compiler: Exclude false scope members from encoded name duplication tracking

[witemple-msft]

This is a very strange bug that likely stems from the way mutators work.

When mutating a model for visibility transforms, we create clones of the model's properties. These clones end up checked/finished, even if they are not actually attached to the model, and vice versa (the original properties may be checked/finished even if the clone ends up attached).

This runs stateful decorators, like @encodedName. Unfortunately, in these cases, it means that both the clone and the original property end up binding an encoded name within the same scope, which the validator later sees as a conflict.

However, only one of these properties is actually a member of the scope at validation time. This PR uses that to exclude "false members". These are properties that exist in the state map (the decorator has been applied to them), but aren't actually bound to the parent they claim to be a member of. We can work around this mutator bug in the validator by checking if the target is actually within the set of member values (in this case, the properties actually present in the parent model, but this will also work for enum members and union variants).

Closes #8599

[azure-sdk]

You can try these changes here

🛝 Playground	🌐 Website	🛝 VSCode Extension

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/microsoft/typespec/@typespec/compiler@8681

commit: fb0002c

[github-actions]

All changed packages have been documented.

• ✅ @typespec/compiler

Show changes

@typespec/compiler - fix ✏️

Addressed a bug that could cause duplicate @encodedName applications to be detected when none actually exist.