Memoize an empty literal's type separately as a union arm

The type mapper maps the type inferred from an empty object literal to the
empty record as an arm of an unreduced union and to a checked-dynamic
value elsewhere, but both answers shared one memo entry. Once the type had
been mapped on its own, a union such as `typeof empty | DataView` reused
the dynamic answer for its arm and refused (SC2009). The memo key now
includes whether the mapping is for such an arm.

packages/compiler/src/frontend/type-mapper.ts

@@ -1655,7 +1655,10 @@ export function mapType(type: ts.Type, ctx: TypeMapperCtx): IrType | null {
   const topLevel = mapTypeDepth === 0;
   const typeId = (type as { id?: number }).id;
   const memoizable = typeId !== undefined && (ctx.canMemoizeType?.() ?? true);
-  const memoKey = memoizable ? `${0}:${typeId}` : null;
+  // An empty literal's type maps to the empty record as an unreduced union's
+  // arm (emptyLiteralArm) and to the checked-dynamic value elsewhere: the two
+  // answers keep separate memo entries.
+  const memoKey = memoizable ? `${ctx.emptyLiteralArm ? 1 : 0}:${typeId}` : null;
   if (memoKey !== null) {
     const memo = ctx.typeMemo?.get(memoKey);
     if (memo !== undefined) return memo;

tests/corpus/6909-empty-object-type-values.ts

@@ -30,3 +30,11 @@ console.log(typeof choose(1), typeof choose(-1));
 
 const empty = {};
 console.log(JSON.stringify(empty), Object.keys(empty).length, empty === empty, String(empty));
+
+// An empty literal's type beside a typed arm of an unreduced union, after
+// the type was already used on its own.
+const viewed = {};
+console.log(JSON.stringify(viewed));
+const views: (typeof viewed | DataView)[] = [viewed, new DataView(new ArrayBuffer(2))];
+for (const view of views)
+  console.log(view instanceof DataView ? `view ${view.byteLength}` : JSON.stringify(view));