Fold navigator member reads under optional chains (#1954)

packages/compiler/src/frontend/lowering/expressions/optional-chains.ts

@@ -1,3 +1,4 @@
+import { isNavigatorGlobal } from "../builtins/navigator.js";
 import { isSafeToDiscard } from "./evaluation-safety.js";
 import { completeFuncValueArgs } from "../call-arguments.js";
 import { dynUndefinedExpr } from "../../../ir/build.js";
@@ -223,6 +224,16 @@ export function lowerOptionalChain(
   // Native handle methods have no first-class value to guard; when the
   // method read itself has no lowering, re-dispatch the plain call (which
   // reports its own diagnostics) instead of failing on the value read.
+  // `navigator?.userAgent`: Node's navigator global is never nullish and
+  // has no value form, so the guarded step is the plain member read.
+  if (isNavigatorGlobal(lowerer, recvNode)) {
+    lowerer.chainHandled.add(dotNode);
+    try {
+      return lowerer.lowerExpr(expr);
+    } finally {
+      lowerer.chainHandled.delete(dotNode);
+    }
+  }
   let loweredReceiver: IrExpr | null;
   if (dotNode === expr && alwaysPresentStdlibMethod(lowerer, recvNode)) {
     loweredReceiver = tryLowerExpression(lowerer, recvNode);

packages/compiler/src/frontend/lowering/expressions/property-access.ts

@@ -306,10 +306,10 @@ export function lowerPropertyAccessExpr(
       };
     }
   }
-  if (!expr.questionDotToken) {
-    const navigator = lowerNavigatorMember(lowerer, expr, NODE_COMPAT_VERSION);
-    if (navigator) return navigator;
-  }
+  // Node's navigator is never nullish, so `navigator?.userAgent` (zod's
+  // Cloudflare probe) folds like `navigator.userAgent`.
+  const navigator = lowerNavigatorMember(lowerer, expr, NODE_COMPAT_VERSION);
+  if (navigator) return navigator;
   // A dgram socket's method read as a value in JavaScript (`emit:
   // socket.emit.bind(socket)`): the socket crosses as its checked-dynamic
   // handle, whose methods are function values that dispatch on their

packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7600-navigator-optional-chain.js.json

@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/7600-navigator-optional-chain.js"
+ ],
+ "diags": []
+}

tests/corpus/7600-navigator-optional-chain.js

@@ -0,0 +1,7 @@
+// Node's navigator global read through optional chains (zod 4's
+// `navigator?.userAgent?.includes("Cloudflare")` probe): navigator is
+// never nullish, so each chain answers like the plain member read.
+const cloudflare = typeof navigator !== "undefined" && navigator?.userAgent?.includes("Cloudflare");
+console.log(cloudflare);
+console.log(navigator?.userAgent === navigator.userAgent, navigator?.userAgent?.startsWith("Node.js/"));
+console.log(navigator?.userAgent?.split("/")[0] ?? "none");

tests/harness/corpus-costs/7600-navigator-optional-chain.js.json

@@ -0,0 +1 @@
+{ "plain": 3000, "san": 22000 }