Throw if the type parameter for ActionResult<T> is an action result (#7942)

Fixes #7931

src/Microsoft.AspNetCore.Mvc.Core/ActionResultOfT.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using Microsoft.AspNetCore.Mvc.Core;
 using Microsoft.AspNetCore.Mvc.Infrastructure;
 
 namespace Microsoft.AspNetCore.Mvc
@@ -18,6 +19,12 @@ namespace Microsoft.AspNetCore.Mvc
         /// <param name="value">The value.</param>
         public ActionResult(TValue value)
         {
+            if (typeof(IActionResult).IsAssignableFrom(typeof(TValue)))
+            {
+                var error = Resources.FormatInvalidTypeTForActionResultOfT(typeof(TValue), "ActionResult<T>");
+                throw new ArgumentException(error);
+            }
+
             Value = value;
         }
 
@@ -27,6 +34,12 @@ namespace Microsoft.AspNetCore.Mvc
         /// <param name="result">The <see cref="ActionResult"/>.</param>
         public ActionResult(ActionResult result)
         {
+            if (typeof(IActionResult).IsAssignableFrom(typeof(TValue)))
+            {
+                var error = Resources.FormatInvalidTypeTForActionResultOfT(typeof(TValue), "ActionResult<T>");
+                throw new ArgumentException(error);
+            }
+
             Result = result ?? throw new ArgumentNullException(nameof(result));
         }
 

src/Microsoft.AspNetCore.Mvc.Core/Properties/Resources.Designer.cs

@@ -1480,6 +1480,20 @@ namespace Microsoft.AspNetCore.Mvc.Core
         internal static string FormatApiConventionMustBeStatic(object p0)
             => string.Format(CultureInfo.CurrentCulture, GetString("ApiConventionMustBeStatic"), p0);
 
+        /// <summary>
+        /// Invalid type parameter '{0}' specified for '{1}'.
+        /// </summary>
+        internal static string InvalidTypeTForActionResultOfT
+        {
+            get => GetString("InvalidTypeTForActionResultOfT");
+        }
+
+        /// <summary>
+        /// Invalid type parameter '{0}' specified for '{1}'.
+        /// </summary>
+        internal static string FormatInvalidTypeTForActionResultOfT(object p0, object p1)
+            => string.Format(CultureInfo.CurrentCulture, GetString("InvalidTypeTForActionResultOfT"), p0, p1);
+
         private static string GetString(string name, params string[] formatterNames)
         {
             var value = _resourceManager.GetString(name);

src/Microsoft.AspNetCore.Mvc.Core/Resources.resx

@@ -445,4 +445,7 @@
   <data name="ApiConventionMustBeStatic" xml:space="preserve">
     <value>API convention type '{0}' must be a static type.</value>
   </data>
+  <data name="InvalidTypeTForActionResultOfT" xml:space="preserve">
+    <value>Invalid type parameter '{0}' specified for '{1}'.</value>
+  </data>
 </root>

test/Microsoft.AspNetCore.Mvc.Core.Test/ActionResultOfTTest.cs

@@ -1,6 +1,8 @@
 // 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 System;
+using System.IO;
 using Microsoft.AspNetCore.Mvc.Infrastructure;
 using Xunit;
 
@@ -8,6 +10,28 @@ namespace Microsoft.AspNetCore.Mvc
 {
     public class ActionResultOfTTest
     {
+        [Fact]
+        public void Constructor_WithValue_ThrowsForInvalidType()
+        {
+            // Arrange
+            var input = new FileStreamResult(Stream.Null, "application/json");
+
+            // Act & Assert
+            var ex = Assert.Throws<ArgumentException>(() => new ActionResult<FileStreamResult>(value: input));
+            Assert.Equal($"Invalid type parameter '{typeof(FileStreamResult)}' specified for 'ActionResult<T>'.", ex.Message);
+        }
+
+        [Fact]
+        public void Constructor_WithActionResult_ThrowsForInvalidType()
+        {
+            // Arrange
+            var actionResult = new OkResult();
+
+            // Act & Assert
+            var ex = Assert.Throws<ArgumentException>(() => new ActionResult<FileStreamResult>(result: actionResult));
+            Assert.Equal($"Invalid type parameter '{typeof(FileStreamResult)}' specified for 'ActionResult<T>'.", ex.Message);
+        }
+
         [Fact]
         public void Convert_ReturnsResultIfSet()
         {