[Bug]: Function could not work correctly with mutation

[tadelesh]

Describe the bug

The function should be called for the mutated type graph when combining use with decorator, etc. Currently, it seems the function will only be called once for the original type graph, which caused the decorator logic in mutation get the wrong type graph from function result. The original issue here.

Reproduction

Playground

Checklist

• Follow our Code of Conduct
• 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.

[github-actions]

🎯 Issue Triage

A bug where functions are only called once for the original type graph and not re-evaluated for mutated type graphs, causing decorators that use mutation to receive incorrect/stale function results.

Category: Bug
Area: compiler:core

⚠️ No typespec.io playground link provided. The reproduction links to the typespec-azure playground — a minimal repro on the [TypeSpec playground]((typespec.io/redacted) would help with diagnosis.

🔍 Analysis

• The issue is in the compiler's type mutation pipeline: when a type is mutated (e.g., via useWith or similar mutation APIs), any associated functions (e.g., $decoratorFn or getFoo-style helpers) are not re-invoked on the mutated copy, so they return results based on the original type graph.
• This causes incorrect behavior when decorators that depend on function results are applied to mutated types.
• Root cause likely lies in how the compiler resolves/caches function results tied to a type — cached results from the original type are reused for mutations without re-execution.
• Related upstream issue: Azure/typespec-azure#4441
• Affected area: compiler type mutation / decorator evaluation pipeline

📝 Suggested next steps

• Identify where function results are cached relative to type identity in the compiler
• Determine whether functions should be re-run on cloned/mutated types or whether results should not be cached across mutations
• Add a minimal repro on typespec.io playground if possible
• Add a regression test once the fix is identified

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