Dispatch splice on arrays of unknown elements (#1935)

packages/compiler/src/frontend/lowering/containers/array-methods.ts

@@ -361,6 +361,21 @@ export function lowerArrayMethodCall(
     if (probe.type.kind === "array") {
       receiverIr = probe.type;
       probedUntyped = true;
+    } else if (
+      probe.type.kind === "dyn" &&
+      !isJsSourceFile(ts.getSourceFileOfNode(access)) &&
+      lowerer.isStdlibMember(access)
+    ) {
+      // An `unknown[]` (or `any[]`) array stored checked-dynamic — a
+      // generic body instantiated at `unknown`, a spread of unknown
+      // elements: the method runs on the runtime array (dynInvoke, JS
+      // exact), and a typed result is checked like any member read.
+      const dispatched = lowerDynDispatchMethodCall(lowerer, call, access, probe, true);
+      if (!dispatched) return null;
+      const result = lowerer.mapTypeOf(lowerer.typeOf(call));
+      return result && result.kind !== "void" && result.kind !== "dyn"
+        ? lowerer.coerceInto(call, dispatched, result)
+        : dispatched;
     }
   }
   if (receiverIr?.kind === "union") {

packages/compiler/src/frontend/lowering/lower-calls.ts

@@ -4261,6 +4261,7 @@ const DYN_DISPATCH_METHODS = new Set([
   "shift",
   "unshift",
   "slice",
+  "splice",
   "at",
   "indexOf",
   "lastIndexOf",

packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7402-unknown-array-methods.ts.json

@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/7402-unknown-array-methods.ts"
+ ],
+ "diags": []
+}

tests/corpus/7402-unknown-array-methods.ts

@@ -0,0 +1,14 @@
+// Methods on arrays of unknown elements, which are stored checked-dynamic:
+// direct `unknown[]` bindings and generic bodies instantiated at `unknown`.
+const values: unknown[] = [1, "two", 3, { four: 4 }];
+const removed = values.splice(1, 2);
+console.log(values, removed, removed.length);
+values.splice(1, 0, "inserted", null);
+console.log(values, values.indexOf(null), values.includes("inserted"));
+console.log(values.slice(1).reverse(), values.join("|"));
+
+function takeAt<T>(items: readonly T[], index: number): T[] {
+  const pool = [...items];
+  return pool.splice(index, 1);
+}
+console.log(takeAt([10, 20, 30], 1), takeAt<unknown>([true, "x", 7], 2));

tests/harness/corpus-costs/7402-unknown-array-methods.ts.json

@@ -0,0 +1 @@
+{ "plain": 2600, "san": 20000 }