await thenable returns the thenable itself instead of calling .then(resolve, reject) #586

Repro

class Thenable {
  [Symbol.toStringTag] = "Thenable";
  then(onFulfilled: any, _onRejected: any) {
    console.log('[then] called');
    return Promise.resolve(42).then(onFulfilled);
  }
}

const t = new Thenable();
const result = await t;
console.log('result =', result);

Bun output (correct, ECMAScript spec):

[then] called
result = 42

Perry output (broken):

result = {}

Perry's await only handles real Promises — it doesn't recognize objects with a .then(resolve, reject) method (the thenable protocol). When you await a thenable, perry returns the thenable object itself unchanged.

Spec

ECMAScript await semantics: if the awaited value is an object with a callable then property, it should be assimilated — perry should call value.then(resolve, reject) and resume the surrounding async function with whatever resolve is called with. Both real Promise instances and user-defined thenables (Q, Bluebird, Drizzle's QueryPromise, etc.) follow this contract.

Why it matters for #488

drizzle-orm's QueryPromise (drizzle-orm/query-promise.js) is the load-bearing pattern:

class QueryPromise {
  static [entityKind] = "QueryPromise";
  [Symbol.toStringTag] = "QueryPromise";
  then(onFulfilled, onRejected) {
    return this.execute().then(onFulfilled, onRejected);  // executes lazily on first await
  }
}

db.select().from(users) returns a PgSelectBase extending QueryPromise. The intent is await db.select().from(users) calls .then(resolve), which triggers .execute() (the actual SQL round-trip), and resolves with rows. Without thenable assimilation, await returns the builder object unchanged — query never runs.

This is the second perry-side blocker for #488 (after #585 cross-module instanceof).

Where I'd start digging

crates/perry-runtime/src/promise.rs (await/resolve path) and the async-to-generator transform in crates/perry-transform/src/generator.rs. The relevant point is wherever the await is evaluated — when the awaited value is not a Promise, perry should:

1. Probe for a .then method via js_object_get_field_by_name(value, "then")
2. If callable, allocate a fresh Promise, call value.then(resolve, reject) where resolve/reject route to the new promise
3. Suspend the async function on that new Promise

The fast path for real Promises stays unchanged.

Workaround

User-side await Promise.resolve(thenable.execute()) works (bypasses the await-via-thenable assimilation by calling .execute() directly and awaiting the result). For drizzle this would mean rewriting every query call site to call .execute() explicitly — invasive enough that fixing the runtime is the right move.

Refs

Blocks #488. Surfaced from #420 sub-issue acceptance run on v0.5.684.