[committed] libstdc++: Remove reliance on unspecified behaviour in std::rethrow_if_nested test

[Jonathan Wakely <jwakely@redhat.com>]

This is the patch resolving the non-portable test that Iain raised in:
https://gcc.gnu.org/pipermail/libstdc++/2023-August/056534.html

Tested x86_64-linux. Pushed to trunk.

Backports would be OK, but I don't think they are needed.

-- >8 --

This test case calls std::set_terminate while there is an active
exception. Since LWG 2111 it is unspecified which terminate handler is
used when std::nested_exception::rethrow_nested() calls std::terminate.
With libsupc++ the global handler changed by std::set_terminate is used,
but libc++abi uses the active exception's handler (the one that was
current when the exception was first thrown).

Adjust the test case so that it works with either implementation choice.
So that the process doesn't exit cleanly if std::terminate happens
sooner than expected, use a global variable to control when the "clean
terminate" behaviour happens.

libstdc++-v3/ChangeLog:

	* testsuite/18_support/nested_exception/rethrow_if_nested-term.cc:
	Call std::set_terminate before throwing the nested exception.
---
 .../nested_exception/rethrow_if_nested-term.cc     | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/libstdc++-v3/testsuite/18_support/nested_exception/rethrow_if_nested-term.cc b/libstdc++-v3/testsuite/18_support/nested_exception/rethrow_if_nested-term.cc
index 3bfc7ab9943..b221eea3178 100644
--- a/libstdc++-v3/testsuite/18_support/nested_exception/rethrow_if_nested-term.cc
+++ b/libstdc++-v3/testsuite/18_support/nested_exception/rethrow_if_nested-term.cc
@@ -4,25 +4,33 @@
 #include <exception>
 #include <cstdlib>
 
-[[noreturn]] void terminate_cleanly() noexcept { std::exit(0); }
+int exit_status = 1;
+[[noreturn]] void terminate_cleanly() noexcept { std::exit(exit_status); }
 
 struct A { virtual ~A() = default; };
 
 int main()
 {
+  std::set_terminate(terminate_cleanly);
   try
   {
     // At this point std::current_exception() == nullptr so the
     // std::nested_exception object is empty.
     std::throw_with_nested(A{});
+
+    // Should not reach this point.
+    std::abort();
   }
   catch (const A& a)
   {
-    std::set_terminate(terminate_cleanly);
+    // This means the expected std::terminate() call will exit cleanly,
+    // so this test will PASS.
+    exit_status = 0;
+
     std::rethrow_if_nested(a);
 #if __cpp_rtti
     // No nested exception, so trying to rethrow it calls std::terminate()
-    // which calls std::exit(0). Shoud not reach this point.
+    // which calls std::exit(0). Should not reach this point.
     std::abort();
 #else
     // Without RTTI we can't dynamic_cast<const std::nested_exception*>(&a)
-- 
2.41.0