Prefer the union arm that keeps every field of a converted record (#2259)

packages/compiler/src/frontend/lowering/coercions/structural-plans.ts

@@ -128,6 +128,25 @@ export function widthLiftPlan(lowerer: Lowerer, src: IrType, dst: IrType): Width
       const route = presenceRoute(lowerer, src.shapeId, selected[0]!, selected[1]!);
       if (route) return route;
     }
+    // A record whose fields some arm keeps EVERY one of, each lifting
+    // (`{ id; limit; opts?: A | B }` into `{ id; limit; opts?: A | B | C } |
+    // { id; opts?: C }`): that arm is the value's own layout, widened; the
+    // others hold it only by dropping fields. Prefer the unique lossless arm.
+    if (selected.length > 1 && src.kind === "record") {
+      const source = lowerer.shapes.get(src.shapeId);
+      const lossless = selected.filter(({ arm }) => {
+        const target = arm.kind === "record" ? lowerer.shapes.get(arm.shapeId) : undefined;
+        return (
+          source !== undefined &&
+          target !== undefined &&
+          !source.tuple &&
+          !target.tuple &&
+          target.fields.length === source.fields.length &&
+          source.fields.every((field) => target.fields.some((kept) => kept.name === field.name))
+        );
+      });
+      if (lossless.length === 1) selected = lossless;
+    }
     if (selected.length !== 1) return null;
     return { how: "liftWrap", tag: selected[0]!.tag, arm: selected[0]!.arm };
   }

packages/compiler/test/ts7/baselines/order-parity/tests/corpus/union-record-lossless-arm.ts.json

@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/union-record-lossless-arm.ts"
+ ],
+ "diags": []
+}

tests/corpus/union-record-lossless-arm.ts

@@ -0,0 +1,55 @@
+// A record flowing into a union where several record arms can hold it by a
+// width conversion: the arm that keeps every one of its fields is the
+// value's own layout, widened, and wins over arms that drop fields.
+
+interface FastOpts {
+  fast?: boolean;
+  strict?: boolean;
+}
+interface DepthOpts {
+  depth?: number;
+}
+interface StrictOpts {
+  strict?: boolean;
+}
+
+interface Wide {
+  id: string;
+  window: number;
+  kind?: string;
+  limit: number;
+  opts?: FastOpts | DepthOpts | StrictOpts;
+}
+
+interface Judge {
+  id: string;
+  window: number;
+  kind: string;
+  opts?: StrictOpts;
+}
+
+interface Narrow {
+  id: string;
+  window: number;
+  kind?: string;
+  limit: number;
+  opts?: FastOpts | DepthOpts;
+}
+
+function show(entry: Wide | Judge): string {
+  const limit = "limit" in entry ? ` limit=${entry.limit}` : "";
+  return `${entry.kind ?? "wide"} ${entry.id} window=${entry.window}${limit} ${JSON.stringify(entry.opts ?? null)}`;
+}
+
+const narrow: Narrow = { id: "n1", window: 4, limit: 2, opts: { depth: 3 } };
+console.log(show(narrow));
+
+const entries: (Wide | Judge)[] = [narrow, { id: "j1", window: 1, kind: "judge", opts: { strict: true } }];
+const plain: Narrow = { id: "n2", window: 8, kind: "custom", limit: 5 };
+entries.push(plain);
+for (const entry of entries) console.log(show(entry));
+
+let latest: Wide | Judge | undefined;
+for (const item of [narrow, plain]) latest = item;
+console.log(latest === undefined ? "none" : show(latest));
+

tests/harness/corpus-costs/union-record-lossless-arm.ts.json

@@ -0,0 +1 @@
+{ "plain": 11809, "san": 13580 }