[release/7.0] Continue using typeof(void) for empty TypedResults

[halter73]

This is fixing a minor regression from a few hours ago. This only has in impact if you're using the new TypedResults with MVC Controllers to populate your ApiExplorer model for things like Swashbuckle.

In #43961, I updated the internal ProducesResponseTypeMetadata type to accept a null Type to match the interface. I think null is still correct for "text/plain" strings returned directly from minimal route handlers. However, I went too far with that change and modified a constructor that our existing TypedResults were already using to start using null instead of typeof(void) even though these are slightly different.

aspnetcore/src/Http/Http.Results/src/Ok.cs

Line 53 in 0bb3d68

builder.Metadata.Add(new ProducesResponseTypeMetadata(StatusCodes.Status200OK));

OpenApiGenerator and EndpointMetadataApiDescriptionProvider handle typeof(void) and null the same, so this has no real impact when minimal route handlers return TypedResults.

aspnetcore/src/OpenApi/src/OpenApiGenerator.cs

Lines 138 to 140 in 0bb3d68

	discoveredTypeAnnotation = discoveredTypeAnnotation == null || discoveredTypeAnnotation == typeof(void)
	? responseType
	: discoveredTypeAnnotation;

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

Lines 360 to 363 in 0bb3d68

	if (apiResponseType.Type is null || apiResponseType.Type == typeof(void))
	{
	apiResponseType.Type = responseType;
	}

However, for some reason, ApiResponseTypeProvider does treat these differently and excludes all results with a null type but not a void type.

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

Lines 208 to 211 in 0bb3d68

	if (apiResponseType.Type != null)
	{
	results[apiResponseType.StatusCode] = apiResponseType;
	}

I don't really understand why ApiResponseTypeProvider treats these differently while the others don't, but it's been that way for 4 years. I'm more comfortable reverting the unnecessary change I made a couple hours ago. Semantically this makes more sense too. typeof(void) means there's no response content and null means there is content but no schema beyond the Content-Type (e.g. "text/plain").

@Pilchie

[ghost]

Hi @halter73. Please make sure you've updated the PR description to use the Shiproom Template. Also, make sure this PR is not marked as a draft and is ready-to-merge.

To learn more about how to prepare a servicing PR click here.

[Pilchie]

Approved for .NET 7 RC2.