diff --git a/src/Compiler/Checking/CheckDeclarations.fs b/src/Compiler/Checking/CheckDeclarations.fs
index 1b85fada4c6..07573e3f7f3 100644
--- a/src/Compiler/Checking/CheckDeclarations.fs
+++ b/src/Compiler/Checking/CheckDeclarations.fs
@@ -969,6 +969,20 @@ module MutRecBindingChecking =
                     | rest -> rest
 
                 let prelimRecValues = [ for x in defnAs do match x with Phase2AMember bind -> yield bind.RecBindingInfo.Val | _ -> () ]
+                
+                let tyconOpt =
+                    if cenv.g.langVersion.SupportsFeature(LanguageFeature.CSharpExtensionAttributeNotRequired) then
+                        tyconOpt
+                        |> Option.map (fun tycon ->
+                            tryAddExtensionAttributeIfNotAlreadyPresent
+                                (fun tryFindExtensionAttribute ->
+                                    tycon.MembersOfFSharpTyconSorted
+                                    |> Seq.tryPick (fun m -> tryFindExtensionAttribute m.Attribs)
+                                )
+                                tycon
+                        )
+                    else
+                        tyconOpt
                 let defnAs = MutRecShape.Tycon(TyconBindingsPhase2A(tyconOpt, declKind, prelimRecValues, tcref, copyOfTyconTypars, thisTy, defnAs))
                 defnAs, (tpenv, recBindIdx, uncheckedBindsRev))
 
@@ -4228,6 +4242,50 @@ module TcDeclarations =
         // Check the members and decide on representations for types with implicit constructors.
         let withBindings, envFinal = TcMutRecDefns_Phase2 cenv envInitial m scopem mutRecNSInfo envMutRecPrelimWithReprs withEnvs isMutRec
 
+        let withBindings =
+            if cenv.g.langVersion.SupportsFeature(LanguageFeature.CSharpExtensionAttributeNotRequired) then
+                // If any of the types has a member with the System.Runtime.CompilerServices.ExtensionAttribute,
+                // or a recursive module has a binding with the System.Runtime.CompilerServices.ExtensionAttribute,
+                // that type/recursive module should also received the ExtensionAttribute if it is not yet present.
+                // Example:
+                // open System.Runtime.CompilerServices
+                //
+                // type Int32Extensions =
+                //      [<Extension>]
+                //      static member PlusOne (a:int) : int = a + 1
+                //
+                // or
+                //
+                // module rec Foo
+                //
+                // [<System.Runtime.CompilerServices.Extension>]
+                // let PlusOne (a:int) = a + 1
+                withBindings
+                |> List.map (function
+                    | MutRecShape.Tycon (Some tycon, bindings) ->
+                        let tycon =
+                            tryAddExtensionAttributeIfNotAlreadyPresent
+                                (fun tryFindExtensionAttribute ->
+                                    tycon.MembersOfFSharpTyconSorted
+                                    |> Seq.tryPick (fun m -> tryFindExtensionAttribute m.Attribs)
+                                )
+                                tycon
+                        MutRecShape.Tycon (Some tycon, bindings)
+                    | MutRecShape.Module ((MutRecDefnsPhase2DataForModule(moduleOrNamespaceType, entity), env), shapes) ->
+                        let entity =
+                            tryAddExtensionAttributeIfNotAlreadyPresent
+                                (fun tryFindExtensionAttribute ->
+                                    moduleOrNamespaceType.Value.AllValsAndMembers
+                                    |> Seq.filter(fun v -> v.IsModuleBinding)
+                                    |> Seq.tryPick (fun v -> tryFindExtensionAttribute v.Attribs)
+                                )
+                                entity
+
+                        MutRecShape.Module ((MutRecDefnsPhase2DataForModule(moduleOrNamespaceType, entity), env), shapes)
+                    | shape -> shape)
+                else
+                    withBindings
+
         // Generate the hash/compare/equality bindings for all tycons.
         //
         // Note: generating these bindings must come after generating the members, since some in the case of structs some fields
@@ -4766,6 +4824,31 @@ let rec TcModuleOrNamespaceElementNonMutRec (cenv: cenv) parent typeNames scopem
 
               // Get the inferred type of the decls and record it in the modul. 
               moduleEntity.entity_modul_type <- MaybeLazy.Strict moduleTyAcc.Value
+
+              let moduleEntity =
+                if cenv.g.langVersion.SupportsFeature(LanguageFeature.CSharpExtensionAttributeNotRequired) then
+                    // If any of the let bindings inside the module has the System.Runtime.CompilerServices.ExtensionAttribute,
+                    // that module should also received the ExtensionAttribute if it is not yet present.
+                    // Example:
+                    // module Foo
+                    //
+                    //[<System.Runtime.CompilerServices.Extension>]
+                    //let PlusOne (a:int) = a + 1
+                    tryAddExtensionAttributeIfNotAlreadyPresent
+                        (fun tryFindExtensionAttribute ->
+                            match moduleContents with
+                            | ModuleOrNamespaceContents.TMDefs(defs) ->
+                                defs
+                                |> Seq.tryPick (function
+                                    | ModuleOrNamespaceContents.TMDefLet (Binding.TBind(var = v),_) ->
+                                        tryFindExtensionAttribute v.Attribs
+                                    | _ -> None)
+                            | _ -> None
+                        )
+                        moduleEntity
+                else
+                    moduleEntity
+                    
               let moduleDef = TMDefRec(false, [], [], [ModuleOrNamespaceBinding.Module(moduleEntity, moduleContents)], m)
 
               PublishModuleDefn cenv env moduleEntity 
diff --git a/src/Compiler/Checking/NameResolution.fs b/src/Compiler/Checking/NameResolution.fs
index 13b0c7c3c14..ad78b5ca5c5 100644
--- a/src/Compiler/Checking/NameResolution.fs
+++ b/src/Compiler/Checking/NameResolution.fs
@@ -530,14 +530,18 @@ let IsMethInfoPlainCSharpStyleExtensionMember g m isEnclExtTy (minfo: MethInfo)
 /// Get the info for all the .NET-style extension members listed as static members in the type.
 let private GetCSharpStyleIndexedExtensionMembersForTyconRef (amap: Import.ImportMap) m  (tcrefOfStaticClass: TyconRef) =
     let g = amap.g
-
-    if IsTyconRefUsedForCSharpStyleExtensionMembers g m tcrefOfStaticClass then
-        let pri = NextExtensionMethodPriority()
+    
+    if g.langVersion.SupportsFeature(LanguageFeature.CSharpExtensionAttributeNotRequired) then
         let ty = generalizedTyconRef g tcrefOfStaticClass
+      
+        let minfos =
+            GetImmediateIntrinsicMethInfosOfType (None, AccessorDomain.AccessibleFromSomeFSharpCode) g amap m ty
+            |> List.filter (IsMethInfoPlainCSharpStyleExtensionMember g m true)
 
-        let minfos = GetImmediateIntrinsicMethInfosOfType (None, AccessorDomain.AccessibleFromSomeFSharpCode) g amap m ty
-        [ for minfo in minfos do
-            if IsMethInfoPlainCSharpStyleExtensionMember g m true minfo then
+        if IsTyconRefUsedForCSharpStyleExtensionMembers g m tcrefOfStaticClass || not minfos.IsEmpty then
+            let pri = NextExtensionMethodPriority()
+
+            [ for minfo in minfos do
                 let ilExtMem = ILExtMem (tcrefOfStaticClass, minfo, pri)
 
                 // The results are indexed by the TyconRef of the first 'this' argument, if any.
@@ -584,9 +588,60 @@ let private GetCSharpStyleIndexedExtensionMembersForTyconRef (amap: Import.Impor
                 | None -> ()
                 | Some (Some tcref) -> yield Choice1Of2(tcref, ilExtMem)
                 | Some None -> yield Choice2Of2 ilExtMem ]
+        else
+            []
     else
-        []
-
+        if IsTyconRefUsedForCSharpStyleExtensionMembers g m tcrefOfStaticClass then
+            let pri = NextExtensionMethodPriority()
+            let ty = generalizedTyconRef g tcrefOfStaticClass
+            let minfos = GetImmediateIntrinsicMethInfosOfType (None, AccessorDomain.AccessibleFromSomeFSharpCode) g amap m ty
+            
+            [ for minfo in minfos do
+                if IsMethInfoPlainCSharpStyleExtensionMember g m true minfo then
+                    let ilExtMem = ILExtMem (tcrefOfStaticClass, minfo, pri)
+                    // The results are indexed by the TyconRef of the first 'this' argument, if any.
+                    // So we need to go and crack the type of the 'this' argument.
+                    //
+                    // This is convoluted because we only need the ILTypeRef of the first argument, and we don't
+                    // want to read any other metadata as it can trigger missing-assembly errors. It turns out ImportILTypeRef
+                    // is less eager in reading metadata than GetParamTypes.
+                    //
+                    // We don't use the index for the IL extension method for tuple of F# function types (e.g. if extension
+                    // methods for tuple occur in C# code)
+                    let thisTyconRef =
+                     try
+                        let rs =
+                            match metadataOfTycon tcrefOfStaticClass.Deref, minfo with
+                            | ILTypeMetadata (TILObjectReprData(scoref, _, _)), ILMeth(_, ILMethInfo(_, _, _, ilMethod, _), _) ->
+                                match ilMethod.ParameterTypes with
+                                | firstTy :: _ ->
+                                    match firstTy with
+                                    | ILType.Boxed  tspec | ILType.Value tspec ->
+                                        let tref = (tspec |> rescopeILTypeSpec scoref).TypeRef
+                                        if Import.CanImportILTypeRef amap m tref then
+                                            let tcref = tref |> Import.ImportILTypeRef amap m
+                                            if isCompiledTupleTyconRef g tcref || tyconRefEq g tcref g.fastFunc_tcr then None
+                                            else Some tcref
+                                        else None
+                                    | _ -> None
+                                | _ -> None
+                            | _ ->
+                                // The results are indexed by the TyconRef of the first 'this' argument, if any.
+                                // So we need to go and crack the type of the 'this' argument.
+                                let thisTy = minfo.GetParamTypes(amap, m, generalizeTypars minfo.FormalMethodTypars).Head.Head
+                                match thisTy with
+                                | AppTy g (tcrefOfTypeExtended, _) when not (isByrefTy g thisTy) -> Some tcrefOfTypeExtended
+                                | _ -> None
+                        Some rs
+                      with e -> // Import of the ILType may fail, if so report the error and skip on
+                        errorRecovery e m
+                        None
+                    match thisTyconRef with
+                    | None -> ()
+                    | Some (Some tcref) -> yield Choice1Of2(tcref, ilExtMem)
+                    | Some None -> yield Choice2Of2 ilExtMem ]
+        else
+            []
 
 /// Query the declared properties of a type (including inherited properties)
 let IntrinsicPropInfosOfTypeInScope (infoReader: InfoReader) optFilter ad findFlag m ty =
diff --git a/src/Compiler/FSComp.txt b/src/Compiler/FSComp.txt
index 7e460abeb92..9c9da192081 100644
--- a/src/Compiler/FSComp.txt
+++ b/src/Compiler/FSComp.txt
@@ -1560,6 +1560,7 @@ featureRequiredProperties,"support for required properties"
 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"
 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."
diff --git a/src/Compiler/Facilities/LanguageFeatures.fs b/src/Compiler/Facilities/LanguageFeatures.fs
index 8644fbe9971..00ddd74b6c5 100644
--- a/src/Compiler/Facilities/LanguageFeatures.fs
+++ b/src/Compiler/Facilities/LanguageFeatures.fs
@@ -55,6 +55,7 @@ type LanguageFeature =
     | InterfacesWithAbstractStaticMembers
     | SelfTypeConstraints
     | MatchNotAllowedForUnionCaseWithNoData
+    | CSharpExtensionAttributeNotRequired
 
 /// LanguageVersion management
 type LanguageVersion(versionText) =
@@ -126,6 +127,8 @@ type LanguageVersion(versionText) =
                 // F# preview
                 LanguageFeature.FromEndSlicing, previewVersion
                 LanguageFeature.MatchNotAllowedForUnionCaseWithNoData, previewVersion
+                LanguageFeature.CSharpExtensionAttributeNotRequired, previewVersion
+
             ]
 
     static let defaultLanguageVersion = LanguageVersion("default")
@@ -233,6 +236,7 @@ type LanguageVersion(versionText) =
         | LanguageFeature.InterfacesWithAbstractStaticMembers -> FSComp.SR.featureInterfacesWithAbstractStaticMembers ()
         | LanguageFeature.SelfTypeConstraints -> FSComp.SR.featureSelfTypeConstraints ()
         | LanguageFeature.MatchNotAllowedForUnionCaseWithNoData -> FSComp.SR.featureMatchNotAllowedForUnionCaseWithNoData ()
+        | LanguageFeature.CSharpExtensionAttributeNotRequired -> FSComp.SR.featureCSharpExtensionAttributeNotRequired  ()
 
     /// Get a version string associated with the given feature.
     static member GetFeatureVersionString feature =
diff --git a/src/Compiler/Facilities/LanguageFeatures.fsi b/src/Compiler/Facilities/LanguageFeatures.fsi
index 694a6ae73fd..1b3d68e8f2a 100644
--- a/src/Compiler/Facilities/LanguageFeatures.fsi
+++ b/src/Compiler/Facilities/LanguageFeatures.fsi
@@ -45,6 +45,7 @@ type LanguageFeature =
     | InterfacesWithAbstractStaticMembers
     | SelfTypeConstraints
     | MatchNotAllowedForUnionCaseWithNoData
+    | CSharpExtensionAttributeNotRequired
 
 /// LanguageVersion management
 type LanguageVersion =
diff --git a/src/Compiler/TypedTree/TypedTreeOps.fs b/src/Compiler/TypedTree/TypedTreeOps.fs
index f57954f7bf5..6fa70a5903e 100644
--- a/src/Compiler/TypedTree/TypedTreeOps.fs
+++ b/src/Compiler/TypedTree/TypedTreeOps.fs
@@ -10410,4 +10410,22 @@ let (|EmptyModuleOrNamespaces|_|) (moduleOrNamespaceContents: ModuleOrNamespaceC
             Some emptyModuleOrNamespaces
         else
             None
-    | _ -> None
\ No newline at end of file
+    | _ -> None
+
+let tryAddExtensionAttributeIfNotAlreadyPresent
+    (tryFindExtensionAttributeIn: (Attrib list -> Attrib option) -> Attrib option)
+    (entity: Entity)
+    : Entity
+    =
+    let tryFindExtensionAttribute (attribs: Attrib list): Attrib option =
+         List.tryFind
+             (fun (a: Attrib) ->
+                a.TyconRef.CompiledRepresentationForNamedType.BasicQualifiedName = "System.Runtime.CompilerServices.ExtensionAttribute")
+             attribs
+
+    if Option.isSome (tryFindExtensionAttribute entity.Attribs) then
+        entity
+    else
+        match tryFindExtensionAttributeIn tryFindExtensionAttribute with
+        | None -> entity
+        | Some extensionAttrib -> { entity with entity_attribs = extensionAttrib :: entity.Attribs }
diff --git a/src/Compiler/TypedTree/TypedTreeOps.fsi b/src/Compiler/TypedTree/TypedTreeOps.fsi
index 00d838e04d4..ae58019683a 100755
--- a/src/Compiler/TypedTree/TypedTreeOps.fsi
+++ b/src/Compiler/TypedTree/TypedTreeOps.fsi
@@ -2687,3 +2687,7 @@ type TraitConstraintInfo with
 /// This will match anything that does not have any types or bindings.
 val (|EmptyModuleOrNamespaces|_|):
     moduleOrNamespaceContents: ModuleOrNamespaceContents -> (ModuleOrNamespace list) option
+
+/// Add an System.Runtime.CompilerServices.ExtensionAttribute to the Entity if found via predicate and not already present.
+val tryAddExtensionAttributeIfNotAlreadyPresent:
+    tryFindExtensionAttributeIn: ((Attrib list -> Attrib option) -> Attrib option) -> entity: Entity -> Entity
diff --git a/src/Compiler/xlf/FSComp.txt.cs.xlf b/src/Compiler/xlf/FSComp.txt.cs.xlf
index 2d981896ab9..8c4a1300a7f 100644
--- a/src/Compiler/xlf/FSComp.txt.cs.xlf
+++ b/src/Compiler/xlf/FSComp.txt.cs.xlf
@@ -152,6 +152,11 @@
         <target state="translated">automatické generování vlastnosti Message pro deklarace exception</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureCSharpExtensionAttributeNotRequired">
+        <source>Allow implicit Extension attribute on declaring types, modules</source>
+        <target state="new">Allow implicit Extension attribute on declaring types, modules</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDefaultInterfaceMemberConsumption">
         <source>default interface member consumption</source>
         <target state="translated">využití člena výchozího rozhraní</target>
diff --git a/src/Compiler/xlf/FSComp.txt.de.xlf b/src/Compiler/xlf/FSComp.txt.de.xlf
index 7b2e86ccd81..688592092e3 100644
--- a/src/Compiler/xlf/FSComp.txt.de.xlf
+++ b/src/Compiler/xlf/FSComp.txt.de.xlf
@@ -152,6 +152,11 @@
         <target state="translated">Automatische Generierung der Eigenschaft „Message“ für „exception“-Deklarationen</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureCSharpExtensionAttributeNotRequired">
+        <source>Allow implicit Extension attribute on declaring types, modules</source>
+        <target state="new">Allow implicit Extension attribute on declaring types, modules</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDefaultInterfaceMemberConsumption">
         <source>default interface member consumption</source>
         <target state="translated">standardmäßige Schnittstellenmembernutzung</target>
diff --git a/src/Compiler/xlf/FSComp.txt.es.xlf b/src/Compiler/xlf/FSComp.txt.es.xlf
index 8aad3d6f781..a06daa1f7f1 100644
--- a/src/Compiler/xlf/FSComp.txt.es.xlf
+++ b/src/Compiler/xlf/FSComp.txt.es.xlf
@@ -152,6 +152,11 @@
         <target state="translated">generación automática de la propiedad 'Message' para declaraciones 'exception'</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureCSharpExtensionAttributeNotRequired">
+        <source>Allow implicit Extension attribute on declaring types, modules</source>
+        <target state="new">Allow implicit Extension attribute on declaring types, modules</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDefaultInterfaceMemberConsumption">
         <source>default interface member consumption</source>
         <target state="translated">consumo de miembros de interfaz predeterminados</target>
diff --git a/src/Compiler/xlf/FSComp.txt.fr.xlf b/src/Compiler/xlf/FSComp.txt.fr.xlf
index ffc5e832198..f8a89904bfa 100644
--- a/src/Compiler/xlf/FSComp.txt.fr.xlf
+++ b/src/Compiler/xlf/FSComp.txt.fr.xlf
@@ -152,6 +152,11 @@
         <target state="translated">génération automatique de la propriété « Message » pour les déclarations « exception »</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureCSharpExtensionAttributeNotRequired">
+        <source>Allow implicit Extension attribute on declaring types, modules</source>
+        <target state="new">Allow implicit Extension attribute on declaring types, modules</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDefaultInterfaceMemberConsumption">
         <source>default interface member consumption</source>
         <target state="translated">consommation par défaut des membres d'interface</target>
diff --git a/src/Compiler/xlf/FSComp.txt.it.xlf b/src/Compiler/xlf/FSComp.txt.it.xlf
index 29240e0d98c..37d11b74636 100644
--- a/src/Compiler/xlf/FSComp.txt.it.xlf
+++ b/src/Compiler/xlf/FSComp.txt.it.xlf
@@ -152,6 +152,11 @@
         <target state="translated">generazione automatica della proprietà 'Messaggio' per le dichiarazioni 'eccezione'</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureCSharpExtensionAttributeNotRequired">
+        <source>Allow implicit Extension attribute on declaring types, modules</source>
+        <target state="new">Allow implicit Extension attribute on declaring types, modules</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDefaultInterfaceMemberConsumption">
         <source>default interface member consumption</source>
         <target state="translated">utilizzo predefinito dei membri di interfaccia</target>
diff --git a/src/Compiler/xlf/FSComp.txt.ja.xlf b/src/Compiler/xlf/FSComp.txt.ja.xlf
index c509bc1328e..c42700c1d18 100644
--- a/src/Compiler/xlf/FSComp.txt.ja.xlf
+++ b/src/Compiler/xlf/FSComp.txt.ja.xlf
@@ -152,6 +152,11 @@
         <target state="translated">`exception` 宣言の `Message` プロパティの自動生成</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureCSharpExtensionAttributeNotRequired">
+        <source>Allow implicit Extension attribute on declaring types, modules</source>
+        <target state="new">Allow implicit Extension attribute on declaring types, modules</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDefaultInterfaceMemberConsumption">
         <source>default interface member consumption</source>
         <target state="translated">既定のインターフェイス メンバーの消費</target>
diff --git a/src/Compiler/xlf/FSComp.txt.ko.xlf b/src/Compiler/xlf/FSComp.txt.ko.xlf
index 9767ff521a6..39e9a33001e 100644
--- a/src/Compiler/xlf/FSComp.txt.ko.xlf
+++ b/src/Compiler/xlf/FSComp.txt.ko.xlf
@@ -152,6 +152,11 @@
         <target state="translated">'exception' 선언에 대한 'Message' 속성 자동 생성</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureCSharpExtensionAttributeNotRequired">
+        <source>Allow implicit Extension attribute on declaring types, modules</source>
+        <target state="new">Allow implicit Extension attribute on declaring types, modules</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDefaultInterfaceMemberConsumption">
         <source>default interface member consumption</source>
         <target state="translated">기본 인터페이스 멤버 사용</target>
diff --git a/src/Compiler/xlf/FSComp.txt.pl.xlf b/src/Compiler/xlf/FSComp.txt.pl.xlf
index 4a565f29eaf..e3a61d7dd58 100644
--- a/src/Compiler/xlf/FSComp.txt.pl.xlf
+++ b/src/Compiler/xlf/FSComp.txt.pl.xlf
@@ -152,6 +152,11 @@
         <target state="translated">Automatyczne generowanie właściwości „Wiadomość“ dla deklaracji „Wyjątek“</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureCSharpExtensionAttributeNotRequired">
+        <source>Allow implicit Extension attribute on declaring types, modules</source>
+        <target state="new">Allow implicit Extension attribute on declaring types, modules</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDefaultInterfaceMemberConsumption">
         <source>default interface member consumption</source>
         <target state="translated">domyślne użycie składowej interfejsu</target>
diff --git a/src/Compiler/xlf/FSComp.txt.pt-BR.xlf b/src/Compiler/xlf/FSComp.txt.pt-BR.xlf
index 140f330414d..78742938012 100644
--- a/src/Compiler/xlf/FSComp.txt.pt-BR.xlf
+++ b/src/Compiler/xlf/FSComp.txt.pt-BR.xlf
@@ -152,6 +152,11 @@
         <target state="translated">geração automática da propriedade 'Message' para declarações de 'exception'</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureCSharpExtensionAttributeNotRequired">
+        <source>Allow implicit Extension attribute on declaring types, modules</source>
+        <target state="new">Allow implicit Extension attribute on declaring types, modules</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDefaultInterfaceMemberConsumption">
         <source>default interface member consumption</source>
         <target state="translated">consumo de membro da interface padrão</target>
diff --git a/src/Compiler/xlf/FSComp.txt.ru.xlf b/src/Compiler/xlf/FSComp.txt.ru.xlf
index 5c8c1ad44e7..837271832bf 100644
--- a/src/Compiler/xlf/FSComp.txt.ru.xlf
+++ b/src/Compiler/xlf/FSComp.txt.ru.xlf
@@ -152,6 +152,11 @@
         <target state="translated">автоматическое создание свойства “Message” для объявлений “exception”</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureCSharpExtensionAttributeNotRequired">
+        <source>Allow implicit Extension attribute on declaring types, modules</source>
+        <target state="new">Allow implicit Extension attribute on declaring types, modules</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDefaultInterfaceMemberConsumption">
         <source>default interface member consumption</source>
         <target state="translated">использование элемента интерфейса по умолчанию</target>
diff --git a/src/Compiler/xlf/FSComp.txt.tr.xlf b/src/Compiler/xlf/FSComp.txt.tr.xlf
index fb8d5406e68..59d04a3fe94 100644
--- a/src/Compiler/xlf/FSComp.txt.tr.xlf
+++ b/src/Compiler/xlf/FSComp.txt.tr.xlf
@@ -152,6 +152,11 @@
         <target state="translated">'exception' bildirimleri için 'Message' özelliğinin otomatik olarak oluşturulması</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureCSharpExtensionAttributeNotRequired">
+        <source>Allow implicit Extension attribute on declaring types, modules</source>
+        <target state="new">Allow implicit Extension attribute on declaring types, modules</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDefaultInterfaceMemberConsumption">
         <source>default interface member consumption</source>
         <target state="translated">varsayılan arabirim üyesi tüketimi</target>
diff --git a/src/Compiler/xlf/FSComp.txt.zh-Hans.xlf b/src/Compiler/xlf/FSComp.txt.zh-Hans.xlf
index baf4e53d041..ebcafbf1852 100644
--- a/src/Compiler/xlf/FSComp.txt.zh-Hans.xlf
+++ b/src/Compiler/xlf/FSComp.txt.zh-Hans.xlf
@@ -152,6 +152,11 @@
         <target state="translated">自动生成“异常”声明的“消息”属性</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureCSharpExtensionAttributeNotRequired">
+        <source>Allow implicit Extension attribute on declaring types, modules</source>
+        <target state="new">Allow implicit Extension attribute on declaring types, modules</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDefaultInterfaceMemberConsumption">
         <source>default interface member consumption</source>
         <target state="translated">默认接口成员消耗</target>
diff --git a/src/Compiler/xlf/FSComp.txt.zh-Hant.xlf b/src/Compiler/xlf/FSComp.txt.zh-Hant.xlf
index 3711de1d214..0606b9b48c4 100644
--- a/src/Compiler/xlf/FSComp.txt.zh-Hant.xlf
+++ b/src/Compiler/xlf/FSComp.txt.zh-Hant.xlf
@@ -152,6 +152,11 @@
         <target state="translated">自動產生 'exception' 宣告的 'Message' 屬性</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureCSharpExtensionAttributeNotRequired">
+        <source>Allow implicit Extension attribute on declaring types, modules</source>
+        <target state="new">Allow implicit Extension attribute on declaring types, modules</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureDefaultInterfaceMemberConsumption">
         <source>default interface member consumption</source>
         <target state="translated">預設介面成員使用</target>
diff --git a/tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj b/tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj
index 29cca6690d0..b6ea2254387 100644
--- a/tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj
+++ b/tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj
@@ -174,6 +174,7 @@
     <Compile Include="Language\ComputationExpressionTests.fs" />
     <Compile Include="Language\CastingTests.fs" />
     <Compile Include="Language\NameofTests.fs" />
+    <Compile Include="Language\ExtensionMethodTests.fs" />
     <Compile Include="ConstraintSolver\PrimitiveConstraints.fs" />
     <Compile Include="ConstraintSolver\MemberConstraints.fs" />
     <None Include="**\*.cs;**\*.fs;**\*.fsx;**\*.fsi" Exclude="@(Compile)">
diff --git a/tests/FSharp.Compiler.ComponentTests/Language/ExtensionMethodTests.fs b/tests/FSharp.Compiler.ComponentTests/Language/ExtensionMethodTests.fs
new file mode 100644
index 00000000000..1ad91f999c3
--- /dev/null
+++ b/tests/FSharp.Compiler.ComponentTests/Language/ExtensionMethodTests.fs
@@ -0,0 +1,611 @@
+namespace FSharp.Compiler.ComponentTests.Language
+
+open FSharp.Test
+open Xunit
+open FSharp.Test.Compiler
+
+module ExtensionMethodTests =
+
+    [<Fact>]
+    let ``Extension method with toplevel attribute on type`` () =
+        Fsx
+            """
+open System.Runtime.CompilerServices
+
+[<Extension>]
+type Foo =
+    [<Extension>]
+    static member PlusOne (a:int) : int = a + 1
+
+let f (b:int) = b.PlusOne()
+            """
+        |> withLangVersionPreview
+        |> compile
+        |> shouldSucceed
+
+    [<Fact>]
+    let ``Extension method without toplevel attribute on type`` () =
+        Fsx
+            """
+open System.Runtime.CompilerServices
+
+type Foo =
+    [<Extension>]
+    static member PlusOne (a:int) : int = a + 1
+
+let f (b:int) = b.PlusOne()
+            """
+        |> withLangVersionPreview
+        |> compile
+        |> shouldSucceed
+        
+    [<Fact>]
+    let ``Extension method without toplevel attribute on type lang version 7`` () =
+        Fsx
+            """
+open System.Runtime.CompilerServices
+
+type Foo =
+    [<Extension>]
+    static member PlusOne (a:int) : int = a + 1
+
+let f (b:int) = b.PlusOne()
+            """
+        |> withLangVersion70
+        |> compile
+        |> shouldFail
+        |> withDiagnostics [
+            (Error 39, Line 8, Col 19, Line 8, Col 26, "The type 'Int32' does not define the field, constructor or member 'PlusOne'.")
+        ]
+
+    [<Fact>]
+    let ``Extension method without toplevel attribute on recursive type`` () =
+        Fsx
+            """
+open System.Runtime.CompilerServices
+
+type Foo =
+    class
+    end
+and Bar =
+    [<Extension>]
+    static member PlusOne (a:int) : int = a + 1
+
+let f (b:int) = b.PlusOne()
+            """
+        |> withLangVersionPreview
+        |> compile
+        |> shouldSucceed
+    
+    [<Fact>]
+    let ``F# CSharpStyleExtensionMethod consumed in C#`` () =
+        let fsharp =
+            FSharp
+                """
+module Hello
+
+open System.Runtime.CompilerServices
+
+type Foo =
+    [<Extension>]
+    static member PlusOne (a:int) : int = a + 1
+"""
+            |> withLangVersionPreview
+            |> withName "FSLib"
+
+        let csharp =
+            CSharp
+                """
+namespace Consumer
+{
+    using static Hello.Foo;
+
+    public class Class1
+    {
+        public Class1()
+        {
+            var meh = 1.PlusOne();
+        }
+    }
+}
+"""
+            |> withName "CSLib"
+            |> withReferences [ fsharp ]
+
+        csharp |> compile |> shouldSucceed
+        
+    [<Fact>]
+    let ``F# lang version 7 CSharpStyleExtensionMethod consumed in C#`` () =
+        let fsharp =
+            FSharp
+                """
+module Hello
+
+open System.Runtime.CompilerServices
+
+type Foo =
+    [<Extension>]
+    static member PlusOne (a:int) : int = a + 1
+"""
+            |> withLangVersion70
+            |> withName "FSLib"
+
+        let csharp =
+            CSharp
+                """
+namespace Consumer
+{
+    using static Hello.Foo;
+
+    public class Class1
+    {
+        public Class1()
+        {
+            var meh = 1.PlusOne();
+        }
+    }
+}
+"""
+            |> withName "CSLib"
+            |> withReferences [ fsharp ]
+
+        csharp
+        |> compile
+        |> shouldFail
+        |> withDiagnostics [
+            (Error 1061, Line 9, Col 25, Line 9, Col 32, "'int' does not contain a definition for 'PlusOne' and no accessible extension method 'PlusOne' accepting a first argument of type 'int' could be found (are you missing a using directive or an assembly reference?)")
+        ]
+
+    [<Fact>]
+    let ``F# CSharpStyleExtensionMethod in recursive type consumed in C#`` () =
+        let fsharp =
+            FSharp
+                """
+module Hello
+
+open System.Runtime.CompilerServices
+
+type Foo =
+    class
+    end
+and Bar =
+    [<Extension>]
+    static member PlusOne (a:int) : int = a + 1
+"""
+            |> withLangVersionPreview
+            |> withName "FSLib"
+
+        let csharp =
+            CSharp
+                """
+namespace Consumer
+{
+    using static Hello.Bar;
+
+    public class Class1
+    {
+        public Class1()
+        {
+            var meh = 1.PlusOne();
+        }
+    }
+}
+"""
+            |> withName "CSLib"
+            |> withReferences [ fsharp ]
+
+        csharp |> compile |> shouldSucceed
+    
+    [<Fact>]
+    let ``F# CSharpStyleExtensionMethod defined in top level module with attribute consumed in C#`` () =
+        let fsharp =
+            FSharp
+                """
+namespace Hello
+
+open System.Runtime.CompilerServices
+
+[<Extension>]
+module Foo =
+    [<Extension>]
+    let PlusOne (a:int) : int = a + 1
+"""
+            |> withLangVersionPreview
+            |> withName "FSLib"
+
+        let csharp =
+            CSharp
+                """
+namespace Consumer
+{
+    using static Hello.Foo;
+
+    public class Class1
+    {
+        public Class1()
+        {
+            var meh = 1.PlusOne();
+        }
+    }
+}
+"""
+            |> withName "CSLib"
+            |> withReferences [ fsharp ]
+
+        csharp |> compile |> shouldSucceed
+
+    [<Fact>]
+    let ``F# CSharpStyleExtensionMethod defined in top level module without attribute consumed in C#`` () =
+        let fsharp =
+            FSharp
+                """
+namespace Hello
+
+open System.Runtime.CompilerServices
+
+module Foo =
+    [<Extension>]
+    let PlusOne (a:int) : int = a + 1
+"""
+            |> withLangVersionPreview
+            |> withName "FSLib"
+
+        let csharp =
+            CSharp
+                """
+namespace Consumer
+{
+    using static Hello.Foo;
+
+    public class Class1
+    {
+        public Class1()
+        {
+            var meh = 1.PlusOne();
+        }
+    }
+}
+"""
+            |> withName "CSLib"
+            |> withReferences [ fsharp ]
+
+        csharp |> compile |> shouldSucceed
+
+    [<Fact>]
+    let ``Toplevel named module with Extension attribute and top level let binding with Extension attribute`` () =
+        let fsharp =
+            FSharp """
+    [<System.Runtime.CompilerServices.Extension>]
+    module Foo
+
+    [<System.Runtime.CompilerServices.Extension>]
+    let PlusOne (a:int) = a + 1
+    """
+           |> withLangVersionPreview
+           |> withName "FSLib"
+        
+        let csharp =
+            CSharp """
+    namespace Consumer
+    {
+        using static Foo;
+
+        public class Class1
+        {
+            public Class1()
+            {
+                var meh = 1.PlusOne();
+            }
+        }
+    }
+    """
+
+            |> withName "CSLib"
+            |> withReferences [ fsharp ]
+        
+        csharp |> compile |> shouldSucceed
+
+    [<Fact>]
+    let ``Toplevel named module without Extension attribute and top level let binding with Extension attribute`` () =
+        let fsharp =
+            FSharp """
+    module Foo
+
+    [<System.Runtime.CompilerServices.Extension>]
+    let PlusOne (a:int) = a + 1
+    """
+           |> withLangVersionPreview
+           |> withName "FSLib"
+        
+        let csharp =
+            CSharp """
+    namespace Consumer
+    {
+        using static Foo;
+
+        public class Class1
+        {
+            public Class1()
+            {
+                var meh = 1.PlusOne();
+            }
+        }
+    }
+    """
+
+            |> withName "CSLib"
+            |> withReferences [ fsharp ]
+        
+        csharp |> compile |> shouldSucceed
+
+    [<Fact>]
+    let ``Recursive toplevel named module with Extension attribute and top level let binding with Extension attribute`` () =
+        let fsharp =
+            FSharp """
+    [<System.Runtime.CompilerServices.Extension>]
+    module rec Foo
+
+    [<System.Runtime.CompilerServices.Extension>]
+    let PlusOne (a:int) = a + 1
+    """
+           |> withLangVersionPreview
+           |> withName "FSLib"
+        
+        let csharp =
+            CSharp """
+    namespace Consumer
+    {
+        using static Foo;
+
+        public class Class1
+        {
+            public Class1()
+            {
+                var meh = 1.PlusOne();
+            }
+        }
+    }
+    """
+
+            |> withName "CSLib"
+            |> withReferences [ fsharp ]
+        
+        csharp |> compile |> shouldSucceed
+    
+    [<Fact>]
+    let ``Recursive toplevel named module without Extension attribute and top level let binding with Extension attribute`` () =
+        let fsharp =
+            FSharp """
+    module rec Foo
+
+    [<System.Runtime.CompilerServices.Extension>]
+    let PlusOne (a:int) = a + 1
+    """
+           |> withLangVersionPreview
+           |> withName "FSLib"
+        
+        let csharp =
+            CSharp """
+    namespace Consumer
+    {
+        using static Foo;
+
+        public class Class1
+        {
+            public Class1()
+            {
+                var meh = 1.PlusOne();
+            }
+        }
+    }
+    """
+
+            |> withName "CSLib"
+            |> withReferences [ fsharp ]
+        
+        csharp |> compile |> shouldSucceed
+
+    [<Fact>]
+    let ``Foobar `` () =
+        let fsharp =
+            FSharp """
+module rec Foo
+
+[<System.Runtime.CompilerServices.Extension>]
+type Bar =
+    [<System.Runtime.CompilerServices.Extension>]
+    static member PlusOne (a:int) = a + 1
+    """
+           |> withLangVersionPreview
+           |> withName "FSLib"
+        
+        let csharp =
+            CSharp """
+    namespace Consumer
+    {
+        using static Foo.Bar;
+
+        public class Class1
+        {
+            public Class1()
+            {
+                var meh = 1.PlusOne();
+            }
+        }
+    }
+    """
+            |> withName "CSLib"
+            |> withReferences [ fsharp ]
+        
+        csharp |> compile |> shouldSucceed
+    
+    [<Fact>]
+    let ``Recursive named module with type with CSharp style extension can be consumed in CSharp`` () =
+        let fsharp =
+            FSharp """
+module rec Foo
+
+type Bar =
+    [<System.Runtime.CompilerServices.Extension>]
+    static member PlusOne (a:int) = a + 1
+    """
+            |> withLangVersionPreview
+            |> withName "FSLib"
+        
+        let csharp =
+            CSharp """
+    namespace Consumer
+    {
+        using static Foo.Bar;
+
+        public class Class1
+        {
+            public Class1()
+            {
+                var meh = 1.PlusOne();
+            }
+        }
+    }
+    """
+            |> withName "CSLib"
+            |> withReferences [ fsharp ]
+        
+        csharp |> compile |> shouldSucceed
+
+    [<Fact>]
+    let ``CSharp style extension method in F# type backed by a signature`` () =
+        let implementation =
+            SourceCodeFileKind.Create(
+                "Source.fs",
+                """
+module Foo
+
+open System.Runtime.CompilerServices
+
+type Bar =
+    [<Extension>]
+    static member PlusOne (a:int) : int = a + 1
+"""
+            )
+
+        let fsharp =
+            Fsi """
+module Foo
+
+open System.Runtime.CompilerServices
+
+[<Class>]
+type Bar =
+    [<Extension>]
+    static member PlusOne: a: int -> int
+"""
+           |> withLangVersionPreview
+           |> withAdditionalSourceFile implementation
+           |> withName "FSLib"
+        
+        let csharp =
+            CSharp """
+    namespace Consumer
+    {
+        using static Foo.Bar;
+
+        public class Class1
+        {
+            public Class1()
+            {
+                var meh = 1.PlusOne();
+            }
+        }
+    }
+    """
+            |> withName "CSLib"
+            |> withReferences [ fsharp ]
+        
+        csharp |> compile |> shouldSucceed
+
+    [<Fact>]
+    let ``CSharp style extension method in F# type backed by a signature in a recursive module`` () =
+        let implementation =
+            SourceCodeFileKind.Create(
+                "Source.fs",
+                """
+module rec Foo
+
+open System.Runtime.CompilerServices
+
+type Bar =
+    [<Extension>]
+    static member PlusOne (a:int) : int = a + 1
+"""
+            )
+
+        let fsharp =
+            Fsi """
+module rec Foo
+
+open System.Runtime.CompilerServices
+
+[<Class>]
+type Bar =
+    [<Extension>]
+    static member PlusOne: a: int -> int
+"""
+           |> withLangVersionPreview
+           |> withAdditionalSourceFile implementation
+           |> withName "FSLib"
+        
+        let csharp =
+            CSharp """
+    namespace Consumer
+    {
+        using static Foo.Bar;
+
+        public class Class1
+        {
+            public Class1()
+            {
+                var meh = 1.PlusOne();
+            }
+        }
+    }
+    """
+            |> withName "CSLib"
+            |> withReferences [ fsharp ]
+        
+        csharp |> compile |> shouldSucceed
+
+    [<Fact>]
+    let ``Multiple top level let binding with Extension attribute`` () =
+        let fsharp =
+            FSharp """
+    module Foo
+
+    [<System.Runtime.CompilerServices.Extension>]
+    let PlusOne (a:int) = a + 1
+    
+    [<System.Runtime.CompilerServices.Extension>]
+    let MinusOne (a:int) = a - 1
+    """
+           |> withLangVersionPreview
+           |> withName "FSLib"
+        
+        let csharp =
+            CSharp """
+    namespace Consumer
+    {
+        using static Foo;
+
+        public class Class1
+        {
+            public Class1()
+            {
+                var meh = 1.PlusOne().MinusOne();
+            }
+        }
+    }
+    """
+
+            |> withName "CSLib"
+            |> withReferences [ fsharp ]
+        
+        csharp |> compile |> shouldSucceed
diff --git a/tests/fsharp/single-test.fs b/tests/fsharp/single-test.fs
index 3e1ab4390bc..6d7b401e003 100644
--- a/tests/fsharp/single-test.fs
+++ b/tests/fsharp/single-test.fs
@@ -392,7 +392,7 @@ let singleVersionedNegTest (cfg: TestConfig) version testname =
 
     let cfg = {
         cfg with
-            fsc_flags = sprintf "%s %s --preferreduilang:en-US --define:NEGATIVE" cfg.fsc_flags options
+            fsc_flags = sprintf """%s %s --preferreduilang:en-US --define:NEGATIVE --simpleresolution /r:"%s" """ cfg.fsc_flags options cfg.FSCOREDLLPATH
             fsi_flags = sprintf "%s --preferreduilang:en-US %s" cfg.fsi_flags options
             }