Pass null for required nullable (#9635)

Author: JoshLove-msft

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/Providers/ScmMethodProviderCollection.cs

@@ -644,6 +644,12 @@ private IReadOnlyList<ValueExpression> GetProtocolMethodArguments(Dictionary<str
                         else
                         {
                             requireNamedArgs = true;
+                            // The protocol parameter might be required due to our need to avoid ambiguity with overloads. In this
+                            // case, the parameter should have also been made nullable.
+                            if (protocolParam.DefaultValue == null && protocolParam.Type.IsNullable)
+                            {
+                                AddArgument(protocolParam, Null);
+                            }
                         }
                         continue;
                     }

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/ScmMethodProviderCollectionTests.cs

@@ -1656,6 +1656,46 @@ public async Task MissingOperationParamsResultInNamedArgsForSubsequent()
             Assert.AreEqual(Helpers.GetExpectedFromFile(), methodBody);
         }
 
+        [Test]
+        public async Task MissingRequiredOperationParamsResultInPassingNull()
+        {
+            var idParam = InputFactory.PathParameter("id", InputPrimitiveType.String, isRequired: true);
+            // This will be made required in the protocol method due to method overload ambiguity
+            var takeParam = InputFactory.QueryParameter("take", InputPrimitiveType.Int32, isRequired: false);
+            var filterParam = InputFactory.QueryParameter("filter", InputPrimitiveType.String, isRequired: false);
+            var orderParam = InputFactory.HeaderParameter("order", InputPrimitiveType.String, isRequired: false);
+
+            var serviceMethod = InputFactory.BasicServiceMethod(
+                "TestOp",
+                InputFactory.Operation(
+                    "TestOp",
+                    parameters: [idParam, filterParam, orderParam, takeParam],
+                    responses: [InputFactory.OperationResponse([200])]),
+                parameters:
+                [
+                    InputFactory.MethodParameter("id", InputPrimitiveType.String, isRequired: true, location: InputRequestLocation.Path),
+                    InputFactory.MethodParameter("take", InputPrimitiveType.Int32, isRequired: false, location: InputRequestLocation.Query),
+                    InputFactory.MethodParameter("order", InputPrimitiveType.Int32, isRequired: false, location: InputRequestLocation.Header),
+                ]);
+
+            var inputClient = InputFactory.Client("TestClient", methods: [serviceMethod]);
+            await MockHelpers.LoadMockGeneratorAsync(clients: () => [inputClient]);
+
+            var client = ScmCodeModelGenerator.Instance.TypeFactory.CreateClient(inputClient);
+            Assert.IsNotNull(client);
+
+            var methodCollection = new ScmMethodProviderCollection(serviceMethod, client!);
+            Assert.IsNotNull(methodCollection);
+
+            var convenienceMethod = methodCollection.FirstOrDefault(m
+                => m.Signature.Parameters.All(p => p.Name != "options")
+                   && m.Signature.Name == "TestOp");
+            Assert.IsNotNull(convenienceMethod);
+
+            var methodBody = convenienceMethod!.BodyStatements!.ToDisplayString();
+            Assert.AreEqual(Helpers.GetExpectedFromFile(), methodBody);
+        }
+
         [Test]
         public async Task CombinedMissingOperationParamsAndNonBodyModelParams()
         {

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/TestData/ScmMethodProviderCollectionTests/MissingRequiredOperationParamsResultInPassingNull.cs

@@ -0,0 +1,3 @@
+global::Sample.Argument.AssertNotNullOrEmpty(id, nameof(id));
+
+return this.TestOp(id, filter: null, order: order, take: take, options: cancellationToken.ToRequestOptions());