Update optionality check in API descriptor

Author: captainsafia

src/Mvc/Mvc.ApiExplorer/src/EndpointMetadataApiDescriptionProvider.cs

@@ -143,7 +143,7 @@ private ApiDescription CreateApiDescription(RouteEndpoint routeEndpoint, string
 
             // Determine the "requiredness" based on nullability, default value or if allowEmpty is set
             var nullability = NullabilityContext.Create(parameter);
-            var isOptional = parameter.HasDefaultValue || nullability.ReadState == NullabilityState.Nullable || allowEmpty;
+            var isOptional = parameter.HasDefaultValue || nullability.ReadState != NullabilityState.NotNull || allowEmpty;
 
             return new ApiParameterDescription
             {

src/Mvc/Mvc.ApiExplorer/test/EndpointMetadataApiDescriptionProviderTest.cs

@@ -413,6 +413,27 @@ public void AddsMetadataFromRouteEndpoint()
             Assert.True(apiExplorerSettings.IgnoreApi);
         }
 
+        [Fact]
+        public void TestParameterIsRequiredForObliviousNullabilityContext()
+        {
+            // In an oblivious nullability context, reference type parameters without
+            // annotations are optional. Value type parameters are always required.
+            var apiDescription = GetApiDescription((string foo, int bar) => { });
+            Assert.Equal(2, apiDescription.ParameterDescriptions.Count);
+
+            var fooParam = apiDescription.ParameterDescriptions[0];
+            Assert.Equal(typeof(string), fooParam.Type);
+            Assert.Equal(typeof(string), fooParam.ModelMetadata.ModelType);
+            Assert.Equal(BindingSource.Query, fooParam.Source);
+            Assert.False(fooParam.IsRequired);
+
+            var barParam = apiDescription.ParameterDescriptions[1];
+            Assert.Equal(typeof(int), barParam.Type);
+            Assert.Equal(typeof(int), barParam.ModelMetadata.ModelType);
+            Assert.Equal(BindingSource.Query, barParam.Source);
+            Assert.True(barParam.IsRequired);
+        }
+
         [Fact]
         public void RespectsProducesProblemExtensionMethod()
         {