diff --git a/docs/release-notes/.FSharp.Compiler.Service/8.0.400.md b/docs/release-notes/.FSharp.Compiler.Service/8.0.400.md
index 8f2038ba3f6..42c2e4fc3a8 100644
--- a/docs/release-notes/.FSharp.Compiler.Service/8.0.400.md
+++ b/docs/release-notes/.FSharp.Compiler.Service/8.0.400.md
@@ -34,3 +34,4 @@
 * Reduce allocations in compiler checking via `ValueOption` usage ([PR #16822](https://github.com/dotnet/fsharp/pull/16822))
 * Use AsyncLocal instead of ThreadStatic to hold Cancellable.Token ([PR #17156](https://github.com/dotnet/fsharp/pull/17156))
 * Showing and inserting correct name of entities from unopened namespace/module ([Issue #14375](https://github.com/dotnet/fsharp/issues/14375), [PR #17261](https://github.com/dotnet/fsharp/pull/17261))
+* Support lazy custom attributes calculation for `ILTypeDef` public API, improve `ExtensionAttribute` presence detecting perf. ([PR #16168](https://github.com/dotnet/fsharp/pull/16168))
diff --git a/src/Compiler/AbstractIL/il.fs b/src/Compiler/AbstractIL/il.fs
index 48cee265fdc..66b736f87c3 100644
--- a/src/Compiler/AbstractIL/il.fs
+++ b/src/Compiler/AbstractIL/il.fs
@@ -1258,6 +1258,9 @@ let storeILCustomAttrs (attrs: ILAttributes) =
     else
         ILAttributesStored.Given attrs
 
+let mkILCustomAttrsComputed f =
+    ILAttributesStored.Reader(fun _ -> f ())
+
 let mkILCustomAttrsReader f = ILAttributesStored.Reader f
 
 type ILCodeLabel = int
@@ -2611,6 +2614,14 @@ let convertInitSemantics (init: ILTypeInit) =
     | ILTypeInit.BeforeField -> TypeAttributes.BeforeFieldInit
     | ILTypeInit.OnAny -> enum 0
 
+[<Flags>]
+type ILTypeDefAdditionalFlags =
+    | None = 0
+    | IsKnownToBeAttribute = 1
+    /// The type can contain extension methods,
+    /// or this information may not be available at the time the ILTypeDef is created
+    | CanContainExtensionMethods = 2
+
 [<NoComparison; NoEquality; StructuredFormatDisplay("{DebugText}")>]
 type ILTypeDef
     (
@@ -2626,7 +2637,7 @@ type ILTypeDef
         methodImpls: ILMethodImplDefs,
         events: ILEventDefs,
         properties: ILPropertyDefs,
-        isKnownToBeAttribute: bool,
+        additionalFlags: ILTypeDefAdditionalFlags,
         securityDeclsStored: ILSecurityDeclsStored,
         customAttrsStored: ILAttributesStored,
         metadataIndex: int32
@@ -2634,6 +2645,8 @@ type ILTypeDef
 
     let mutable customAttrsStored = customAttrsStored
 
+    let hasFlag flag = additionalFlags &&& flag = flag
+
     new(name,
         attributes,
         layout,
@@ -2646,7 +2659,7 @@ type ILTypeDef
         methodImpls,
         events,
         properties,
-        isKnownToBeAttribute,
+        additionalFlags,
         securityDecls,
         customAttrs) =
         ILTypeDef(
@@ -2662,9 +2675,9 @@ type ILTypeDef
             methodImpls,
             events,
             properties,
-            isKnownToBeAttribute,
+            additionalFlags,
             storeILSecurityDecls securityDecls,
-            storeILCustomAttrs customAttrs,
+            customAttrs,
             NoMetadataIdx
         )
 
@@ -2694,7 +2707,10 @@ type ILTypeDef
 
     member _.Properties = properties
 
-    member _.IsKnownToBeAttribute = isKnownToBeAttribute
+    member _.IsKnownToBeAttribute = hasFlag ILTypeDefAdditionalFlags.IsKnownToBeAttribute
+
+    member _.CanContainExtensionMethods =
+        hasFlag ILTypeDefAdditionalFlags.CanContainExtensionMethods
 
     member _.CustomAttrsStored = customAttrsStored
 
@@ -2714,7 +2730,7 @@ type ILTypeDef
             ?methodImpls,
             ?events,
             ?properties,
-            ?isKnownToBeAttribute,
+            ?newAdditionalFlags,
             ?customAttrs,
             ?securityDecls
         ) =
@@ -2732,11 +2748,11 @@ type ILTypeDef
             methodImpls = defaultArg methodImpls x.MethodImpls,
             events = defaultArg events x.Events,
             properties = defaultArg properties x.Properties,
-            isKnownToBeAttribute = defaultArg isKnownToBeAttribute x.IsKnownToBeAttribute,
-            customAttrs = defaultArg customAttrs x.CustomAttrs
+            additionalFlags = defaultArg newAdditionalFlags additionalFlags,
+            customAttrs = defaultArg customAttrs (storeILCustomAttrs x.CustomAttrs)
         )
 
-    member x.CustomAttrs =
+    member x.CustomAttrs: ILAttributes =
         match customAttrsStored with
         | ILAttributesStored.Reader f ->
             let res = ILAttributes(f x.MetadataIndex)
@@ -4220,11 +4236,11 @@ let mkILGenericClass (nm, access, genparams, extends, impl, methods, fields, nes
         methods = methods,
         fields = fields,
         nestedTypes = nestedTypes,
-        customAttrs = attrs,
+        customAttrs = storeILCustomAttrs attrs,
         methodImpls = emptyILMethodImpls,
         properties = props,
         events = events,
-        isKnownToBeAttribute = false,
+        additionalFlags = ILTypeDefAdditionalFlags.None,
         securityDecls = emptyILSecurityDecls
     )
 
@@ -4244,11 +4260,11 @@ let mkRawDataValueTypeDef (iltyp_ValueType: ILType) (nm, size, pack) =
         methods = emptyILMethods,
         fields = emptyILFields,
         nestedTypes = emptyILTypeDefs,
-        customAttrs = emptyILCustomAttrs,
+        customAttrs = emptyILCustomAttrsStored,
         methodImpls = emptyILMethodImpls,
         properties = emptyILProperties,
         events = emptyILEvents,
-        isKnownToBeAttribute = false,
+        additionalFlags = ILTypeDefAdditionalFlags.None,
         securityDecls = emptyILSecurityDecls
     )
 
diff --git a/src/Compiler/AbstractIL/il.fsi b/src/Compiler/AbstractIL/il.fsi
index 5e02f4c0c1e..d6673131060 100644
--- a/src/Compiler/AbstractIL/il.fsi
+++ b/src/Compiler/AbstractIL/il.fsi
@@ -4,6 +4,7 @@
 
 module rec FSharp.Compiler.AbstractIL.IL
 
+open System
 open FSharp.Compiler.IO
 open System.Collections.Generic
 open System.Reflection
@@ -1481,6 +1482,12 @@ type ILTypeDefs =
     /// Calls to <c>ExistsByName</c> will result in all the ILPreTypeDefs being read.
     member internal ExistsByName: string -> bool
 
+[<Flags>]
+type ILTypeDefAdditionalFlags =
+    | None = 0
+    | IsKnownToBeAttribute = 1
+    | CanContainExtensionMethods = 2
+
 /// Represents IL Type Definitions.
 [<NoComparison; NoEquality>]
 type ILTypeDef =
@@ -1499,7 +1506,7 @@ type ILTypeDef =
         methodImpls: ILMethodImplDefs *
         events: ILEventDefs *
         properties: ILPropertyDefs *
-        isKnownToBeAttribute: bool *
+        additionalFlags: ILTypeDefAdditionalFlags *
         securityDeclsStored: ILSecurityDeclsStored *
         customAttrsStored: ILAttributesStored *
         metadataIndex: int32 ->
@@ -1519,9 +1526,9 @@ type ILTypeDef =
         methodImpls: ILMethodImplDefs *
         events: ILEventDefs *
         properties: ILPropertyDefs *
-        isKnownToBeAttribute: bool *
+        additionalFlags: ILTypeDefAdditionalFlags *
         securityDecls: ILSecurityDecls *
-        customAttrs: ILAttributes ->
+        customAttrs: ILAttributesStored ->
             ILTypeDef
 
     member Name: string
@@ -1556,6 +1563,7 @@ type ILTypeDef =
     member HasSecurity: bool
     member Encoding: ILDefaultPInvokeEncoding
     member IsKnownToBeAttribute: bool
+    member CanContainExtensionMethods: bool
 
     member internal WithAccess: ILTypeDefAccess -> ILTypeDef
     member internal WithNestedAccess: ILMemberAccess -> ILTypeDef
@@ -1584,8 +1592,8 @@ type ILTypeDef =
         ?methodImpls: ILMethodImplDefs *
         ?events: ILEventDefs *
         ?properties: ILPropertyDefs *
-        ?isKnownToBeAttribute: bool *
-        ?customAttrs: ILAttributes *
+        ?newAdditionalFlags: ILTypeDefAdditionalFlags *
+        ?customAttrs: ILAttributesStored *
         ?securityDecls: ILSecurityDecls ->
             ILTypeDef
 
@@ -2212,8 +2220,10 @@ val internal mkILTypeForGlobalFunctions: ILScopeRef -> ILType
 val mkILCustomAttrs: ILAttribute list -> ILAttributes
 val mkILCustomAttrsFromArray: ILAttribute[] -> ILAttributes
 val storeILCustomAttrs: ILAttributes -> ILAttributesStored
+val mkILCustomAttrsComputed: (unit -> ILAttribute[]) -> ILAttributesStored
 val internal mkILCustomAttrsReader: (int32 -> ILAttribute[]) -> ILAttributesStored
 val emptyILCustomAttrs: ILAttributes
+val emptyILCustomAttrsStored: ILAttributesStored
 
 val mkILSecurityDecls: ILSecurityDecl list -> ILSecurityDecls
 val emptyILSecurityDecls: ILSecurityDecls
diff --git a/src/Compiler/AbstractIL/ilmorph.fs b/src/Compiler/AbstractIL/ilmorph.fs
index b4305791076..334ed93d212 100644
--- a/src/Compiler/AbstractIL/ilmorph.fs
+++ b/src/Compiler/AbstractIL/ilmorph.fs
@@ -378,7 +378,7 @@ let rec tdef_ty2ty_ilmbody2ilmbody_mdefs2mdefs isInKnownSet enc fs (tdef: ILType
         methodImpls = mimpls_ty2ty fTyInCtxtR tdef.MethodImpls,
         events = edefs_ty2ty fTyInCtxtR tdef.Events,
         properties = pdefs_ty2ty fTyInCtxtR tdef.Properties,
-        customAttrs = cattrs_ty2ty fTyInCtxtR tdef.CustomAttrs
+        customAttrs = storeILCustomAttrs (cattrs_ty2ty fTyInCtxtR tdef.CustomAttrs)
     )
 
 and tdefs_ty2ty_ilmbody2ilmbody_mdefs2mdefs isInKnownSet enc fs tdefs =
diff --git a/src/Compiler/AbstractIL/ilread.fs b/src/Compiler/AbstractIL/ilread.fs
index 61c04f8e4ba..1535078bfe0 100644
--- a/src/Compiler/AbstractIL/ilread.fs
+++ b/src/Compiler/AbstractIL/ilread.fs
@@ -858,10 +858,10 @@ let hsCompare (TaggedIndex(t1: HasSemanticsTag, idx1: int)) (TaggedIndex(t2: Has
     elif idx1 > idx2 then 1
     else compare t1.Tag t2.Tag
 
-let hcaCompare (TaggedIndex(t1: HasCustomAttributeTag, idx1: int)) (TaggedIndex(t2: HasCustomAttributeTag, idx2)) =
-    if idx1 < idx2 then -1
-    elif idx1 > idx2 then 1
-    else compare t1.Tag t2.Tag
+let inline hcaCompare (t1: TaggedIndex<HasCustomAttributeTag>) (t2: TaggedIndex<HasCustomAttributeTag>) =
+    if t1.index < t2.index then -1
+    elif t1.index > t2.index then 1
+    else compare t1.tag t2.tag
 
 let mfCompare (TaggedIndex(t1: MemberForwardedTag, idx1: int)) (TaggedIndex(t2: MemberForwardedTag, idx2)) =
     if idx1 < idx2 then -1
@@ -2112,9 +2112,82 @@ and typeDefReader ctxtH : ILTypeDefStored =
         let layout = typeLayoutOfFlags ctxt mdv flags idx
 
         let hasLayout =
-            (match layout with
-             | ILTypeDefLayout.Explicit _ -> true
-             | _ -> false)
+            match layout with
+            | ILTypeDefLayout.Explicit _ -> true
+            | _ -> false
+
+        let containsExtensionMethods =
+            let mutable containsExtensionMethods = false
+            let searchedKey = TaggedIndex(hca_TypeDef, idx)
+
+            let attributesSearcher =
+                { new ISeekReadIndexedRowReader<int, int, int> with
+                    member _.GetRow(i, rowIndex) = rowIndex <- i
+                    member _.GetKey(rowIndex) = rowIndex
+
+                    member _.CompareKey(rowIndex) =
+                        let mutable addr = ctxt.rowAddr TableNames.CustomAttribute rowIndex
+                        // read parentIndex
+                        let key = seekReadHasCustomAttributeIdx ctxt mdv &addr
+                        hcaCompare searchedKey key
+
+                    member _.ConvertRow(i) = i
+                }
+
+            let attrsStartIdx, attrsEndIdx =
+                seekReadIndexedRowsRange
+                    (ctxt.getNumRows TableNames.CustomAttribute)
+                    (isSorted ctxt TableNames.CustomAttribute)
+                    attributesSearcher
+
+            if attrsStartIdx <= 0 || attrsEndIdx < attrsStartIdx then
+                false
+            else
+                let mutable attrIdx = attrsStartIdx
+
+                let looksLikeSystemAssembly =
+                    ctxt.fileName.EndsWith("System.Runtime.dll")
+                    || ctxt.fileName.EndsWith("mscorlib.dll")
+                    || ctxt.fileName.EndsWith("netstandard.dll")
+
+                while attrIdx <= attrsEndIdx && not containsExtensionMethods do
+                    let mutable addr = ctxt.rowAddr TableNames.CustomAttribute attrIdx
+                    // skip parentIndex to read typeIndex
+                    seekReadHasCustomAttributeIdx ctxt mdv &addr |> ignore
+                    let attrTypeIndex = seekReadCustomAttributeTypeIdx ctxt mdv &addr
+                    let attrCtorIdx = attrTypeIndex.index
+
+                    let name =
+                        if attrTypeIndex.tag = cat_MethodDef then
+                            // the ExtensionAttribute constructor can be cat_MethodDef if the metadata is read from the assembly
+                            // in which the corresponding attribute is defined -- from the system library
+                            if not looksLikeSystemAssembly then
+                                ""
+                            else
+                                let _, (_, nameIdx, namespaceIdx, _, _, _) = seekMethodDefParent ctxt attrCtorIdx
+                                readBlobHeapAsTypeName ctxt (nameIdx, namespaceIdx)
+                        else
+                            let mutable addr = ctxt.rowAddr TableNames.MemberRef attrCtorIdx
+                            let mrpTag = seekReadMemberRefParentIdx ctxt mdv &addr
+
+                            if mrpTag.tag <> mrp_TypeRef then
+                                ""
+                            else
+                                let _, nameIdx, namespaceIdx = seekReadTypeRefRow ctxt mdv mrpTag.index
+                                readBlobHeapAsTypeName ctxt (nameIdx, namespaceIdx)
+
+                    if name = "System.Runtime.CompilerServices.ExtensionAttribute" then
+                        containsExtensionMethods <- true
+
+                    attrIdx <- attrIdx + 1
+
+                containsExtensionMethods
+
+        let additionalFlags =
+            if containsExtensionMethods then
+                ILTypeDefAdditionalFlags.CanContainExtensionMethods
+            else
+                ILTypeDefAdditionalFlags.None
 
         let mdefs = seekReadMethods ctxt numTypars methodsIdx endMethodsIdx
         let fdefs = seekReadFields ctxt (numTypars, hasLayout) fieldsIdx endFieldsIdx
@@ -2138,7 +2211,7 @@ and typeDefReader ctxtH : ILTypeDefStored =
             methodImpls = mimpls,
             events = events,
             properties = props,
-            isKnownToBeAttribute = false,
+            additionalFlags = additionalFlags,
             customAttrsStored = ctxt.customAttrsReader_TypeDef,
             metadataIndex = idx
         ))
@@ -2797,22 +2870,26 @@ and seekReadMemberRefAsFieldSpecUncached ctxtH (MemberRefAsFspecIdx(numTypars, i
 // method-range and field-range start/finish indexes
 and seekReadMethodDefAsMethodData ctxt idx = ctxt.seekReadMethodDefAsMethodData idx
 
+and seekMethodDefParent (ctxt: ILMetadataReader) methodIdx =
+    seekReadIndexedRow (
+        ctxt.getNumRows TableNames.TypeDef,
+        (fun i -> i, seekReadTypeDefRow ctxt i),
+        id,
+        (fun (i, (_, _, _, _, _, methodsIdx as info)) ->
+            if methodsIdx > methodIdx then
+                -1
+            else
+                let struct (_, endMethodsIdx) = seekReadTypeDefRowExtents ctxt info i
+                if endMethodsIdx <= methodIdx then 1 else 0),
+        true,
+        id
+    )
+
 and seekReadMethodDefAsMethodDataUncached ctxtH idx =
     let (ctxt: ILMetadataReader) = getHole ctxtH
     let mdv = ctxt.mdfile.GetView()
     // Look for the method def parent.
-    let tidx =
-        seekReadIndexedRow (
-            ctxt.getNumRows TableNames.TypeDef,
-            (fun i -> i, seekReadTypeDefRowWithExtents ctxt i),
-            id,
-            (fun (_, ((_, _, _, _, _, methodsIdx), (_, endMethodsIdx))) ->
-                if endMethodsIdx <= idx then 1
-                elif methodsIdx <= idx && idx < endMethodsIdx then 0
-                else -1),
-            true,
-            fst
-        )
+    let tidx, _ = seekMethodDefParent ctxt idx
     // Create a formal instantiation if needed
     let typeGenericArgs = seekReadGenericParams ctxt 0 (tomd_TypeDef, tidx)
     let typeGenericArgsCount = typeGenericArgs.Length
diff --git a/src/Compiler/Checking/NameResolution.fs b/src/Compiler/Checking/NameResolution.fs
index 780824e3ead..6605ae861f2 100644
--- a/src/Compiler/Checking/NameResolution.fs
+++ b/src/Compiler/Checking/NameResolution.fs
@@ -524,7 +524,10 @@ let NextExtensionMethodPriority() = uint64 (newStamp())
 /// Checks if the type is used for C# style extension members.
 let IsTyconRefUsedForCSharpStyleExtensionMembers g m (tcref: TyconRef) =
     // Type must be non-generic and have 'Extension' attribute
-    isNil(tcref.Typars m) && TyconRefHasAttribute g m g.attrib_ExtensionAttribute tcref
+    match metadataOfTycon tcref.Deref with
+    | ILTypeMetadata(TILObjectReprData(_, _, tdef)) -> tdef.CanContainExtensionMethods
+    | _ -> true
+    && isNil(tcref.Typars m) && TyconRefHasAttribute g m g.attrib_ExtensionAttribute tcref
 
 /// Checks if the type is used for C# style extension members.
 let IsTypeUsedForCSharpStyleExtensionMembers g m ty =
diff --git a/src/Compiler/CodeGen/EraseClosures.fs b/src/Compiler/CodeGen/EraseClosures.fs
index cf1499f0c30..7eddfd9b820 100644
--- a/src/Compiler/CodeGen/EraseClosures.fs
+++ b/src/Compiler/CodeGen/EraseClosures.fs
@@ -578,11 +578,11 @@ let rec convIlxClosureDef cenv encl (td: ILTypeDef) clo =
                         extends = Some cenv.mkILTyFuncTy,
                         methods = mkILMethods (ctorMethodDef :: nowApplyMethDef :: nowMethods),
                         fields = mkILFields (mkILCloFldDefs cenv nowFields @ td.Fields.AsList()),
-                        customAttrs = emptyILCustomAttrs,
+                        customAttrs = emptyILCustomAttrsStored,
                         methodImpls = emptyILMethodImpls,
                         properties = emptyILProperties,
                         events = emptyILEvents,
-                        isKnownToBeAttribute = false,
+                        additionalFlags = ILTypeDefAdditionalFlags.None,
                         securityDecls = emptyILSecurityDecls
                     )
                         .WithSpecialName(false)
@@ -712,11 +712,11 @@ let rec convIlxClosureDef cenv encl (td: ILTypeDef) clo =
                         extends = Some nowEnvParentClass,
                         methods = mkILMethods (ctorMethodDef :: nowApplyMethDef :: nowMethods),
                         fields = mkILFields (mkILCloFldDefs cenv nowFields @ td.Fields.AsList()),
-                        customAttrs = emptyILCustomAttrs,
+                        customAttrs = emptyILCustomAttrsStored,
                         methodImpls = emptyILMethodImpls,
                         properties = emptyILProperties,
                         events = emptyILEvents,
-                        isKnownToBeAttribute = false,
+                        additionalFlags = ILTypeDefAdditionalFlags.None,
                         securityDecls = emptyILSecurityDecls
                     )
                         .WithHasSecurity(false)
diff --git a/src/Compiler/CodeGen/EraseUnions.fs b/src/Compiler/CodeGen/EraseUnions.fs
index e21f76b3071..d5670ed3fdf 100644
--- a/src/Compiler/CodeGen/EraseUnions.fs
+++ b/src/Compiler/CodeGen/EraseUnions.fs
@@ -1452,8 +1452,8 @@ let mkClassUnionDef
                     methodImpls = emptyILMethodImpls,
                     events = emptyILEvents,
                     properties = emptyILProperties,
-                    isKnownToBeAttribute = false,
-                    customAttrs = emptyILCustomAttrs
+                    additionalFlags = ILTypeDefAdditionalFlags.None,
+                    customAttrs = emptyILCustomAttrsStored
                 )
                     .WithNestedAccess(cud.UnionCasesAccessibility)
                     .WithAbstract(true)
diff --git a/src/Compiler/CodeGen/IlxGen.fs b/src/Compiler/CodeGen/IlxGen.fs
index 9be96f680b7..c152a0f239a 100644
--- a/src/Compiler/CodeGen/IlxGen.fs
+++ b/src/Compiler/CodeGen/IlxGen.fs
@@ -6202,19 +6202,21 @@ and GenStructStateMachine cenv cgbuf eenvouter (res: LoweredStateMachine) sequel
                 yield fdef
         ]
 
+    let customAttrs =
+        [
+            g.CompilerGeneratedAttribute
+            mkCompilationMappingAttr g (int SourceConstructFlags.Closure)
+        ]
+        |> mkILCustomAttrs
+        |> storeILCustomAttrs
+
     let cloTypeDef =
         ILTypeDef(
             name = ilCloTypeRef.Name,
             layout = ILTypeDefLayout.Auto,
             attributes = enum 0,
             genericParams = ilCloGenericFormals,
-            customAttrs =
-                mkILCustomAttrs (
-                    [
-                        g.CompilerGeneratedAttribute
-                        mkCompilationMappingAttr g (int SourceConstructFlags.Closure)
-                    ]
-                ),
+            customAttrs = customAttrs,
             fields = mkILFields fdefs,
             events = emptyILEvents,
             properties = emptyILProperties,
@@ -6223,7 +6225,7 @@ and GenStructStateMachine cenv cgbuf eenvouter (res: LoweredStateMachine) sequel
             nestedTypes = emptyILTypeDefs,
             implements = ilInterfaceTys,
             extends = Some super,
-            isKnownToBeAttribute = false,
+            additionalFlags = ILTypeDefAdditionalFlags.None,
             securityDecls = emptyILSecurityDecls
         )
             .WithSealed(true)
@@ -6636,13 +6638,18 @@ and GenClosureTypeDefs
         else
             mdefs, []
 
+    let customAttrs =
+        attrs @ [ mkCompilationMappingAttr g (int SourceConstructFlags.Closure) ]
+        |> mkILCustomAttrs
+        |> storeILCustomAttrs
+
     let tdef =
         ILTypeDef(
             name = tref.Name,
             layout = ILTypeDefLayout.Auto,
             attributes = enum 0,
             genericParams = ilGenParams,
-            customAttrs = mkILCustomAttrs (attrs @ [ mkCompilationMappingAttr g (int SourceConstructFlags.Closure) ]),
+            customAttrs = customAttrs,
             fields = mkILFields fdefs,
             events = emptyILEvents,
             properties = emptyILProperties,
@@ -6651,7 +6658,7 @@ and GenClosureTypeDefs
             nestedTypes = emptyILTypeDefs,
             implements = ilIntfTys,
             extends = Some ext,
-            isKnownToBeAttribute = false,
+            additionalFlags = ILTypeDefAdditionalFlags.None,
             securityDecls = emptyILSecurityDecls
         )
             .WithSealed(true)
@@ -11327,8 +11334,9 @@ and GenTypeDef cenv mgbuf lazyInitInfo eenv m (tycon: Tycon) : ILTypeRef option
                 | TILObjectRepr _ ->
                     let tdef = tycon.ILTyconRawMetadata.WithAccess tyconAccess
 
-                    let tdef =
-                        tdef.With(customAttrs = mkILCustomAttrs ilCustomAttrs, genericParams = ilGenParams)
+                    let customAttrs = ilCustomAttrs |> mkILCustomAttrs |> storeILCustomAttrs
+
+                    let tdef = tdef.With(customAttrs = customAttrs, genericParams = ilGenParams)
 
                     tdef, None
 
@@ -11364,6 +11372,12 @@ and GenTypeDef cenv mgbuf lazyInitInfo eenv m (tycon: Tycon) : ILTypeRef option
                     let isKnownToBeAttribute =
                         ExistsSameHeadTypeInHierarchy g cenv.amap m super g.mk_Attribute_ty
 
+                    let additionalFlags =
+                        if isKnownToBeAttribute then
+                            ILTypeDefAdditionalFlags.IsKnownToBeAttribute
+                        else
+                            ILTypeDefAdditionalFlags.None
+
                     let tdef =
                         mkILGenericClass (
                             ilTypeName,
@@ -11389,7 +11403,7 @@ and GenTypeDef cenv mgbuf lazyInitInfo eenv m (tycon: Tycon) : ILTypeRef option
                             .WithSerializable(isSerializable)
                             .WithAbstract(isAbstract)
                             .WithImport(isComInteropTy g thisTy)
-                            .With(methodImpls = mkILMethodImpls methodImpls, isKnownToBeAttribute = isKnownToBeAttribute)
+                            .With(methodImpls = mkILMethodImpls methodImpls, newAdditionalFlags = additionalFlags)
 
                     let tdLayout, tdEncoding =
                         match TryFindFSharpAttribute g g.attrib_StructLayoutAttribute tycon.Attribs with
@@ -11524,19 +11538,19 @@ and GenTypeDef cenv mgbuf lazyInitInfo eenv m (tycon: Tycon) : ILTypeRef option
                             ILTypeDefLayout.Auto
 
                     let cattrs =
-                        mkILCustomAttrs (
-                            ilCustomAttrs
-                            @ [
-                                mkCompilationMappingAttr
-                                    g
-                                    (int (
-                                        if hiddenRepr then
-                                            SourceConstructFlags.SumType ||| SourceConstructFlags.NonPublicRepresentation
-                                        else
-                                            SourceConstructFlags.SumType
-                                    ))
-                            ]
-                        )
+                        ilCustomAttrs
+                        @ [
+                            mkCompilationMappingAttr
+                                g
+                                (int (
+                                    if hiddenRepr then
+                                        SourceConstructFlags.SumType ||| SourceConstructFlags.NonPublicRepresentation
+                                    else
+                                        SourceConstructFlags.SumType
+                                ))
+                        ]
+                        |> mkILCustomAttrs
+                        |> storeILCustomAttrs
 
                     let tdef =
                         ILTypeDef(
@@ -11559,7 +11573,7 @@ and GenTypeDef cenv mgbuf lazyInitInfo eenv m (tycon: Tycon) : ILTypeRef option
                                     else
                                         g.ilg.typ_Object
                                 ),
-                            isKnownToBeAttribute = false,
+                            additionalFlags = ILTypeDefAdditionalFlags.None,
                             securityDecls = emptyILSecurityDecls
                         )
                             .WithLayout(layout)
diff --git a/tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.SurfaceArea.netstandard20.debug.bsl b/tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.SurfaceArea.netstandard20.debug.bsl
index 447ebd82026..63a16671bca 100644
--- a/tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.SurfaceArea.netstandard20.debug.bsl
+++ b/tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.SurfaceArea.netstandard20.debug.bsl
@@ -1483,6 +1483,7 @@ FSharp.Compiler.AbstractIL.IL+ILType: System.String QualifiedName
 FSharp.Compiler.AbstractIL.IL+ILType: System.String ToString()
 FSharp.Compiler.AbstractIL.IL+ILType: System.String get_BasicQualifiedName()
 FSharp.Compiler.AbstractIL.IL+ILType: System.String get_QualifiedName()
+FSharp.Compiler.AbstractIL.IL+ILTypeDef: Boolean CanContainExtensionMethods
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Boolean HasSecurity
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Boolean IsAbstract
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Boolean IsClass
@@ -1496,6 +1497,7 @@ FSharp.Compiler.AbstractIL.IL+ILTypeDef: Boolean IsSerializable
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Boolean IsSpecialName
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Boolean IsStruct
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Boolean IsStructOrEnum
+FSharp.Compiler.AbstractIL.IL+ILTypeDef: Boolean get_CanContainExtensionMethods()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Boolean get_HasSecurity()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Boolean get_IsAbstract()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Boolean get_IsClass()
@@ -1525,7 +1527,7 @@ FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILPropertyDefs Properties
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILPropertyDefs get_Properties()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILSecurityDecls SecurityDecls
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILSecurityDecls get_SecurityDecls()
-FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILTypeDef With(Microsoft.FSharp.Core.FSharpOption`1[System.String], Microsoft.FSharp.Core.FSharpOption`1[System.Reflection.TypeAttributes], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILTypeDefLayout], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILType]], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILGenericParameterDef]], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILType]], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILMethodDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILTypeDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILFieldDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILMethodImplDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILEventDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILPropertyDefs], Microsoft.FSharp.Core.FSharpOption`1[System.Boolean], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILAttributes], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILSecurityDecls])
+FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILTypeDef With(Microsoft.FSharp.Core.FSharpOption`1[System.String], Microsoft.FSharp.Core.FSharpOption`1[System.Reflection.TypeAttributes], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILTypeDefLayout], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILType]], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILGenericParameterDef]], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILType]], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILMethodDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILTypeDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILFieldDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILMethodImplDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILEventDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILPropertyDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILTypeDefAdditionalFlags], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILAttributesStored], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILSecurityDecls])
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILTypeDefAccess Access
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILTypeDefAccess get_Access()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILTypeDefLayout Layout
@@ -1543,7 +1545,7 @@ FSharp.Compiler.AbstractIL.IL+ILTypeDef: System.Reflection.TypeAttributes get_At
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: System.String Name
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: System.String ToString()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: System.String get_Name()
-FSharp.Compiler.AbstractIL.IL+ILTypeDef: Void .ctor(System.String, System.Reflection.TypeAttributes, ILTypeDefLayout, Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILType], Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILGenericParameterDef], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILType], ILMethodDefs, ILTypeDefs, ILFieldDefs, ILMethodImplDefs, ILEventDefs, ILPropertyDefs, Boolean, ILSecurityDecls, ILAttributes)
+FSharp.Compiler.AbstractIL.IL+ILTypeDef: Void .ctor(System.String, System.Reflection.TypeAttributes, ILTypeDefLayout, Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILType], Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILGenericParameterDef], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILType], ILMethodDefs, ILTypeDefs, ILFieldDefs, ILMethodImplDefs, ILEventDefs, ILPropertyDefs, ILTypeDefAdditionalFlags, ILSecurityDecls, ILAttributesStored)
 FSharp.Compiler.AbstractIL.IL+ILTypeDefAccess+Nested: ILMemberAccess Item
 FSharp.Compiler.AbstractIL.IL+ILTypeDefAccess+Nested: ILMemberAccess get_Item()
 FSharp.Compiler.AbstractIL.IL+ILTypeDefAccess+Tags: Int32 Nested
@@ -1574,6 +1576,10 @@ FSharp.Compiler.AbstractIL.IL+ILTypeDefAccess: Int32 GetHashCode(System.Collecti
 FSharp.Compiler.AbstractIL.IL+ILTypeDefAccess: Int32 Tag
 FSharp.Compiler.AbstractIL.IL+ILTypeDefAccess: Int32 get_Tag()
 FSharp.Compiler.AbstractIL.IL+ILTypeDefAccess: System.String ToString()
+FSharp.Compiler.AbstractIL.IL+ILTypeDefAdditionalFlags: ILTypeDefAdditionalFlags CanContainExtensionMethods
+FSharp.Compiler.AbstractIL.IL+ILTypeDefAdditionalFlags: ILTypeDefAdditionalFlags IsKnownToBeAttribute
+FSharp.Compiler.AbstractIL.IL+ILTypeDefAdditionalFlags: ILTypeDefAdditionalFlags None
+FSharp.Compiler.AbstractIL.IL+ILTypeDefAdditionalFlags: Int32 value__
 FSharp.Compiler.AbstractIL.IL+ILTypeDefKind+Tags: Int32 Class
 FSharp.Compiler.AbstractIL.IL+ILTypeDefKind+Tags: Int32 Delegate
 FSharp.Compiler.AbstractIL.IL+ILTypeDefKind+Tags: Int32 Enum
@@ -1853,6 +1859,7 @@ FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+ILThisConvention
 FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+ILType
 FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+ILTypeDef
 FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+ILTypeDefAccess
+FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+ILTypeDefAdditionalFlags
 FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+ILTypeDefKind
 FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+ILTypeDefLayout
 FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+ILTypeDefs
@@ -1866,6 +1873,9 @@ FSharp.Compiler.AbstractIL.IL: ILAttributes emptyILCustomAttrs
 FSharp.Compiler.AbstractIL.IL: ILAttributes get_emptyILCustomAttrs()
 FSharp.Compiler.AbstractIL.IL: ILAttributes mkILCustomAttrs(Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILAttribute])
 FSharp.Compiler.AbstractIL.IL: ILAttributes mkILCustomAttrsFromArray(ILAttribute[])
+FSharp.Compiler.AbstractIL.IL: ILAttributesStored emptyILCustomAttrsStored
+FSharp.Compiler.AbstractIL.IL: ILAttributesStored get_emptyILCustomAttrsStored()
+FSharp.Compiler.AbstractIL.IL: ILAttributesStored mkILCustomAttrsComputed(Microsoft.FSharp.Core.FSharpFunc`2[Microsoft.FSharp.Core.Unit,FSharp.Compiler.AbstractIL.IL+ILAttribute[]])
 FSharp.Compiler.AbstractIL.IL: ILAttributesStored storeILCustomAttrs(ILAttributes)
 FSharp.Compiler.AbstractIL.IL: ILEventDefs emptyILEvents
 FSharp.Compiler.AbstractIL.IL: ILEventDefs get_emptyILEvents()
diff --git a/tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.SurfaceArea.netstandard20.release.bsl b/tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.SurfaceArea.netstandard20.release.bsl
index 4dc729720c0..646e067251e 100644
--- a/tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.SurfaceArea.netstandard20.release.bsl
+++ b/tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.SurfaceArea.netstandard20.release.bsl
@@ -1483,6 +1483,7 @@ FSharp.Compiler.AbstractIL.IL+ILType: System.String QualifiedName
 FSharp.Compiler.AbstractIL.IL+ILType: System.String ToString()
 FSharp.Compiler.AbstractIL.IL+ILType: System.String get_BasicQualifiedName()
 FSharp.Compiler.AbstractIL.IL+ILType: System.String get_QualifiedName()
+FSharp.Compiler.AbstractIL.IL+ILTypeDef: Boolean CanContainExtensionMethods
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Boolean HasSecurity
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Boolean IsAbstract
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Boolean IsClass
@@ -1496,6 +1497,7 @@ FSharp.Compiler.AbstractIL.IL+ILTypeDef: Boolean IsSerializable
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Boolean IsSpecialName
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Boolean IsStruct
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Boolean IsStructOrEnum
+FSharp.Compiler.AbstractIL.IL+ILTypeDef: Boolean get_CanContainExtensionMethods()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Boolean get_HasSecurity()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Boolean get_IsAbstract()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Boolean get_IsClass()
@@ -1525,7 +1527,7 @@ FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILPropertyDefs Properties
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILPropertyDefs get_Properties()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILSecurityDecls SecurityDecls
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILSecurityDecls get_SecurityDecls()
-FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILTypeDef With(Microsoft.FSharp.Core.FSharpOption`1[System.String], Microsoft.FSharp.Core.FSharpOption`1[System.Reflection.TypeAttributes], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILTypeDefLayout], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILType]], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILGenericParameterDef]], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILType]], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILMethodDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILTypeDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILFieldDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILMethodImplDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILEventDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILPropertyDefs], Microsoft.FSharp.Core.FSharpOption`1[System.Boolean], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILAttributes], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILSecurityDecls])
+FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILTypeDef With(Microsoft.FSharp.Core.FSharpOption`1[System.String], Microsoft.FSharp.Core.FSharpOption`1[System.Reflection.TypeAttributes], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILTypeDefLayout], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILType]], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILGenericParameterDef]], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILType]], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILMethodDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILTypeDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILFieldDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILMethodImplDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILEventDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILPropertyDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILTypeDefAdditionalFlags], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILAttributesStored], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILSecurityDecls])
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILTypeDefAccess Access
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILTypeDefAccess get_Access()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILTypeDefLayout Layout
@@ -1543,7 +1545,7 @@ FSharp.Compiler.AbstractIL.IL+ILTypeDef: System.Reflection.TypeAttributes get_At
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: System.String Name
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: System.String ToString()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: System.String get_Name()
-FSharp.Compiler.AbstractIL.IL+ILTypeDef: Void .ctor(System.String, System.Reflection.TypeAttributes, ILTypeDefLayout, Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILType], Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILGenericParameterDef], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILType], ILMethodDefs, ILTypeDefs, ILFieldDefs, ILMethodImplDefs, ILEventDefs, ILPropertyDefs, Boolean, ILSecurityDecls, ILAttributes)
+FSharp.Compiler.AbstractIL.IL+ILTypeDef: Void .ctor(System.String, System.Reflection.TypeAttributes, ILTypeDefLayout, Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILType], Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILGenericParameterDef], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILType], ILMethodDefs, ILTypeDefs, ILFieldDefs, ILMethodImplDefs, ILEventDefs, ILPropertyDefs, ILTypeDefAdditionalFlags, ILSecurityDecls, ILAttributesStored)
 FSharp.Compiler.AbstractIL.IL+ILTypeDefAccess+Nested: ILMemberAccess Item
 FSharp.Compiler.AbstractIL.IL+ILTypeDefAccess+Nested: ILMemberAccess get_Item()
 FSharp.Compiler.AbstractIL.IL+ILTypeDefAccess+Tags: Int32 Nested
@@ -1574,6 +1576,10 @@ FSharp.Compiler.AbstractIL.IL+ILTypeDefAccess: Int32 GetHashCode(System.Collecti
 FSharp.Compiler.AbstractIL.IL+ILTypeDefAccess: Int32 Tag
 FSharp.Compiler.AbstractIL.IL+ILTypeDefAccess: Int32 get_Tag()
 FSharp.Compiler.AbstractIL.IL+ILTypeDefAccess: System.String ToString()
+FSharp.Compiler.AbstractIL.IL+ILTypeDefAdditionalFlags: ILTypeDefAdditionalFlags CanContainExtensionMethods
+FSharp.Compiler.AbstractIL.IL+ILTypeDefAdditionalFlags: ILTypeDefAdditionalFlags IsKnownToBeAttribute
+FSharp.Compiler.AbstractIL.IL+ILTypeDefAdditionalFlags: ILTypeDefAdditionalFlags None
+FSharp.Compiler.AbstractIL.IL+ILTypeDefAdditionalFlags: Int32 value__
 FSharp.Compiler.AbstractIL.IL+ILTypeDefKind+Tags: Int32 Class
 FSharp.Compiler.AbstractIL.IL+ILTypeDefKind+Tags: Int32 Delegate
 FSharp.Compiler.AbstractIL.IL+ILTypeDefKind+Tags: Int32 Enum
@@ -1853,6 +1859,7 @@ FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+ILThisConvention
 FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+ILType
 FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+ILTypeDef
 FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+ILTypeDefAccess
+FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+ILTypeDefAdditionalFlags
 FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+ILTypeDefKind
 FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+ILTypeDefLayout
 FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+ILTypeDefs
@@ -1866,6 +1873,9 @@ FSharp.Compiler.AbstractIL.IL: ILAttributes emptyILCustomAttrs
 FSharp.Compiler.AbstractIL.IL: ILAttributes get_emptyILCustomAttrs()
 FSharp.Compiler.AbstractIL.IL: ILAttributes mkILCustomAttrs(Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILAttribute])
 FSharp.Compiler.AbstractIL.IL: ILAttributes mkILCustomAttrsFromArray(ILAttribute[])
+FSharp.Compiler.AbstractIL.IL: ILAttributesStored emptyILCustomAttrsStored
+FSharp.Compiler.AbstractIL.IL: ILAttributesStored get_emptyILCustomAttrsStored()
+FSharp.Compiler.AbstractIL.IL: ILAttributesStored mkILCustomAttrsComputed(Microsoft.FSharp.Core.FSharpFunc`2[Microsoft.FSharp.Core.Unit,FSharp.Compiler.AbstractIL.IL+ILAttribute[]])
 FSharp.Compiler.AbstractIL.IL: ILAttributesStored storeILCustomAttrs(ILAttributes)
 FSharp.Compiler.AbstractIL.IL: ILEventDefs emptyILEvents
 FSharp.Compiler.AbstractIL.IL: ILEventDefs get_emptyILEvents()
diff --git a/tests/FSharp.Compiler.Service.Tests/ModuleReaderCancellationTests.fs b/tests/FSharp.Compiler.Service.Tests/ModuleReaderCancellationTests.fs
index 560dbceeb66..8038cb8ad38 100644
--- a/tests/FSharp.Compiler.Service.Tests/ModuleReaderCancellationTests.fs
+++ b/tests/FSharp.Compiler.Service.Tests/ModuleReaderCancellationTests.fs
@@ -115,10 +115,10 @@ type PreTypeDefData =
                 mkILMethods []
 
         let typeAttributes = TypeAttributes.Public
-        let customAttrs = mkILCustomAttrs []
+
         ILTypeDef(this.Name, typeAttributes, ILTypeDefLayout.Auto, [], [],
-            None, methodsDefs, mkILTypeDefs [], mkILFields [], emptyILMethodImpls, mkILEvents [], mkILProperties [], false,
-            emptyILSecurityDecls, customAttrs)
+            None, methodsDefs, mkILTypeDefs [], mkILFields [], emptyILMethodImpls, mkILEvents [], mkILProperties [],
+            ILTypeDefAdditionalFlags.None, emptyILSecurityDecls, emptyILCustomAttrsStored)
 
 type PreTypeDef(data: PreTypeDefData) =
     let typeDef = data.TypeDef