From e198a9eec235a36fdca3b954c27c4eedd249939a Mon Sep 17 00:00:00 2001
From: Malte Ubl <malte@vercel.com>
Date: Wed, 28 Oct 2026 14:04:06 -0700
Subject: [PATCH] Dispatch a `keyof` read whose member is `unknown` per
 declared key
MIME-Version: 1.0
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`settings[key]` over a settings record with an `unknown` member
(`type: unknown`) refuses even though every field reads through a
checked-dynamic slot: the checker's answer collapses to `unknown`
because the union of the field types absorbs into it, so there is no
typed join for the keyed read to dispatch into.

Recognize that shape — the read's type is `unknown`, the shape has no
index signature, and every declared field's value is checked-dynamic
representable — and dispatch per declared key answering a checked-
dynamic read. The key type is the record's own `keyof`, so the answer
is always present.

Corpus 8523 pins the read differentially.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 .../lowering/expressions/element-access.ts    | 14 +++++++
 .../8523-settings-keyof-dynamic-read.ts.json  |  6 +++
 .../8523-settings-keyof-dynamic-read.ts       | 40 +++++++++++++++++++
 .../8523-settings-keyof-dynamic-read.ts.json  |  1 +
 4 files changed, 61 insertions(+)
 create mode 100644 packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8523-settings-keyof-dynamic-read.ts.json
 create mode 100644 tests/corpus/8523-settings-keyof-dynamic-read.ts
 create mode 100644 tests/harness/corpus-costs/8523-settings-keyof-dynamic-read.ts.json

diff --git a/packages/compiler/src/frontend/lowering/expressions/element-access.ts b/packages/compiler/src/frontend/lowering/expressions/element-access.ts
index 5cb9776f6d..1ce873853d 100644
--- a/packages/compiler/src/frontend/lowering/expressions/element-access.ts
+++ b/packages/compiler/src/frontend/lowering/expressions/element-access.ts
@@ -1534,6 +1534,20 @@ export function lowerRecordKeyRead(
     recordKeyResultOk(lowerer, shape, DYN)
   )
     declared = DYN;
+  // A `keyof` read over a record that holds an 'unknown' member: the
+  // checker's answer collapses to 'unknown' (the union of every field
+  // type absorbs into it), so there is no typed join to dispatch into.
+  // Each declared field still reads through its checked-dynamic slot —
+  // the very value the checker's 'unknown' can hold — so dispatch per
+  // declared key and answer a checked-dynamic read. The key type is the
+  // record's own `keyof`, so the answer is present whenever the read is.
+  if (
+    !declared &&
+    (lowerer.typeOf(expr).flags & ts.TypeFlags.Unknown) !== 0 &&
+    !shape.indexValue &&
+    recordKeyResultOk(lowerer, shape, DYN)
+  )
+    declared = DYN;
   if (!declared) {
     lowerer.unsupported(
       "SC1090",
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8523-settings-keyof-dynamic-read.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8523-settings-keyof-dynamic-read.ts.json
new file mode 100644
index 0000000000..e42bdab216
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8523-settings-keyof-dynamic-read.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/8523-settings-keyof-dynamic-read.ts"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/8523-settings-keyof-dynamic-read.ts b/tests/corpus/8523-settings-keyof-dynamic-read.ts
new file mode 100644
index 0000000000..d0460d6dff
--- /dev/null
+++ b/tests/corpus/8523-settings-keyof-dynamic-read.ts
@@ -0,0 +1,40 @@
+// A `keyof` read over a record that holds an `unknown` member: the
+// checker's answer collapses to `unknown` (the union of every field type
+// absorbs into it), so there is no typed join. The read dispatches per
+// declared key through the checked-dynamic slot instead.
+interface Retry {
+  baseDelayMs?: number;
+  enabled?: boolean;
+}
+interface Settings {
+  $schema?: string;
+  enabledModels?: string[];
+  branchSummary: unknown;
+  fullscreenWheelScrollLines?: number | string;
+  modelThinkingLevels?: { [key: string]: string };
+  retry?: Retry;
+  outputPad?: number;
+}
+function read(settings: Settings, key: keyof Settings): unknown {
+  return settings[key];
+}
+const settings: Settings = {
+  branchSummary: { note: "x", count: 2 },
+  outputPad: 4,
+  enabledModels: ["m"],
+  fullscreenWheelScrollLines: "wide",
+  modelThinkingLevels: { high: "big" },
+  retry: { enabled: true },
+};
+const keys: (keyof Settings)[] = [
+  "outputPad",
+  "enabledModels",
+  "branchSummary",
+  "retry",
+  "$schema",
+  "fullscreenWheelScrollLines",
+];
+for (const key of keys) {
+  const value = read(settings, key);
+  console.log(key, typeof value, value === undefined ? "absent" : JSON.stringify(value));
+}
diff --git a/tests/harness/corpus-costs/8523-settings-keyof-dynamic-read.ts.json b/tests/harness/corpus-costs/8523-settings-keyof-dynamic-read.ts.json
new file mode 100644
index 0000000000..10b67e419b
--- /dev/null
+++ b/tests/harness/corpus-costs/8523-settings-keyof-dynamic-read.ts.json
@@ -0,0 +1 @@
+{ "plain": 1300, "san": 2700 }