Fix ProducesResponseType's Description not being set for Minimal API's when attribute and inferred types aren't an exact match

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

@@ -405,14 +405,28 @@ private static void AddSupportedResponseTypes(
             foreach (var metadata in responseMetadataTypes)
             {
                 if (metadata.StatusCode == apiResponseType.StatusCode &&
-                    metadata.Type == apiResponseType.Type &&
+                    TypesAreCompatible(apiResponseType.Type, metadata.Type) &&
                     metadata.Description is not null)
                 {
                     matchingDescription = metadata.Description;
                 }
             }
             return matchingDescription;
         }
+
+        static bool TypesAreCompatible(Type? apiResponseType, Type? metadataType)
+        {
+            // We need to a special check for cases where the inferred type is different than the one specified in attributes.
+            // For example, an endpoint that defines [ProducesResponseType<IEnumerable<WeatherForecast>>],
+            // but the endpoint returns weatherForecasts.ToList(). Because List<> is a different type than IEnumerable<>, it would incorrectly set OpenAPI metadata incorrectly.
+            // We use a conservative unidirectional check where the attribute type must be assignable from the inferred type.
+            // This handles inheritance (BaseClass ← DerivedClass) and interface implementation (IEnumerable<T> ← List<T>).
+            // This should be sufficient, as it's more common to specify an interface or base class type in the attribute and a concrete type in the endpoint implementation,
+            // compared to doing the opposite.
+            // For more information, check the related bug: https://github.com/dotnet/aspnetcore/issues/60518
+            return apiResponseType == metadataType ||
+                metadataType?.IsAssignableFrom(apiResponseType) == true;
+        }
     }
 
     private static ApiResponseType CreateDefaultApiResponseType(Type responseType)

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

@@ -331,6 +331,108 @@ public void AddsResponseDescription()
         Assert.Equal(expectedBadRequestDescription, badRequestResponseType.Description);
     }
 
+    [Fact]
+    public void AddsResponseDescription_WorksWithGenerics()
+    {
+        const string expectedOkDescription = "The weather forecast for the next 5 days.";
+
+        var apiDescription = GetApiDescription([ProducesResponseType<GenericClass<TimeSpan>>(StatusCodes.Status200OK, Description = expectedOkDescription)]
+        () => new GenericClass<TimeSpan> { Value = new TimeSpan() });
+
+        var okResponseType = Assert.Single(apiDescription.SupportedResponseTypes);
+
+        Assert.Equal(200, okResponseType.StatusCode);
+        Assert.Equal(typeof(GenericClass<TimeSpan>), okResponseType.Type);
+        Assert.Equal(typeof(GenericClass<TimeSpan>), okResponseType.ModelMetadata?.ModelType);
+        Assert.Equal(expectedOkDescription, okResponseType.Description);
+
+        var createdOkFormat = Assert.Single(okResponseType.ApiResponseFormats);
+        Assert.Equal("application/json", createdOkFormat.MediaType);
+    }
+
+    [Fact]
+    public void AddsResponseDescription_WorksWithGenericsAndTypedResults()
+    {
+        const string expectedOkDescription = "The weather forecast for the next 5 days.";
+
+        var apiDescription = GetApiDescription([ProducesResponseType<GenericClass<TimeSpan>>(StatusCodes.Status200OK, Description = expectedOkDescription)]
+        () => TypedResults.Ok(new GenericClass<TimeSpan> { Value = new TimeSpan() }));
+
+        var okResponseType = Assert.Single(apiDescription.SupportedResponseTypes);
+
+        Assert.Equal(200, okResponseType.StatusCode);
+        Assert.Equal(typeof(GenericClass<TimeSpan>), okResponseType.Type);
+        Assert.Equal(typeof(GenericClass<TimeSpan>), okResponseType.ModelMetadata?.ModelType);
+        Assert.Equal(expectedOkDescription, okResponseType.Description);
+
+        var createdOkFormat = Assert.Single(okResponseType.ApiResponseFormats);
+        Assert.Equal("application/json", createdOkFormat.MediaType);
+    }
+
+    [Fact]
+    public void AddsResponseDescription_WorksWithCollections()
+    {
+        const string expectedOkDescription = "The weather forecast for the next 5 days.";
+
+        var apiDescription = GetApiDescription([ProducesResponseType<IEnumerable<TimeSpan>>(StatusCodes.Status200OK, Description = expectedOkDescription)]
+        () => new List<TimeSpan> { new() });
+
+        var okResponseType = Assert.Single(apiDescription.SupportedResponseTypes);
+
+        Assert.Equal(200, okResponseType.StatusCode);
+        Assert.Equal(typeof(List<TimeSpan>), okResponseType.Type); // We use List as the inferred type has higher priority than those set by metadata (attributes)
+        Assert.Equal(typeof(List<TimeSpan>), okResponseType.ModelMetadata?.ModelType);
+        Assert.Equal(expectedOkDescription, okResponseType.Description);
+
+        var createdOkFormat = Assert.Single(okResponseType.ApiResponseFormats);
+        Assert.Equal("application/json", createdOkFormat.MediaType);
+    }
+
+    [Fact]
+    public void AddsResponseDescription_WorksWithCollectionsAndTypedResults()
+    {
+        const string expectedOkDescription = "The weather forecast for the next 5 days.";
+
+        var apiDescription = GetApiDescription([ProducesResponseType<IEnumerable<TimeSpan>>(StatusCodes.Status200OK, Description = expectedOkDescription)]
+        () => TypedResults.Ok(new List<TimeSpan> { new() }));
+
+        var okResponseType = Assert.Single(apiDescription.SupportedResponseTypes);
+
+        Assert.Equal(200, okResponseType.StatusCode);
+        Assert.Equal(typeof(List<TimeSpan>), okResponseType.Type); // We use List as the inferred type has higher priority than those set by metadata (attributes)
+        Assert.Equal(typeof(List<TimeSpan>), okResponseType.ModelMetadata?.ModelType);
+        Assert.Equal(expectedOkDescription, okResponseType.Description);
+
+        var createdOkFormat = Assert.Single(okResponseType.ApiResponseFormats);
+        Assert.Equal("application/json", createdOkFormat.MediaType);
+    }
+
+    /// <summary>
+    /// EndpointMetadataApiDescriptionProvider performs a one way type check for discovering response types to match the description that's set in [ProducesResponseType]
+    /// The reason we do a one-way check instead of a bidirectional check is to prevent too many positive matches.
+    /// </summary>
+    /// <remarks>
+    /// Example: If we did a bidirectional check, we would match something scenarios like this, which can cause confusion:
+    /// [ProducesResponseType<string>(StatusCodes.Status200OK, Description = "Returned with a string")] -> TypedResults.Ok(new object())
+    /// This would match because object is assignable to string,
+    /// but it doesn't make sense to add the Description to the object type because the attribute says we should return a string.
+    ///
+    /// This test documents this desired behavior and will fail if the behavior changes, so the developer can double check if their change is intentional.
+    /// </summary>
+    [Fact]
+    public void AddsResponseDescription_ShouldFailWhenInferredTypeIsNotDirectlyAssignableToAttributeType()
+    {
+        var apiDescription = GetApiDescription([ProducesResponseType<string>(StatusCodes.Status200OK, Description = "Only returned with a string")]
+        () => TypedResults.Ok(new object()));
+
+        var okResponseType = Assert.Single(apiDescription.SupportedResponseTypes);
+
+        Assert.Equal(200, okResponseType.StatusCode);
+        Assert.Equal(typeof(object), okResponseType.Type);
+        Assert.Equal(typeof(object), okResponseType.ModelMetadata?.ModelType);
+        Assert.Null(okResponseType.Description);
+    }
+
     [Fact]
     public void WithEmptyMethodBody_AddsResponseDescription()
     {
@@ -1814,4 +1916,6 @@ private class TestServiceProvider : IServiceProvider
             return null;
         }
     }
+
+    private class GenericClass<TType> { public required TType Value { get; set; } }
 }