Make DefaultApiDescriptionProvider understand ActionResult<T>

Fixes #6784

src/Microsoft.AspNetCore.Mvc.ApiExplorer/DefaultApiDescriptionProvider.cs

@@ -193,8 +193,7 @@ namespace Microsoft.AspNetCore.Mvc.ApiExplorer
                     parameter.Source == BindingSource.ModelBinding ||
                     parameter.Source == BindingSource.Custom)
                 {
-                    ApiParameterRouteInfo routeInfo;
-                    if (routeParameters.TryGetValue(parameter.Name, out routeInfo))
+                    if (routeParameters.TryGetValue(parameter.Name, out var routeInfo))
                     {
                         parameter.RouteInfo = routeInfo;
                         routeParameters.Remove(parameter.Name);
@@ -322,8 +321,7 @@ namespace Microsoft.AspNetCore.Mvc.ApiExplorer
             {
                 foreach (var formatter in _inputFormatters)
                 {
-                    var requestFormatMetadataProvider = formatter as IApiRequestFormatMetadataProvider;
-                    if (requestFormatMetadataProvider != null)
+                    if (formatter is IApiRequestFormatMetadataProvider requestFormatMetadataProvider)
                     {
                         var supportedTypes = requestFormatMetadataProvider.GetSupportedContentTypes(contentType, type);
 
@@ -445,7 +443,10 @@ namespace Microsoft.AspNetCore.Mvc.ApiExplorer
             }
 
             // Unwrap the type if it's a Task<T>. The Task (non-generic) case was already handled.
-            var unwrappedType = GetTaskInnerTypeOrNull(declaredReturnType) ?? declaredReturnType;
+            var unwrappedType = UnwrapGenericType(declaredReturnType, typeof(Task<>));
+
+            // Unwrap the type if it's ActionResult<T> or Task<ActionResult<T>>.
+            unwrappedType = UnwrapGenericType(unwrappedType, typeof(ActionResult<>));
 
             // If the method is declared to return IActionResult or a derived class, that information
             // isn't valuable to the formatter.
@@ -457,13 +458,12 @@ namespace Microsoft.AspNetCore.Mvc.ApiExplorer
             {
                 return unwrappedType;
             }
-        }
 
-        private static Type GetTaskInnerTypeOrNull(Type type)
-        {
-            var genericType = ClosedGenericMatcher.ExtractGenericInterface(type, typeof(Task<>));
-
-            return genericType?.GenericTypeArguments[0];
+            Type UnwrapGenericType(Type type, Type queryType)
+            {
+                var genericType = ClosedGenericMatcher.ExtractGenericInterface(type, queryType);
+                return genericType?.GenericTypeArguments[0] ?? type;
+            }
         }
 
         private Type GetRuntimeReturnType(Type declaredReturnType)

test/Microsoft.AspNetCore.Mvc.ApiExplorer.Test/DefaultApiDescriptionProviderTest.cs

@@ -394,6 +394,42 @@ namespace Microsoft.AspNetCore.Mvc.Description
             Assert.NotNull(responseType.ModelMetadata);
         }
 
+        [Theory]
+        [InlineData(nameof(ReturnsActionResultOfProduct))]
+        [InlineData(nameof(ReturnsTaskOfActionResultOfProduct))]
+        public void GetApiDescription_PopulatesResponseType_ForActionResultOfT(string methodName)
+        {
+            // Arrange
+            var action = CreateActionDescriptor(methodName);
+
+            // Act
+            var descriptions = GetApiDescriptions(action);
+
+            // Assert
+            var description = Assert.Single(descriptions);
+            var responseType = Assert.Single(description.SupportedResponseTypes);
+            Assert.Equal(typeof(Product), responseType.Type);
+            Assert.NotNull(responseType.ModelMetadata);
+        }
+
+        [Theory]
+        [InlineData(nameof(ReturnsActionResultOfSequenceOfProducts))]
+        [InlineData(nameof(ReturnsTaskOfActionResultOfSequenceOfProducts))]
+        public void GetApiDescription_PopulatesResponseType_ForActionResultOfSequenceOfT(string methodName)
+        {
+            // Arrange
+            var action = CreateActionDescriptor(methodName);
+
+            // Act
+            var descriptions = GetApiDescriptions(action);
+
+            // Assert
+            var description = Assert.Single(descriptions);
+            var responseType = Assert.Single(description.SupportedResponseTypes);
+            Assert.Equal(typeof(IEnumerable<Product>), responseType.Type);
+            Assert.NotNull(responseType.ModelMetadata);
+        }
+
         [Fact]
         public void GetApiDescription_PopulatesResponseType_WithTaskOfProduct()
         {
@@ -1478,6 +1514,14 @@ namespace Microsoft.AspNetCore.Mvc.Description
             return null;
         }
 
+        private ActionResult<Product> ReturnsActionResultOfProduct() => null;
+
+        private ActionResult<IEnumerable<Product>> ReturnsActionResultOfSequenceOfProducts() => null;
+
+        private Task<ActionResult<Product>> ReturnsTaskOfActionResultOfProduct() => null;
+
+        private Task<ActionResult<IEnumerable<Product>>> ReturnsTaskOfActionResultOfSequenceOfProducts() => null;
+
         private void AcceptsProduct(Product product)
         {
         }

test/Microsoft.AspNetCore.Mvc.FunctionalTests/ApiExplorerTest.cs

@@ -478,6 +478,7 @@ namespace Microsoft.AspNetCore.Mvc.FunctionalTests
 
         [Theory]
         [InlineData("GetProduct", "ApiExplorerWebSite.Product")]
+        [InlineData("GetActionResultProduct", "ApiExplorerWebSite.Product")]
         [InlineData("GetInt", "System.Int32")]
         [InlineData("GetTaskOfProduct", "ApiExplorerWebSite.Product")]
         [InlineData("GetTaskOfInt", "System.Int32")]

test/WebSites/ApiExplorerWebSite/Controllers/ApiExplorerResponseTypeWithoutAttributeController.cs

@@ -38,6 +38,9 @@ namespace ApiExplorerWebSite
             return null;
         }
 
+        [HttpGet]
+        public ActionResult<Product> GetActionResultProduct() => null;
+
         [HttpGet]
         public int GetInt()
         {