[strong] Allow mutually recursive classes.

test/mjsunit/strong/mutually-recursive-classes.js

Index: test/mjsunit/strong/mutually-recursive-classes.js
diff --git a/test/mjsunit/strong/mutually-recursive-classes.js b/test/mjsunit/strong/mutually-recursive-classes.js
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+++ b/test/mjsunit/strong/mutually-recursive-classes.js
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+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --strong-mode
+
+// Note that it's essential for these tests that the reference is inside dead
+// code (because we already produce ReferenceErrors for run-time unresolved
+// variables and don't want to confuse those with strong mode errors). But the
+// errors should *not* be inside lazy, unexecuted functions, since lazy parsing
+// doesn't produce strong mode scoping errors).
+
+// In addition, assertThrows will call eval and that changes variable binding
+// types (see e.g., UNBOUND_EVAL_SHADOWED). We can avoid unwanted side effects
+// by wrapping the code to be tested inside an outer function.
+function assertThrowsHelper(code) {
+  "use strict";
+  let prologue_dead = "(function outer() { if (false) { ";
+  let epilogue_dead = " } })();";
+
+  assertThrows("'use strong'; " + prologue_dead + code + epilogue_dead, ReferenceError);
+
+  // Make sure the error happens only in strong mode (note that we need strict
+  // mode here because of let).
+  assertDoesNotThrow("'use strict'; " + prologue_dead + code + epilogue_dead);
+
+  // But if we don't put the references inside a dead code, it throws a run-time
+  // error (also in strict mode).
+  let prologue_live = "(function outer() { ";
+  let epilogue_live = "})();";
+
+  assertThrows("'use strong'; " + prologue_live + code + epilogue_live, ReferenceError);
+  assertThrows("'use strict'; " + prologue_live + code + epilogue_live, ReferenceError);
+}
+
+(function InitTimeReferenceForward() {
+  // It's never OK to have an init time reference to a class which hasn't been
+  // declared.
+  assertThrowsHelper(
+      `class A extends B { };
+      class B {}`);
+
+  assertThrowsHelper(
+      `class A {
+        [B.sm()]() { }
+      };
+      class B {
+        static sm() { return 0; }
+      }`);
+})();
+
+(function InitTimeReferenceBackward() {
+  // Backwards is of course fine.
+  "use strong";
+  class A {
+    static sm() { return 0; }
+  };
+  let i = "making these classes non-consecutive";
+  class B extends A {};
+  "by inserting statements and declarations in between";
+  class C {
+    [A.sm()]() { }
+  };
+})();
+
+(function BasicMutualRecursion() {
+  "use strong";
+  class A {
+    m() { B; }
+    static sm() { B; }
+  };
+  // No statements or declarations between the classes.
+  class B {
+    m() { A; }
+    static sm() { A; }
+  };
+})();