[Bug]: Implicit visibility with `@discriminated` unions can create duplicate type names

[witemple-msft]

Describe the bug

import "@typespec/http";

using TypeSpec.Http;

model M {
  @visibility(Lifecycle.Create)
  prop: string;
}

@discriminated
union U {
  variant: M,
}

@put op createOrUpdate(@body body: U): U;

Because the @discriminated union creates its own envelope shapes, it wants to synthesize a schema named UVariant for variant's envelope shape. However, because the inner property prop is only visible in Create phases, implicit visibility is triggered. This causes OpenAPI to create two different schemas both named UVariant. Note that you have to use Create or Update visibility to trigger the bug. If you use Read visibility, the bug is not triggered because the same schema can be reused via. readOnly.

Reproduction

Minimal repro: TypeSpec Playground

Checklist

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• Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion.
• The provided reproduction is a minimal reproducible example of the bug.