1 files changed, 80 insertions, 0 deletions
diff --git a/libcxx/test/libcxx/input.output/iostreams.base/ios.base/ios.base.cons/dtor.uninitialized.pass.cpp b/libcxx/test/libcxx/input.output/iostreams.base/ios.base/ios.base.cons/dtor.uninitialized.pass.cpp
new file mode 100644
index 000000000000..ea42203d0544
--- /dev/null
+++ b/libcxx/test/libcxx/input.output/iostreams.base/ios.base/ios.base.cons/dtor.uninitialized.pass.cpp
@@ -0,0 +1,80 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: no-exceptions
+
+// The fix for issue 57964 requires an updated dylib due to explicit
+// instantiations. That means Apple backdeployment targets remain broken.
+// UNSUPPORTED: using-built-library-before-llvm-19
+
+// <ios>
+
+// class ios_base
+
+// ~ios_base()
+//
+// Destroying a constructed ios_base object that has not been
+// initialized by basic_ios::init is undefined behaviour. This can
+// happen in practice, make sure the undefined behaviour is handled
+// gracefully.
+//
+//
+// [ios.base.cons]/1
+//
+// ios_base();
+// Effects: Each ios_base member has an indeterminate value after construction.
+// The object's members shall be initialized by calling basic_ios::init before
+// the object's first use or before it is destroyed, whichever comes first;
+// otherwise the behavior is undefined.
+//
+// [basic.ios.cons]/2
+//
+// basic_ios();
+// Effects: Leaves its member objects uninitialized.  The object shall be
+// initialized by calling basic_ios::init before its first use or before it is
+// destroyed, whichever comes first; otherwise the behavior is undefined.
+//
+// ostream and friends have a basic_ios virtual base.
+// [class.base.init]/13
+// In a non-delegating constructor, initialization proceeds in the
+// following order:
+// - First, and only for the constructor of the most derived class
+//   ([intro.object]), virtual base classes are initialized ...
+//
+// So in this example
+// struct Foo : AlwaysThrows, std::ostream {
+//    Foo() : AlwaysThrows{}, std::ostream{nullptr} {}
+// };
+//
+// Here
+// - the ios_base object is constructed
+// - the AlwaysThrows object is constructed and throws an exception
+// - the AlwaysThrows object is destrodyed
+// - the ios_base object is destroyed
+//
+// The ios_base object is destroyed before it has been initialized and runs
+// into undefined behavior. By using __loc_ as a sentinel we can avoid
+// accessing uninitialized memory in the destructor.
+
+#include <ostream>
+
+struct AlwaysThrows {
+  AlwaysThrows() { throw 1; }
+};
+
+struct Foo : AlwaysThrows, std::ostream {
+  Foo() : AlwaysThrows(), std::ostream(nullptr) {}
+};
+
+int main(int, char**) {
+  try {
+    Foo foo;
+  } catch (...) {
+  };
+  return 0;
+}