Template member access allow access to late bound properties

[timotheeguerin]

While it works in some cases it errors with circular issues with very confusing error. And as we don't allow it for non template constraint this feels inconsitent. We should revert that for now until we figure out that solution globally

Playground Link

[github-actions]

🎯 Issue Triage

Template member access (. operator on template parameters) currently allows accessing late-bound properties in some cases, but produces confusing circular reference errors. The proposed fix is to revert/restrict this behavior for consistency with non-template constraints until a global solution is designed.

Category: Bug

Area: compiler:core

🔗 [Playground Reproduction]((cadlplayground.z22.web.core.windows.net/redacted)

📋 Reproduction code

namespace Test;

model Reflect<T> {
  ...T;
}

model Property
  is Reflect<  T: string>;
  t: Template<{ a: T. // alias A = .}> extends

🔍 Analysis

• Template member access (e.g., T.PropertyName) works in some scenarios but triggers circular reference errors in others, making the behavior inconsistent and the errors confusing to users.
• This inconsistency exists because non-template constraints don't allow this access pattern, creating an asymmetry.
• The proposed resolution is to revert/disable late-bound template member access until a consistent global solution is available.
• Affected component: TypeSpec compiler core — template instantiation and member access resolution.

📝 Suggested next steps

• Revert the template member access feature for late-bound properties in the compiler
• Add a clear, descriptive diagnostic error when this pattern is used
• Track the broader design work for consistent member access in a follow-up issue

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