi

@@ -46,6 +46,7 @@ type LanguageFeature =
     | SelfTypeConstraints
     | MatchNotAllowedForUnionCaseWithNoData
     | CSharpExtensionAttributeNotRequired
+    | ErrorForNonVirtualMembersOverrides
 
 /// LanguageVersion management
 type LanguageVersion =

src/Compiler/xlf/FSComp.txt.cs.xlf

@@ -177,6 +177,11 @@
         <target state="translated">literál float32 bez tečky</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureErrorForNonVirtualMembersOverrides">
+        <source>Raises errors for non-virtual members overrides</source>
+        <target state="new">Raises errors for non-virtual members overrides</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureErrorOnDeprecatedRequireQualifiedAccess">
         <source>give error on deprecated access of construct with RequireQualifiedAccess attribute</source>
         <target state="translated">chyba při zastaralém přístupu konstruktoru s atributem RequireQualifiedAccess</target>

src/Compiler/xlf/FSComp.txt.de.xlf

@@ -177,6 +177,11 @@
         <target state="translated">punktloses float32-Literal</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureErrorForNonVirtualMembersOverrides">
+        <source>Raises errors for non-virtual members overrides</source>
+        <target state="new">Raises errors for non-virtual members overrides</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureErrorOnDeprecatedRequireQualifiedAccess">
         <source>give error on deprecated access of construct with RequireQualifiedAccess attribute</source>
         <target state="translated">Beim veralteten Zugriff auf das Konstrukt mit dem RequireQualifiedAccess-Attribut wird ein Fehler ausgegeben.</target>

src/Compiler/xlf/FSComp.txt.es.xlf

@@ -177,6 +177,11 @@
         <target state="translated">literal float32 sin punto</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureErrorForNonVirtualMembersOverrides">
+        <source>Raises errors for non-virtual members overrides</source>
+        <target state="new">Raises errors for non-virtual members overrides</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureErrorOnDeprecatedRequireQualifiedAccess">
         <source>give error on deprecated access of construct with RequireQualifiedAccess attribute</source>
         <target state="translated">error en el acceso en desuso de la construcción con el atributo RequireQualifiedAccess</target>

src/Compiler/xlf/FSComp.txt.fr.xlf

@@ -177,6 +177,11 @@
         <target state="translated">littéral float32 sans point</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureErrorForNonVirtualMembersOverrides">
+        <source>Raises errors for non-virtual members overrides</source>
+        <target state="new">Raises errors for non-virtual members overrides</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureErrorOnDeprecatedRequireQualifiedAccess">
         <source>give error on deprecated access of construct with RequireQualifiedAccess attribute</source>
         <target state="translated">donner une erreur sur l’accès déconseillé de la construction avec l’attribut RequireQualifiedAccess</target>

src/Compiler/xlf/FSComp.txt.it.xlf

@@ -177,6 +177,11 @@
         <target state="translated">valore letterale float32 senza punti</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureErrorForNonVirtualMembersOverrides">
+        <source>Raises errors for non-virtual members overrides</source>
+        <target state="new">Raises errors for non-virtual members overrides</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureErrorOnDeprecatedRequireQualifiedAccess">
         <source>give error on deprecated access of construct with RequireQualifiedAccess attribute</source>
         <target state="translated">errore durante l'accesso deprecato del costrutto con l'attributo RequireQualifiedAccess</target>

src/Compiler/xlf/FSComp.txt.ja.xlf

@@ -177,6 +177,11 @@
         <target state="translated">ドットなしの float32 リテラル</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureErrorForNonVirtualMembersOverrides">
+        <source>Raises errors for non-virtual members overrides</source>
+        <target state="new">Raises errors for non-virtual members overrides</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureErrorOnDeprecatedRequireQualifiedAccess">
         <source>give error on deprecated access of construct with RequireQualifiedAccess attribute</source>
         <target state="translated">RequireQualifiedAccess 属性を持つコンストラクトの非推奨アクセスでエラーが発生しました</target>

src/Compiler/xlf/FSComp.txt.ko.xlf

@@ -177,6 +177,11 @@
         <target state="translated">점이 없는 float32 리터럴</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureErrorForNonVirtualMembersOverrides">
+        <source>Raises errors for non-virtual members overrides</source>
+        <target state="new">Raises errors for non-virtual members overrides</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureErrorOnDeprecatedRequireQualifiedAccess">
         <source>give error on deprecated access of construct with RequireQualifiedAccess attribute</source>
         <target state="translated">RequireQualifiedAccess 특성을 사용하여 사용되지 않는 구문 액세스에 대한 오류 제공</target>

src/Compiler/xlf/FSComp.txt.pl.xlf

@@ -177,6 +177,11 @@
         <target state="translated">bezkropkowy literał float32</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureErrorForNonVirtualMembersOverrides">
+        <source>Raises errors for non-virtual members overrides</source>
+        <target state="new">Raises errors for non-virtual members overrides</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureErrorOnDeprecatedRequireQualifiedAccess">
         <source>give error on deprecated access of construct with RequireQualifiedAccess attribute</source>
         <target state="translated">wskazywanie błędu w przypadku przestarzałego dostępu do konstrukcji z atrybutem RequireQualifiedAccess</target>

src/Compiler/xlf/FSComp.txt.pt-BR.xlf

@@ -177,6 +177,11 @@
         <target state="translated">literal float32 sem ponto</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureErrorForNonVirtualMembersOverrides">
+        <source>Raises errors for non-virtual members overrides</source>
+        <target state="new">Raises errors for non-virtual members overrides</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureErrorOnDeprecatedRequireQualifiedAccess">
         <source>give error on deprecated access of construct with RequireQualifiedAccess attribute</source>
         <target state="translated">fornecer erro no acesso preterido do constructo com o atributo RequireQualifiedAccess</target>

src/Compiler/xlf/FSComp.txt.ru.xlf

@@ -177,6 +177,11 @@
         <target state="translated">литерал float32 без точки</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureErrorForNonVirtualMembersOverrides">
+        <source>Raises errors for non-virtual members overrides</source>
+        <target state="new">Raises errors for non-virtual members overrides</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureErrorOnDeprecatedRequireQualifiedAccess">
         <source>give error on deprecated access of construct with RequireQualifiedAccess attribute</source>
         <target state="translated">выдать ошибку при устаревшем доступе к конструкции с атрибутом RequireQualifiedAccess</target>

src/Compiler/xlf/FSComp.txt.tr.xlf

@@ -177,6 +177,11 @@
         <target state="translated">noktasız float32 sabit değeri</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureErrorForNonVirtualMembersOverrides">
+        <source>Raises errors for non-virtual members overrides</source>
+        <target state="new">Raises errors for non-virtual members overrides</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureErrorOnDeprecatedRequireQualifiedAccess">
         <source>give error on deprecated access of construct with RequireQualifiedAccess attribute</source>
         <target state="translated">RequireQualifiedAccess özniteliğine sahip yapının kullanım dışı erişiminde hata</target>

src/Compiler/xlf/FSComp.txt.zh-Hans.xlf

@@ -177,6 +177,11 @@
         <target state="translated">无点 float32 文本</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureErrorForNonVirtualMembersOverrides">
+        <source>Raises errors for non-virtual members overrides</source>
+        <target state="new">Raises errors for non-virtual members overrides</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureErrorOnDeprecatedRequireQualifiedAccess">
         <source>give error on deprecated access of construct with RequireQualifiedAccess attribute</source>
         <target state="translated">对具有 RequireQualifiedAccess 属性的构造进行弃用的访问时出错</target>

src/Compiler/xlf/FSComp.txt.zh-Hant.xlf

@@ -177,6 +177,11 @@
         <target state="translated">無點號的 float32 常值</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureErrorForNonVirtualMembersOverrides">
+        <source>Raises errors for non-virtual members overrides</source>
+        <target state="new">Raises errors for non-virtual members overrides</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureErrorOnDeprecatedRequireQualifiedAccess">
         <source>give error on deprecated access of construct with RequireQualifiedAccess attribute</source>
         <target state="translated">對具有 RequireQualifiedAccess 屬性的建構的已取代存取發出錯誤</target>

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

@@ -120,4 +120,426 @@ type X() =
         |> withDiagnostics [
             (Error 531, Line 4, Col 5,  Line 4, Col 12, "Accessibility modifiers should come immediately prior to the identifier naming a construct")
             (Error 512, Line 4, Col 13, Line 4, Col 18, "Accessibility modifiers are not permitted on 'do' bindings, but 'Private' was given.")
-            (Error 222, Line 2, Col 1,  Line 3, Col 1,  "Files in libraries or multiple-file applications must begin with a namespace or module declaration, e.g. 'namespace SomeNamespace.SubNamespace' or 'module SomeNamespace.SomeModule'. Only the last source file of an application may omit such a declaration.")]
+            (Error 222, Line 2, Col 1,  Line 3, Col 1,  "Files in libraries or multiple-file applications must begin with a namespace or module declaration, e.g. 'namespace SomeNamespace.SubNamespace' or 'module SomeNamespace.SomeModule'. Only the last source file of an application may omit such a declaration.")
+        ]
+        
+    [<Fact>]
+    let ``No abstract or interface member was found that corresponds to this override with lang preview``() =
+        Fsx """
+type A =
+    abstract member M1: unit -> unit
+    abstract member M2: unit -> unit 
+    abstract member M3: unit -> unit 
+    abstract member M4: unit -> unit
+    
+type B() =
+    interface A with
+        override this.M1() = ()
+        override this.M2() = () // error is expected
+        override this.M3() = () // error is expected
+        override this.M4() = ()
+    
+type C() =
+    inherit B()
+    override this.M1() = ()
+    override this.M2() = ()
+    override this.M3() = ()
+    override this.M4() = ()
+    member this.M5() = ()
+        """
+        |> withLangVersionPreview
+        |> compile
+        |> shouldFail
+        |> withDiagnostics [
+            (Error 855, Line 17, Col 19, Line 17, Col 21, "No abstract or interface member was found that corresponds to this override")
+            (Error 855, Line 18, Col 19, Line 18, Col 21, "No abstract or interface member was found that corresponds to this override")
+            (Error 855, Line 19, Col 19, Line 19, Col 21, "No abstract or interface member was found that corresponds to this override")
+            (Error 855, Line 20, Col 19, Line 20, Col 21, "No abstract or interface member was found that corresponds to this override")
+        ]
+    
+    [<Fact>]
+    let ``No abstract or interface member was found that corresponds to this override with lang version70``() =
+        Fsx """
+type A =
+    abstract member M1: unit -> unit
+    abstract member M2: unit -> unit 
+    abstract member M3: unit -> unit 
+    abstract member M4: unit -> unit
+    
+type B() =
+    interface A with
+        override this.M1() = ()
+        override this.M2() = () // error is expected
+        override this.M3() = () // error is expected
+        override this.M4() = ()
+    
+type C() =
+    inherit B()
+    override this.M1() = ()
+    override this.M2() = ()
+    override this.M3() = ()
+    override this.M4() = ()
+    member this.M5() = ()
+        """
+        |> withLangVersion70
+        |> compile
+        |> shouldFail
+        |> withDiagnostics [
+            (Error 855, Line 17, Col 19, Line 17, Col 21, "No abstract or interface member was found that corresponds to this override")
+            (Error 855, Line 18, Col 19, Line 18, Col 21, "No abstract or interface member was found that corresponds to this override")
+            (Error 855, Line 19, Col 19, Line 19, Col 21, "No abstract or interface member was found that corresponds to this override")
+            (Error 855, Line 20, Col 19, Line 20, Col 21, "No abstract or interface member was found that corresponds to this override")
+        ]
+        
+    [<Fact>]
+    let ``Virtual member was found that corresponds to this override with lang version70`` () =
+        let CSLib =
+            CSharp """
+public class A
+{
+    public virtual void M1() { }
+    public virtual void M2() { }
+    public virtual void M3() { }
+    public virtual void M4() { }
+}
+        """ |> withName "CSLib"
+
+        let app =
+            FSharp """
+module ClassTests
+type B() =
+    inherit A()
+    override this.M1() = ()
+    override this.M2() = ()
+    override this.M3() = ()
+    override this.M4() = ()
+    member this.M5() = ()
+        """ |> withReferences [CSLib]
+        app
+        |> withLangVersion70
+        |> compile
+        |> shouldSucceed
+        
+    [<Fact>]
+    let ``Virtual member was found that corresponds to this override with lang preview`` () =
+        let CSLib =
+            CSharp """
+public class A
+{
+    public virtual void M1() { }
+    public virtual void M2() { }
+    public virtual void M3() { }
+    public virtual void M4() { }
+}
+        """ |> withName "CSLib"
+
+        let app =
+            FSharp """
+module ClassTests
+type B() =
+    inherit A()
+    override this.M1() = ()
+    override this.M2() = ()
+    override this.M3() = ()
+    override this.M4() = ()
+    member this.M5() = ()
+        """ |> withReferences [CSLib]
+        app
+        |> withLangVersionPreview
+        |> compile
+        |> shouldSucceed
+
+
+    [<Fact>]
+    let ``Virtual member was not found that corresponds to this override simple base class with lang version preview`` () =
+        let CSLib =
+            CSharp """
+public class A
+{
+    public virtual void M1() { }
+    public void M2() { }
+    public virtual void M3() { }
+}
+        """ |> withName "CSLib"
+
+        let app =
+            FSharp """
+module ClassTests
+type B() =
+    inherit A()
+    
+    override this.M1() = ()
+    override this.M2() = () // error expected
+    override this.M3() = ()
+    member this.M4() = ()
+        """ |> withReferences [CSLib]
+        app
+        |> withLangVersionPreview
+        |> compile
+        |> shouldFail
+        |> withDiagnostics [
+            (Error 855, Line 7, Col 19, Line 7, Col 21, "No abstract or interface member was found that corresponds to this override")
+        ]
+
+    [<Fact>]
+    let ``Virtual member was not found that corresponds to this override simple base class with lang version70`` () =
+        let CSLib =
+            CSharp """
+public class A
+{
+    public virtual void M1() { }
+    public void M2() { }
+    public virtual void M3() { }
+}
+        """ |> withName "CSLib"
+
+        let app =
+            FSharp """
+module ClassTests
+type B() =
+    inherit A()
+    
+    override this.M1() = ()
+    override this.M2() = ()
+    override this.M3() = ()
+    member this.M4() = ()
+        """ |> withReferences [CSLib]
+        app
+        |> withLangVersion70
+        |> compile
+        |> shouldFail
+        |> withDiagnostics [
+            (Error 855, Line 7, Col 19, Line 7, Col 21, "No abstract or interface member was found that corresponds to this override")
+        ]
+        
+    [<Fact>]
+    let ``Virtual member was not found that corresponds to this override nested base class with lang version preview`` () =
+        let CSLib =
+            CSharp """
+public class A
+{
+    public virtual void M1() { }
+    public virtual void M2() { }
+    public virtual void M3() { }
+    public virtual void M4() { }
+}
+
+public class B : A
+{
+    public override void M1() { }
+    public void M2() { }
+    public new void M3() { }
+    public new virtual void M4() { }
+}
+        """ |> withName "CSLib"
+
+        let app =
+            FSharp """
+module ClassTests
+type C() =
+    inherit B()
+    
+    override this.M1() = ()
+    override this.M2() = () // error expected
+    override this.M3() = () // error expected
+    override this.M4() = ()
+    member this.M5() = ()
+        """ |> withReferences [CSLib]
+        app
+        |> withLangVersionPreview
+        |> compile
+        |> shouldFail
+        |> withDiagnostics [
+            (Error 855, Line 7, Col 19, Line 7, Col 21, "No abstract or interface member was found that corresponds to this override")
+            (Error 855, Line 8, Col 19, Line 8, Col 21, "No abstract or interface member was found that corresponds to this override")
+        ]
+        
+    [<Fact>]
+    let ``Virtual member was not found that corresponds to this override nested base class with lang version70`` () =
+        let CSLib =
+            CSharp """
+public class A
+{
+    public virtual void M1() { }
+    public virtual void M2() { }
+    public virtual void M3() { }
+    public virtual void M4() { }
+}
+
+public class B : A
+{
+    public override void M1() { }
+    public void M2() { }
+    public new void M3() { }
+    public new virtual void M4() { }
+}
+        """ |> withName "CSLib"
+
+        let app =
+            FSharp """
+module ClassTests
+type C() =
+    inherit B()
+    
+    override this.M1() = ()
+    override this.M2() = () // error expected
+    override this.M3() = () // error expected
+    override this.M4() = ()
+    member this.M5() = ()
+        """ |> withReferences [CSLib]
+        app
+        |> withLangVersion70
+        |> compile
+        |> shouldSucceed
+
+    [<Fact>]
+    let ``Virtual member was not found that corresponds to this override nested 2 base class with lang preview`` () =
+        let CSLib =
+            CSharp """
+public class A
+{
+    public virtual void M1() { }
+    public virtual void M2() { }
+    public virtual void M3() { }
+    public virtual void M4() { }
+}
+
+public class B : A
+{
+    public void M2() { }
+    public new void M3() { }
+}
+        """ |> withName "CSLib"
+
+        let app =
+            FSharp """
+module ClassTests
+type C() =
+    inherit B()
+    
+    override this.M1() = ()
+    override this.M2() = () // error is expected
+    override this.M3() = () // error is expected
+    override this.M4() = ()
+    member this.M5() = ()
+        """ |> withReferences [CSLib]
+        app
+        |> withLangVersionPreview
+        |> compile
+        |> shouldFail
+        |> withDiagnostics [
+            (Error 855, Line 7, Col 19, Line 7, Col 21, "No abstract or interface member was found that corresponds to this override")
+            (Error 855, Line 8, Col 19, Line 8, Col 21, "No abstract or interface member was found that corresponds to this override")
+        ]
+        
+    [<Fact>]
+    let ``Virtual member was not found that corresponds to this override nested 2 base classes with lang version70`` () =
+        let CSLib =
+            CSharp """
+public class A
+{
+    public virtual void M1() { }
+    public virtual void M2() { }
+    public virtual void M3() { }
+    public virtual void M4() { }
+}
+
+public class B : A
+{
+    public void M2() { }
+    public new void M3() { }
+}
+        """ |> withName "CSLib"
+
+        let app =
+            FSharp """
+module ClassTests
+type C() =
+    inherit B()
+    
+    override this.M1() = ()
+    override this.M2() = () // error is expected
+    override this.M3() = () // error is expected
+    override this.M4() = ()
+    member this.M5() = ()
+        """ |> withReferences [CSLib]
+        app
+        |> withLangVersion70
+        |> compile
+        |> shouldSucceed
+        
+    [<Fact>]
+    let ``Virtual member was not found that corresponds to this override nested 2 base classes and mixed methods with lang preview`` () =
+        let CSLib =
+            CSharp """
+public class A
+{
+    public virtual void M1() { }
+    public void M2() { }
+}
+
+public class B : A
+{
+    public virtual void M3() { }
+    public new void M4() { }
+}
+        """ |> withName "CSLib"
+
+        let app =
+            FSharp """
+module ClassTests
+type C() =
+    inherit B()
+    
+    override this.M1() = ()
+    override this.M2() = () // error is expected
+    override this.M3() = ()
+    override this.M4() = () // error is expected
+    member this.M5() = ()
+        """ |> withReferences [CSLib]
+        app
+        |> withLangVersionPreview
+        |> compile
+        |> shouldFail
+        |> withDiagnostics [
+            (Error 855, Line 7, Col 19, Line 7, Col 21, "No abstract or interface member was found that corresponds to this override")
+            (Error 855, Line 9, Col 19, Line 9, Col 21, "No abstract or interface member was found that corresponds to this override")
+        ]
+
+    [<Fact>]
+    let ``Virtual member was not found that corresponds to this override nested 2 base classes and mixed methods with lang version70`` () =
+        let CSLib =
+            CSharp """
+public class A
+{
+    public virtual void M1() { }
+    public void M2() { }
+}
+
+public class B : A
+{
+    public virtual void M3() { }
+    public new void M4() { }
+}
+        """ |> withName "CSLib"
+
+        let app =
+            FSharp """
+module ClassTests
+type C() =
+    inherit B()
+    
+    override this.M1() = ()
+    override this.M2() = () // error is expected
+    override this.M3() = ()
+    override this.M4() = ()  // error is expected
+    member this.M5() = ()
+        """ |> withReferences [CSLib]
+        app
+        |> withLangVersion70
+        |> compile
+        |> shouldFail
+        |> withDiagnostics [
+            (Error 855, Line 7, Col 19, Line 7, Col 21, "No abstract or interface member was found that corresponds to this override")
+            (Error 855, Line 9, Col 19, Line 9, Col 21, "No abstract or interface member was found that corresponds to this override")
+        ]
\ No newline at end of file