From ec973e722194a4bb828ec26f349be896aa210f05 Mon Sep 17 00:00:00 2001
From: Malte Ubl <malte@vercel.com>
Date: Tue, 27 Oct 2026 15:22:06 -0700
Subject: [PATCH] Pin generic method members constrained by the enclosing type
 parameter

An interface's generic method whose own type parameter is constrained by
the enclosing type's parameter (run<T extends TApi>(item: Item<T>)),
implemented by a plain contextually typed lambda, compiles since #2274:
the lambda takes the contextual signature's type parameters and lowers
once, erased. The merged corpus only covers unconstrained contextual
generics, so this program pins the constrained shape with a conditional
member on the argument type, explicit and inferred call type arguments,
the class-body form, and record-held receivers.

Verified against the 2026-10-23 base checkout, where the same shape
refused with SC2009 (Item<T>'s member 'api' has type 'T'), and the pi
models.ts:1159 Model<T> 'compat' shape refused the same way.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 ...ic-member-constrained-lambda-impls.ts.json |  6 ++
 ...generic-member-constrained-lambda-impls.ts | 73 +++++++++++++++++++
 ...ic-member-constrained-lambda-impls.ts.json |  1 +
 3 files changed, 80 insertions(+)
 create mode 100644 packages/compiler/test/ts7/baselines/order-parity/tests/corpus/generic-member-constrained-lambda-impls.ts.json
 create mode 100644 tests/corpus/generic-member-constrained-lambda-impls.ts
 create mode 100644 tests/harness/corpus-costs/generic-member-constrained-lambda-impls.ts.json

diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/generic-member-constrained-lambda-impls.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/generic-member-constrained-lambda-impls.ts.json
new file mode 100644
index 000000000..4c7bdccc5
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/generic-member-constrained-lambda-impls.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/generic-member-constrained-lambda-impls.ts"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/generic-member-constrained-lambda-impls.ts b/tests/corpus/generic-member-constrained-lambda-impls.ts
new file mode 100644
index 000000000..b34d46c15
--- /dev/null
+++ b/tests/corpus/generic-member-constrained-lambda-impls.ts
@@ -0,0 +1,73 @@
+// A generic method member whose own type parameter is CONSTRAINED by the
+// enclosing type's type parameter (`run<T extends TApi>(item: Item<T>)`),
+// implemented by a plain lambda: the lambda takes the interface signature's
+// type parameters as its own and lowers once, erased like a generic lambda.
+// `Item<T>`'s conditional member depends on the type argument each call
+// passes, so the erased parameter must hold whatever the call sends.
+
+type Api = "alpha" | "beta" | "gamma";
+
+interface AlphaOptions {
+  fast?: boolean;
+}
+interface BetaOptions {
+  depth?: number;
+}
+
+interface Item<TApi extends Api> {
+  id: string;
+  api: TApi;
+  options?: TApi extends "alpha" ? AlphaOptions : TApi extends "beta" ? BetaOptions : never;
+}
+
+interface Runner<TApi extends Api> {
+  name: string;
+  run<T extends TApi>(item: Item<T>): string;
+}
+
+function describe(item: Item<Api>): string {
+  const options = item.options === undefined ? "-" : JSON.stringify(item.options);
+  return `${item.id}:${item.api}:${options}`;
+}
+
+function makeRunner<TApi extends Api>(name: string): Runner<TApi> {
+  return {
+    name,
+    run: (item) => `${name}:${describe(item)}`,
+  };
+}
+
+const both = makeRunner<"alpha" | "beta">("both");
+console.log(both.name);
+console.log(both.run({ id: "a", api: "alpha", options: { fast: true } }));
+console.log(both.run({ id: "b", api: "beta", options: { depth: 3 } }));
+console.log(both.run({ id: "c", api: "alpha" }));
+console.log(both.run<"beta">({ id: "d", api: "beta" }));
+
+const all = makeRunner<Api>("all");
+console.log(all.run({ id: "e", api: "gamma" }));
+console.log(all.run({ id: "f", api: "beta", options: { depth: 1 } }));
+
+// The same contract through a class body: the generic method lowers under
+// the class instantiation's binding joined with its own type parameter.
+class ClassRunner<TApi extends Api> implements Runner<TApi> {
+  name: string;
+  constructor(name: string) {
+    this.name = name;
+  }
+  run<T extends TApi>(item: Item<T>): string {
+    return `${this.name}:${describe(item)}`;
+  }
+}
+
+const cls = new ClassRunner<"alpha" | "beta">("cls");
+console.log(cls.run({ id: "g", api: "alpha", options: { fast: false } }));
+console.log(cls.run({ id: "h", api: "beta" }));
+
+// Record-held receivers call the erased slot the literal stored.
+const runners: Runner<"alpha" | "beta">[] = [both, cls];
+let combined = "";
+for (const runner of runners) {
+  combined += `[${runner.run({ id: "i", api: "alpha" })}]`;
+}
+console.log(combined);
diff --git a/tests/harness/corpus-costs/generic-member-constrained-lambda-impls.ts.json b/tests/harness/corpus-costs/generic-member-constrained-lambda-impls.ts.json
new file mode 100644
index 000000000..fe5fca1ae
--- /dev/null
+++ b/tests/harness/corpus-costs/generic-member-constrained-lambda-impls.ts.json
@@ -0,0 +1 @@
+{ "plain": 4100 }