[pushed] c++: adjust testcase to reveal failure [PR107919]

[Jason Merrill]

Tested x86_64-pc-linux-gnu, applying to trunk.

-- 8< --

This test appeared to be passing, but only because the warning was
suppressed by #pragma system_header.

        PR tree-optimization/107919

gcc/testsuite/ChangeLog:

        * g++.dg/warn/Wuninitialized-pr107919-1.C: Add -Wsystem-headers and
        xfail.
---
 gcc/testsuite/g++.dg/warn/Wuninitialized-pr107919-1.C | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/gcc/testsuite/g++.dg/warn/Wuninitialized-pr107919-1.C 
b/gcc/testsuite/g++.dg/warn/Wuninitialized-pr107919-1.C
index 067a44a462e..049fa4d307a 100644
--- a/gcc/testsuite/g++.dg/warn/Wuninitialized-pr107919-1.C
+++ b/gcc/testsuite/g++.dg/warn/Wuninitialized-pr107919-1.C
@@ -1,6 +1,6 @@
 // { dg-do compile }
 // { dg-require-effective-target c++17 }
-// { dg-options "-O2 -Wuninitialized" }
+// { dg-options "-O2 -Wuninitialized -Wsystem-headers" }
 
 #include <memory>
 #include <variant>
@@ -13,3 +13,5 @@ void do_something(void* storage)
   auto& swappedValue = *reinterpret_cast<Event*>(storage);
   std::swap(event, swappedValue);
 }
+
+// { dg-bogus "may be used uninitialized" "" { xfail *-*-* } 0 }

base-commit: e4b64bea337d9ac936c555154f9d60c4876b65d3
-- 
2.46.0