1 files changed, 24 insertions, 0 deletions
diff --git a/js/src/jit-test/tests/environments/bug1671563.js b/js/src/jit-test/tests/environments/bug1671563.js
new file mode 100644
index 0000000000..29a24fbcd9
--- /dev/null
+++ b/js/src/jit-test/tests/environments/bug1671563.js
@@ -0,0 +1,24 @@
+// The value of a `var` on entering a function is `undefined`, even if a
+// default expression uses the same stack slot for something else.
+
+load(libdir + "asserts.js");
+
+function f(a = class C{}) {
+  var x;
+  return x;
+}
+assertEq(f(), undefined);
+
+function* g1(a = class C {}) {
+  var x;
+  assertEq(x, undefined);
+}
+g1().next();
+
+function* g2(a = class C {}) {
+  x;
+  let x;
+}
+assertThrowsInstanceOf(() => g2().next(), ReferenceError);
+
+