add extension member check to CheckIWSAM function

Author: shoyu-ramen

src/Compiler/Checking/CheckExpressions.fs

@@ -4378,7 +4378,7 @@ and CheckIWSAM (cenv: cenv) (env: TcEnv) checkConstraints iwsam m tcref =
     if iwsam = WarnOnIWSAM.Yes && isInterfaceTy g ty && checkConstraints = CheckCxs then
         let tcref = tcrefOfAppTy g ty
         let meths = AllMethInfosOfTypeInScope ResultCollectionSettings.AllResults cenv.infoReader env.NameEnv None ad IgnoreOverrides m ty
-        if meths |> List.exists (fun meth -> not meth.IsInstance && meth.IsDispatchSlot) then
+        if meths |> List.exists (fun meth -> not meth.IsInstance && meth.IsDispatchSlot && not meth.IsExtensionMember) then
             warning(Error(FSComp.SR.tcUsingInterfaceWithStaticAbstractMethodAsType(tcref.DisplayNameWithStaticParametersAndUnderscoreTypars), m))
 
 and TcLongIdentType kindOpt (cenv: cenv) newOk checkConstraints occ iwsam env tpenv synLongId =

tests/FSharp.Compiler.ComponentTests/Conformance/TypesAndTypeConstraints/IWSAMsAndSRTPs/IWSAMsAndSRTPsTests.fs

@@ -320,6 +320,33 @@ module ``Equivalence of properties and getters`` =
             IL_000e:  ret
           }"""]
 
+module ``Type checking behavior`` =   
+
+    #if !NETCOREAPP
+    [<Theory(Skip = "IWSAMs are not supported by NET472.")>]
+    #else   
+    [<InlineData("""
+        type INormalInterface =
+            abstract member IntMember: int
+
+        module INormalInterfaceExtensions =
+            type INormalInterface with
+                static member ExtMethod (a: INormalInterface) =
+                    ()""")>]
+    [<Theory>]
+    #endif
+    let ``Extension method on interface without SAM does not produce a warning`` code =
+        Fsx code
+        |> withLangVersion60
+        |> compile
+        |> shouldSucceed
+        |> ignore
+
+        Fsx code
+        |> withLangVersion70
+        |> compile
+        |> shouldSucceed
+        |> ignore
 
 module Negative =
 
@@ -375,7 +402,6 @@ module Negative =
         |> withDiagnosticMessage "The trait 'A' invoked by this call has multiple support types. This invocation syntax is not permitted for such traits. See https://aka.ms/fsharp-srtp for guidance."
         |> ignore
 
-
 module InvocationBehavior =
 
     [<Fact>]