diff --git a/docs/release-notes/.FSharp.Compiler.Service/10.0.100.md b/docs/release-notes/.FSharp.Compiler.Service/10.0.100.md
index c87819d01ba..1419988c5e6 100644
--- a/docs/release-notes/.FSharp.Compiler.Service/10.0.100.md
+++ b/docs/release-notes/.FSharp.Compiler.Service/10.0.100.md
@@ -26,6 +26,7 @@
 * Fix nullable types formatting in `FSharpType.Format` and tooltips to include parentheses. ([PR #18842](https://github.com/dotnet/fsharp/pull/18842))
 * TypeMismatchDiagnosticExtendedData: fix expected and actual types calculation. ([Issue ](https://github.com/dotnet/fsharp/pull/18851))
 * Parser: fix range for computed binding expressions ([PR #18903](https://github.com/dotnet/fsharp/pull/18903))
+* Checker: fix declaring type for abbreviated types extensions ([PR #18909](https://github.com/dotnet/fsharp/pull/18909))
 
 ### Changed
 * Use `errorR` instead of `error` in `CheckDeclarations.fs` when possible. ([PR #18645](https://github.com/dotnet/fsharp/pull/18645))
diff --git a/src/Compiler/Checking/CheckDeclarations.fs b/src/Compiler/Checking/CheckDeclarations.fs
index e6a39af0a7b..cbf79fbdfb3 100644
--- a/src/Compiler/Checking/CheckDeclarations.fs
+++ b/src/Compiler/Checking/CheckDeclarations.fs
@@ -4250,11 +4250,9 @@ module TcDeclarations =
         // a) For interfaces, only if it is in the original defn.
         //    Augmentations to interfaces via partial type defns will always be extensions, e.g. extension members on interfaces.
         // b) For other types, if the type is isInSameModuleOrNamespace
-        let declKind, typars = 
-          if isAtOriginalTyconDefn then 
-              ModuleOrMemberBinding, reqTypars
-
-          else
+        if isAtOriginalTyconDefn then
+            ModuleOrMemberBinding, tcref, reqTypars
+        else
             let isInSameModuleOrNamespace = 
                  match envForDecls.eModuleOrNamespaceTypeAccumulator.Value.TypesByMangledName.TryGetValue tcref.LogicalName with 
                  | true, tycon -> tyconOrder.Compare(tcref.Deref, tycon) = 0
@@ -4270,14 +4268,16 @@ module TcDeclarations =
             let _tpenv = TcTyparConstraints cenv NoNewTypars CheckCxs ItemOccurrence.UseInType envForTycon emptyUnscopedTyparEnv synTyparCxs
             declaredTypars |> List.iter (SetTyparRigid envForDecls.DisplayEnv m)
 
-            if isInSameModuleOrNamespace && not isInterfaceOrDelegateOrEnum then 
+            if tcref.TypeAbbrev.IsSome then
+                ExtrinsicExtensionBinding, tcref, declaredTypars
+            elif isInSameModuleOrNamespace && not isInterfaceOrDelegateOrEnum then 
                 // For historical reasons we only give a warning for incorrect type parameters on intrinsic extensions
                 if nReqTypars <> synTypars.Length then 
                     errorR(Error(FSComp.SR.tcDeclaredTypeParametersForExtensionDoNotMatchOriginal(tcref.DisplayNameWithStaticParametersAndUnderscoreTypars), m))
                 if not (typarsAEquiv g (TypeEquivEnv.EmptyWithNullChecks g) reqTypars declaredTypars) then 
                     warning(Error(FSComp.SR.tcDeclaredTypeParametersForExtensionDoNotMatchOriginal(tcref.DisplayNameWithStaticParametersAndUnderscoreTypars), m))
                 // Note we return 'reqTypars' for intrinsic extensions since we may only have given warnings
-                IntrinsicExtensionBinding, reqTypars
+                IntrinsicExtensionBinding, tcref, reqTypars
             else 
                 if isInSameModuleOrNamespace && isDelegateOrEnum then 
                     errorR(Error(FSComp.SR.tcMembersThatExtendInterfaceMustBePlacedInSeparateModule(), tcref.Range))
@@ -4285,10 +4285,7 @@ module TcDeclarations =
                     error(Error(FSComp.SR.tcDeclaredTypeParametersForExtensionDoNotMatchOriginal(tcref.DisplayNameWithStaticParametersAndUnderscoreTypars), m))
                 if not (typarsAEquiv g (TypeEquivEnv.EmptyWithNullChecks g) reqTypars declaredTypars) then 
                     errorR(Error(FSComp.SR.tcDeclaredTypeParametersForExtensionDoNotMatchOriginal(tcref.DisplayNameWithStaticParametersAndUnderscoreTypars), m))
-                ExtrinsicExtensionBinding, declaredTypars
-
-
-        declKind, tcref, typars
+                ExtrinsicExtensionBinding, tcref, declaredTypars
 
 
     let private isAugmentationTyconDefnRepr = function SynTypeDefnSimpleRepr.General(kind=SynTypeDefnKind.Augmentation _) -> true | _ -> false
diff --git a/tests/FSharp.Compiler.ComponentTests/ErrorMessages/AbbreviationTests.fs b/tests/FSharp.Compiler.ComponentTests/ErrorMessages/AbbreviationTests.fs
index 26966e72782..41d7a86677a 100644
--- a/tests/FSharp.Compiler.ComponentTests/ErrorMessages/AbbreviationTests.fs
+++ b/tests/FSharp.Compiler.ComponentTests/ErrorMessages/AbbreviationTests.fs
@@ -54,6 +54,23 @@ type FloatAlias with
             (Error 909, Line 6, Col 15, Line 6, Col 34, "All implemented interfaces should be declared on the initial declaration of the type")
         ]
 
+[<Fact>]
+let ``Members 04 - Error in expr`` () =
+    Fsx """
+type StringAlias = string
+
+type StringAlias with
+    member x.Length = x.Length + ""
+        """
+        |> typecheck
+        |> shouldFail
+        |> withDiagnostics [
+            (Error 964, Line 4, Col 6, Line 4, Col 17, "Type abbreviations cannot have augmentations")
+            (Error 895, Line 5, Col 5, Line 5, Col 36, "Type abbreviations cannot have members")
+            (Error 1, Line 5, Col 34, Line 5, Col 36, "The type 'string' does not match the type 'int'")
+            (Error 43, Line 5, Col 32, Line 5, Col 33, "The type 'string' does not match the type 'int'")
+        ]
+
 [<Fact>]
 let ``Multiple types 01`` () =
     Fsx """