[Bug]: TCGC reports wrong diagnostics "conflicting-multipart-model-usage" for valid multipart cases

[msyyc]

Describe the bug

For the valid cases, TCGC reports wrong diagnostics "conflicting-multipart-model-usage"

Reproduction

    @service({title: "Test Service"}) namespace TestService;

    model MultiPartRequest {
      id: string;
      profileImage: bytes;
    }
    
    @post op basic1(@header contentType: "multipart/form-data", @body body: MultiPartRequest): NoContentResponse;
    @put op basic2(@header contentType: "multipart/form-data", @body body: MultiPartRequest): NoContentResponse;

Checklist

• Follow our Code of Conduct
• Check that this issue is about the Azure libraries for typespec. For bug in the typespec language or core libraries file it in the TypeSpec repo
• Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• The provided reproduction is a minimal reproducible example of the bug.

[msyyc]

The root cause is that TCGC always set a model with UsageFlags.Input:

typespec-azure/packages/typespec-client-generator-core/src/types.ts

Line 1658 in a6365a1

updateUsageOrAccessOfModel(context, UsageFlags.Input, sdkType);

so it is not proper to distinguish whether a model is used both in common content-type and multipart content-type:

typespec-azure/packages/typespec-client-generator-core/src/types.ts

Lines 1633 to 1642 in a6365a1

	// this part should be put before setting current body's usage because it is based on the previous usage
	if (
	sdkType.kind === "model" &&
	((!multipartOperation && (sdkType.usage & UsageFlags.MultipartFormData) > 0) ||
	(multipartOperation &&
	(sdkType.usage & UsageFlags.Input) > 0 &&
	(sdkType.usage & UsageFlags.Input & UsageFlags.MultipartFormData) === 0))
	) {
	// This means we have a model that is used both for formdata input and for regular body input
	diagnostics.add(

.

There are 2 solutions:

• make usage Input and MultipartFormData excluded
• add new usage like NonMultiPart

[weidongxu-microsoft]

Personal opinion:
Make MultipartFormData exclusive to JSON/XML, as all of these relates to the serialization of the model.

[msyyc]

Personal opinion: Make MultipartFormData exclusive to JSON/XML, as all of these relates to the serialization of the model.

After discussion, we pretend to use this solution instead of adding new usage kind. Let me make a PR to fix it later.