Respect user set Type for client errors via ProducesResponseType

src/Mvc/Mvc.ApiExplorer/src/ApiResponseTypeProvider.cs

@@ -109,9 +109,15 @@ private ICollection<ApiResponseType> GetApiResponseTypes(
                             // from the return type.
                             apiResponseType.Type = type;
                         }
-                        else if (IsClientError(statusCode) || apiResponseType.IsDefaultResponse)
+                        else if (IsClientError(statusCode))
+                        {
+                            // Determine whether or not the type was provided by the user. If so, favor it over the default
+                            // error type for 4xx client errors if no response type is specified..
+                            var setByDefault = metadataAttribute is ProducesResponseTypeAttribute { IsResponseTypeSetByDefault: true };
+                            apiResponseType.Type = setByDefault ? defaultErrorType : apiResponseType.Type;
+                        }
+                        else if (apiResponseType.IsDefaultResponse)
                         {
-                            // Use the default error type for "default" responses or 4xx client errors if no response type is specified.
                             apiResponseType.Type = defaultErrorType;
                         }
                     }

src/Mvc/Mvc.Core/src/ProducesResponseTypeAttribute.cs

@@ -15,11 +15,12 @@ public class ProducesResponseTypeAttribute : Attribute, IApiResponseMetadataProv
     {
         /// <summary>
         /// Initializes an instance of <see cref="ProducesResponseTypeAttribute"/>.
-        /// </summary>       
+        /// </summary>
         /// <param name="statusCode">The HTTP response status code.</param>
         public ProducesResponseTypeAttribute(int statusCode)
             : this(typeof(void), statusCode)
         {
+            IsResponseTypeSetByDefault = true;
         }
 
         /// <summary>
@@ -31,6 +32,7 @@ public ProducesResponseTypeAttribute(Type type, int statusCode)
         {
             Type = type ?? throw new ArgumentNullException(nameof(type));
             StatusCode = statusCode;
+            IsResponseTypeSetByDefault = false;
         }
 
         /// <summary>
@@ -43,6 +45,18 @@ public ProducesResponseTypeAttribute(Type type, int statusCode)
         /// </summary>
         public int StatusCode { get; set; }
 
+        /// <summary>
+        /// Used to distinguish a `Type` set by default in the constructor versus
+        /// one provided by the user.
+        ///
+        /// When <see langword="false"/>, then <see cref="Type"/> is set by user.
+        ///
+        /// When <see langword="true"/>, then <see cref="Type"/> is set by by
+        /// default in the constructor
+        /// </summary>
+        /// <value></value>
+        internal bool IsResponseTypeSetByDefault { get; }
+
         /// <inheritdoc />
         void IApiResponseMetadataProvider.SetContentTypes(MediaTypeCollection contentTypes)
         {

src/Mvc/test/Mvc.FunctionalTests/ApiExplorerTest.cs

@@ -1461,6 +1461,29 @@ public async Task ApiConvention_ForActionWithApiConventionMethod()
                 });
         }
 
+        [Theory]
+        [InlineData("ActionWithNoExplicitType", typeof(ProblemDetails))]
+        [InlineData("ActionWithVoidType", typeof(void))]
+        public async Task ApiAction_ForActionWithVoidResponseType(string path, Type type)
+        {
+            // Act
+            var response = await Client.GetAsync($"http://localhost/ApiExplorerVoid/{path}");
+
+            var responseBody = await response.EnsureSuccessStatusCode().Content.ReadAsStringAsync();
+            var result = JsonConvert.DeserializeObject<List<ApiExplorerData>>(responseBody);
+
+            // Assert
+            var description = Assert.Single(result);
+            Assert.Collection(
+                description.SupportedResponseTypes.OrderBy(r => r.StatusCode),
+                responseType =>
+                {
+                    Assert.Equal(type.FullName, responseType.ResponseType);
+                    Assert.Equal(401, responseType.StatusCode);
+                    Assert.False(responseType.IsDefaultResponse);
+                });
+        }
+
         private IEnumerable<string> GetSortedMediaTypes(ApiExplorerResponseType apiResponseType)
         {
             return apiResponseType.ResponseFormats

src/Mvc/test/WebSites/ApiExplorerWebSite/Controllers/ApiExplorerSystemVoid.cs

@@ -0,0 +1,20 @@
+
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using Microsoft.AspNetCore.Mvc;
+
+namespace ApiExplorerWebSite
+{
+    [Route("ApiExplorerVoid/[action]")]
+    [ApiController]
+    public class ApiExplorerVoidController : Controller
+    {
+        [ProducesResponseType(typeof(void), 401)]
+        public IActionResult ActionWithVoidType() => Ok();
+
+        [ProducesResponseType(401)]
+        public IActionResult ActionWithNoExplicitType() => Ok();
+
+    }
+}