From 9cdbc83aee7e85428c44bff096663e16b94fd6b4 Mon Sep 17 00:00:00 2001
From: Malte Ubl <malte@vercel.com>
Date: Sun, 25 Oct 2026 10:07:59 -0700
Subject: [PATCH] Prefer the union arm that keeps every field of a converted
 record

A record flowing into a union where several record arms could hold it by a
width conversion was refused with SC2003 as ambiguous, even when one arm
keeps every field of the record and the others hold it only by dropping
fields (a model record whose option union is narrower than the catalog's,
beside a classifier arm with a subset of its fields). That arm is the
value's own layout, widened: it now wins when it is the only one. Plans
that already resolved (a single candidate, the absent-field preference,
discriminant and presence routes) are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 .../lowering/coercions/structural-plans.ts    | 19 +++++++
 .../corpus/union-record-lossless-arm.ts.json  |  6 ++
 tests/corpus/union-record-lossless-arm.ts     | 55 +++++++++++++++++++
 .../union-record-lossless-arm.ts.json         |  1 +
 4 files changed, 81 insertions(+)
 create mode 100644 packages/compiler/test/ts7/baselines/order-parity/tests/corpus/union-record-lossless-arm.ts.json
 create mode 100644 tests/corpus/union-record-lossless-arm.ts
 create mode 100644 tests/harness/corpus-costs/union-record-lossless-arm.ts.json

diff --git a/packages/compiler/src/frontend/lowering/coercions/structural-plans.ts b/packages/compiler/src/frontend/lowering/coercions/structural-plans.ts
index f42663a13..bff419d48 100644
--- a/packages/compiler/src/frontend/lowering/coercions/structural-plans.ts
+++ b/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 };
   }
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/union-record-lossless-arm.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/union-record-lossless-arm.ts.json
new file mode 100644
index 000000000..143cda7d0
--- /dev/null
+++ b/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": []
+}
diff --git a/tests/corpus/union-record-lossless-arm.ts b/tests/corpus/union-record-lossless-arm.ts
new file mode 100644
index 000000000..b90d5486e
--- /dev/null
+++ b/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));
+
diff --git a/tests/harness/corpus-costs/union-record-lossless-arm.ts.json b/tests/harness/corpus-costs/union-record-lossless-arm.ts.json
new file mode 100644
index 000000000..011a8fbbf
--- /dev/null
+++ b/tests/harness/corpus-costs/union-record-lossless-arm.ts.json
@@ -0,0 +1 @@
+{ "plain": 11809, "san": 13580 }