Fixed destNullableTy internal error (#8655)

* Fixed internal error for destNullableTy by removing rule for optional

* Added test

Author: TIHan

src/fsharp/MethodCalls.fs

@@ -189,7 +189,7 @@ let AdjustCalledArgTypeForOptionals (g: TcGlobals) enforceNullableOptionalsKnown
         // CSharpMethod(x = arg), optional C#-style argument, may have type Nullable<ty>. 
         // The arg should have type ty. However for backwards compat, we also allow arg to have type Nullable<ty>
         | CallerSide _ ->
-            if isNullableTy g calledArgTy  && g.langVersion.SupportsFeature LanguageFeature.NullableOptionalInterop  then 
+            if isNullableTy g calledArgTy && g.langVersion.SupportsFeature LanguageFeature.NullableOptionalInterop then 
                 // If inference has worked out it's a nullable then use this
                 if isNullableTy g callerArg.CallerArgumentType then
                     calledArgTy
@@ -1186,11 +1186,6 @@ let AdjustCallerArgForOptional tcFieldInit eCallerMemberName (infoReader: InfoRe
                             let minfo = GetIntrinsicConstructorInfosOfType infoReader m calledArgTy |> List.head
                             let callerArgExprCoerced = mkCoerceIfNeeded g calledNonOptTy callerArgTy callerArgExpr
                             MakeMethInfoCall amap m minfo [] [callerArgExprCoerced]
-                    elif isOptionTy g calledArgTy then 
-                        // CSharpMethod(x=b) when 'b' has nullable type and 'x' has optional type --> CSharpMethod(Some b.Value)
-                        let calledNonOptTy = destOptionTy g calledArgTy 
-                        let callerArgExprCoerced = mkCoerceIfNeeded g calledNonOptTy callerArgTy callerArgExpr
-                        mkSome g (destNullableTy g callerArgTy) callerArgExprCoerced m
                     else 
                         // CSharpMethod(x=b) --> CSharpMethod(?x=b)
                         callerArgExpr

tests/fsharp/Compiler/Language/OptionalInteropTests.fs

@@ -0,0 +1,46 @@
+// Copyright (c) Microsoft Corporation.  All Rights Reserved.  See License.txt in the project root for license information.
+
+namespace FSharp.Compiler.UnitTests
+
+open System.Collections.Immutable
+open NUnit.Framework
+open FSharp.Compiler.SourceCodeServices
+open FSharp.Compiler.UnitTests.Utilities
+open Microsoft.CodeAnalysis
+
+[<TestFixture>]
+module OptionalInteropTests =
+
+    [<Test>]
+    let ``C# method with an optional parameter and called with an option type should compile`` () =
+        let csSrc =
+            """
+using Microsoft.FSharp.Core;
+
+namespace CSharpTest
+{
+    public static class Test
+    {
+        public static void M(FSharpOption<int> x = null) { }
+    }
+}
+            """
+
+        let fsSrc =
+            """
+open System
+open CSharpTest
+
+Test.M(x = Some 1)
+            """
+
+        let fsharpCoreAssembly =
+            typeof<_ list>.Assembly.Location
+            |> MetadataReference.CreateFromFile
+
+        let cs =
+            CompilationUtil.CreateCSharpCompilation(csSrc, CSharpLanguageVersion.CSharp8, TargetFramework.NetStandard20, additionalReferences = ImmutableArray.CreateRange [fsharpCoreAssembly])
+            |> CompilationReference.Create
+
+        let fs = Compilation.Create(fsSrc, SourceKind.Fsx, CompileOutput.Exe, options = [|"--langversion:preview"|], cmplRefs = [cs])
+        CompilerAssert.Compile fs

tests/fsharp/FSharpSuite.Tests.fsproj

@@ -60,6 +60,7 @@
     <Compile Include="Compiler\Warnings\ExperimentalAttributeTests.fs" />
     <Compile Include="Compiler\Warnings\PatternMatchingWarningTests.fs" />
     <Compile Include="Compiler\SourceTextTests.fs" />
+    <Compile Include="Compiler\Language\OptionalInteropTests.fs" />
     <Compile Include="Compiler\Language\UIntTests.fs" />
     <Compile Include="Compiler\Language\FixedIndexSliceTests.fs" />
     <Compile Include="Compiler\Language\SlicingQuotationTests.fs" />