Compiler crash: decorator called with return type Model when extern fn argument is an invalid reference

[timotheeguerin]

Bug Report

When an extern fn is called with an invalid reference as an argument, the compiler does not call the function (correct), but still calls the decorator with the function's declared return type (the Reflection.Operation Model) instead of skipping the decorator application or reporting an error gracefully.

This causes an internal compiler crash.

Minimal Repro

main.tsp

import "./test.js";

#suppress "experimental-feature" "testing extern fn crash with invalid ref"
extern fn myFn(target: Reflection.Operation): Reflection.Operation;

extern dec myDec(target: Reflection.Operation, override: Reflection.Operation);

op test(): void;

@@myDec(test, myFn(invalidref));

test.js

// @ts-check

export const $functions = {
  "": {
    myFn: (context, target) => {
      console.log("BUG: myFn was called despite invalid reference argument");
      return target;
    },
  },
};

export const $myDec = (context, target, override) => {
  console.log("$myDec called");
  console.log("  target:", target?.kind, target?.name);
  console.log("  override:", override?.kind, override?.name);
  override.parameters.properties;
};

Observed Behavior

$myDec called
  target: Operation test
  override: Model Operation       ← WRONG: should be an Operation, not the type Model
Internal compiler error!
TypeError: Cannot read properties of undefined (reading 'properties')
    at $myDec (test.js)
    at applyDecoratorToType (checker.ts:8646)

The decorator $myDec is called with override being the Reflection.Operation Model type (kind: "Model", name: "Operation") — the declared return type of myFn — instead of an actual Operation instance.

Expected Behavior

When the function call myFn(invalidref) fails due to an invalid reference:

• The decorator should not be applied (since its argument could not be resolved), OR
• An error diagnostic should be reported and the decorator call skipped

The compiler should never pass the raw return type declaration as a decorator argument.

Stack Trace

TypeError: Cannot read properties of undefined (reading 'properties')
    at $myDec (test.js:20:23)
    at applyDecoratorToType (checker.ts:8646:12)
    at applyDecoratorsToType (checker.ts:7919:26)
    at finishType (checker.ts:7903:30)
    at finishOperation (checker.ts:2667:14)

Real-world Impact

This is triggered by @@override(test, reorderParameters(abcde, #["b", "a"])) in @azure-tools/typespec-client-generator-core where abcde is a typo/invalid reference. The $override decorator crashes at override.parameters.properties.

Version

TypeSpec compiler v1.12.0

[github-actions]

🎯 Issue Triage

When an extern fn is called with an invalid reference argument, the compiler passes the function's declared return type (a Model) as the decorator argument instead of skipping the decorator or reporting a diagnostic, causing an internal crash.

Category: Bug
Area: compiler:core

🔗 No playground link (requires JS file side-by-side).

📋 Reproduction code

import "./test.js";

#suppress "experimental-feature" "testing extern fn crash with invalid ref"
extern fn myFn(target: Reflection.Operation): Reflection.Operation;

extern dec myDec(target: Reflection.Operation, override: Reflection.Operation);

op test(): void;

@`@myDec`(test, myFn(invalidref));

🔍 Analysis

• Root cause: In checker.ts, when evaluating a call to an extern fn with an invalid argument reference, the checker appears to fall back to the function's declared return type (the Reflection.Operation Model) rather than producing an error type or undefined. This value is then passed directly to the decorator implementation.
• Affected code: applyDecoratorToType (checker.ts:8646) and finishOperation (checker.ts:2667).
• Expected fix: The checker should detect the failed extern fn invocation (due to invalid ref argument), emit a diagnostic, and skip applying the decorator entirely.
• Real-world impact: Triggered in @azure-tools/typespec-client-generator-core via @@OverRide(test, reorderParameters(abcde, ...)) where abcde is a typo.

📝 Suggested next steps

• In the checker, when resolving extern fn call arguments, ensure an invalid reference causes the whole call expression to resolve to an error/unknown type (not the declared return type).
• Add a diagnostic when a decorator argument cannot be resolved due to a failed function call.
• Add a regression test with the provided minimal repro.

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