[libcxx-commits] [libcxx] [libc++] Improves UB handling in ios_base destructor. (PR #76525)

[Louis Dionne via libcxx-commits]

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@@ -0,0 +1,42 @@
+//===----------------------------------------------------------------------===//
+//
+// 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. See ios_base::ios_base() for the details.
+
+#include <ostream>
+
+struct AlwaysThrows {
+  AlwaysThrows() { throw 1; }
+};
+
+struct Foo : AlwaysThrows, std::ostream {
+  Foo() : AlwaysThrows(), std::ostream(nullptr) {}
+};
+
+int main() {
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ldionne wrote:

```suggestion
int main(int, char**) {
```

https://github.com/llvm/llvm-project/pull/76525