Testing plan for Thenable-Curtailment (Safe Promise Resolve)

[mgaudet]

Testing Plan for https://tc39.es/proposal-thenable-curtailment/

Setup:

• Add a safeResolvePromise host-defined function to allow testing the proposal (which otherwise has no user access in its current shape)

Synchronous Resolve

• Primitive values resolve synchronously
• Object with no then property resolve synchronously
• Object with un-callable then resolve synchronously.
• Object with un-callable then on prototype resolves synchronously
• Object with then setter, but no getter, resolve synchronously
• Object with then setter, but no getter, on the prototype. resolves synchronously.
• Object with own-property then which is not callable shadows a callable then, resolves synchronously.

Deferred Resolve & Execution of User Code

• Resolving with a Proxy (even with empty traps) resolves deferred.
• Resolving with a Proxy with a get trap is resolved.
  • The get trap runs in deferral.
• Resolving an object which has a proxy on its prototype chain is deferred.
• Resolving with a module namespace object is deferred
• Object with callable then own-property is deferred
  • The then call is made, deferred.
• Object with callable then prototype is deferred
  • The then call is made deferred
• A deferred then throwing should cause the promise to be rejected.
• Object with then getter
  • The then getter is run, deferred.
• Object with then getter on prototype.
• The then getter is run, deferred.
• then getter which throws rejects promise, but deferred
• Resolving with a revoked proxy rejects deferred
• Callable Object.prototype.then causes object resolution to be deferred
  • unless the object has a null prototype.
• Ensure deferred then call has correct this binding.
• Tick count of safe resolving a promise with another native promise (resolved, unresolved).
• Delete the callable then after a safeResolve job is enqueued but before it's processed.
• Ensure the Get of then is run twice when

Latching behaviour

• Attempting to resolve a deferred promise of any sort cannot overwrite or otherwise interfere with the deferred promise
• Late rejection also fails.
• Repeated safe resolution: Latched by first and second is no-op.
• Attempting to safe resolve an already resolved promise is a no-op.

Edge Cases

• (If possible) test HostObjectHasImpureLookupInternalMethods.
• Self-resolve: (note: Spec currently will defer depending on the inspection of current promise.. but I actually think maybe we should change the spec to synchronously throw here?)

[bakkot]

I'd also add

• Object with a Proxy in its prototype chain but which has an own-property then which is not callable, resolves synchronously.