Compare unions that hold promises as untyped values

A promise converted into a union's promise arm is a new promise, so two
optional slots set to one promise each hold their own conversion, and a
value narrowed out of such a slot may be one too. Union equality compared
those arm payloads by pointer and answered false where Node answers true.

Any comparison between a union with a promise arm and a promise or another
such union now compares both sides as untyped values, where a converted
promise answers with its origin's identity.

packages/compiler/src/frontend/lowering/expressions/binary.ts

@@ -2388,19 +2388,22 @@ export function lowerStrictEquality(
         loc,
       };
     }
-    // A plain promise against a union whose promise arm has a different
-    // payload (`chains.get(id) === tail` with a `Map<K, Promise<unknown>>`):
-    // wrapping would convert the promise into that arm, and the converted
-    // promise is a new object. Promises are objects to ===, so compare both
-    // sides as untyped values, where a converted promise answers with its
-    // origin's identity (the plain promise-vs-promise rule below).
-    const plainPromise =
-      left.type.kind === "promise" ? left : right.type.kind === "promise" ? right : null;
+    // A promise against a union with a promise arm, or two such unions:
+    // converting a promise into a union's promise arm of another payload
+    // (`chains.get(id) === tail` with a `Map<K, Promise<unknown>>`, or
+    // `a.pending === b.pending` after both were set to one promise) builds a
+    // new promise, so either side may hold a conversion of the other's
+    // promise, and a value narrowed out of the union may be one too.
+    // Promises are objects to ===, so compare both sides as untyped values,
+    // where a converted promise answers with its origin's identity (the plain
+    // promise-vs-promise rule below).
+    const holdsPromise = (type: IrType): boolean =>
+      type.kind === "promise" ||
+      (type.kind === "union" &&
+        (lowerer.unions.get(type.unionId)?.arms ?? []).some((arm) => arm.kind === "promise"));
     if (
-      !bothUnion &&
-      plainPromise !== null &&
-      lowerer.armTag(ut.unionId, plainPromise.type) < 0 &&
-      unionArms.some((arm) => arm.kind === "promise") &&
+      holdsPromise(left.type) &&
+      holdsPromise(right.type) &&
       lowerer.dynConvertible(left.type) &&
       lowerer.dynConvertible(right.type)
     ) {

tests/corpus/union-promise-function-arm-widening.ts

@@ -58,6 +58,15 @@ async function main(): Promise<void> {
   console.log(store.pending === other);
   console.log(await store.pending);
 
+  // One promise in two optional slots: each holds its own conversion.
+  const second: Store = {};
+  second.pending = work;
+  console.log(store.pending === second.pending, store.pending !== second.pending);
+  const slots: Store[] = [store, second, { pending: other }];
+  console.log(slots.map((slot) => slot.pending === slots[0]!.pending).join(","));
+  second.lookup = findName;
+  console.log(store.lookup === second.lookup);
+
   const lookup = store.lookup;
   if (lookup) console.log(await lookup("abcd"), await lookup("ab"));
   console.log(store.scale?.(21));