From c3474cfa7ae6d0ef1e6620040aa5237ecce641f8 Mon Sep 17 00:00:00 2001
From: Chris Tate <chris@ctate.dev>
Date: Wed, 28 Oct 2026 22:30:01 -0500
Subject: [PATCH] Test that receivers are evaluated once and before effectful
 keys

Adds the corpus program receiver-before-effectful-key.ts, with its corpus
cost and order baseline:
- a tuple read with a runtime index that rebinds the tuple or replaces the
  field holding it;
- a keyed read through a finite union of field names whose key rebinds the
  record or replaces the field holding it;
- an inherited-key `in` test on a computed null-prototype receiver, which
  runs the receiver once.

Each case reads the receiver as evaluated before the key, as in Node.
---
 .../receiver-before-effectful-key.ts.json     |  6 ++
 tests/corpus/receiver-before-effectful-key.ts | 59 +++++++++++++++++++
 .../receiver-before-effectful-key.ts.json     |  1 +
 3 files changed, 66 insertions(+)
 create mode 100644 packages/compiler/test/ts7/baselines/order-parity/tests/corpus/receiver-before-effectful-key.ts.json
 create mode 100644 tests/corpus/receiver-before-effectful-key.ts
 create mode 100644 tests/harness/corpus-costs/receiver-before-effectful-key.ts.json

diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/receiver-before-effectful-key.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/receiver-before-effectful-key.ts.json
new file mode 100644
index 000000000..b44d5a127
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/receiver-before-effectful-key.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/receiver-before-effectful-key.ts"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/receiver-before-effectful-key.ts b/tests/corpus/receiver-before-effectful-key.ts
new file mode 100644
index 000000000..496c9c4c3
--- /dev/null
+++ b/tests/corpus/receiver-before-effectful-key.ts
@@ -0,0 +1,59 @@
+// A receiver is evaluated once, before its key, even when the key's
+// evaluation rebinds the receiver's binding or has other effects: a tuple
+// read with a runtime index, a keyed read through a finite union of field
+// names, and an inherited-key `in` test on a computed index-signature
+// receiver.
+
+const log: string[] = [];
+
+// Tuple read with a runtime index that rebinds the tuple.
+let pair: [string, number] = ["first", 1];
+function rebindPair(i: number): number {
+  log.push(`rebind pair ${i}`);
+  pair = ["second", 2];
+  return i;
+}
+const viaIndex = pair[rebindPair(0)];
+console.log("tuple", viaIndex, pair[0]);
+
+// A field holding a tuple, replaced by the index.
+const holder: { t: [number, number, number] } = { t: [10, 20, 30] };
+function replaceHeld(i: number): number {
+  holder.t = [-1, -2, -3];
+  return i;
+}
+console.log("tuple field", holder.t[replaceHeld(2)], holder.t[2]);
+
+// Keyed read through a finite union of keys naming a subset of the fields,
+// with a key that rebinds the local or replaces the field holding the record.
+type Row = { width?: number; label: string; tags: string[] };
+function finiteKeys(): void {
+  let row: Row = { width: 640, label: "old", tags: [] };
+  const pick = (): "width" | "label" => {
+    log.push("pick key");
+    row = { width: 1, label: "new", tags: ["x"] };
+    return "label";
+  };
+  const picked = row[pick()];
+  console.log("keyed", picked, row.label);
+  const box: { row: Row } = { row: { label: "boxed", tags: [] } };
+  const replace = (): "width" | "label" => {
+    box.row = { width: 7, label: "replaced", tags: [] };
+    return "width";
+  };
+  const width = box.row[replace()];
+  console.log("keyed field", width, box.row.width);
+}
+finiteKeys();
+
+// `in` on an inherited key of a computed null-prototype record: the
+// receiver call runs once.
+let groups = 0;
+function grouped(): Partial<Record<string, number[]>> {
+  groups++;
+  log.push("grouped");
+  return Object.groupBy([1, 2, 3], (n) => (n % 2 === 0 ? "even" : "odd"));
+}
+console.log("in", "toString" in grouped(), "hasOwnProperty" in grouped(), groups);
+
+console.log(log.join(" | "));
diff --git a/tests/harness/corpus-costs/receiver-before-effectful-key.ts.json b/tests/harness/corpus-costs/receiver-before-effectful-key.ts.json
new file mode 100644
index 000000000..bb380e241
--- /dev/null
+++ b/tests/harness/corpus-costs/receiver-before-effectful-key.ts.json
@@ -0,0 +1 @@
+{ "plain": 2500, "san": 3000 }