libstdc++: Give std::memory_order a fixed underlying type [PR89624]

Prior to C++20 this enum type doesn't have a fixed underlying type,
which means it can be modified by -fshort-enums, which then means the
HLE bits are outside the range of valid values for the type.

As it has a fixed type of int in C++20 and later, do the same for
earlier standards too. This is technically a change for C++17 down,
because the implicit underlying type (without -fshort-enums) was
unsigned before. I doubt it matters in practice. That incompatibility
already exists between C++17 and C++20 and nobody has noticed or
complained. Now at least the underlying type will be int for all -std
modes.

libstdc++-v3/ChangeLog:

	PR libstdc++/89624
	* include/bits/atomic_base.h (memory_order): Use int as
	underlying type.
	* testsuite/29_atomics/atomic/89624.cc: New test.

2 files changed, 11 insertions, 2 deletions

diff --git a/libstdc++-v3/include/bits/atomic_base.h b/libstdc++-v3/include/bits/atomic_base.h
index dd36030..062f154 100644
--- a/libstdc++-v3/include/bits/atomic_base.h
+++ b/libstdc++-v3/include/bits/atomic_base.h
@@ -78,7 +78,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   inline constexpr memory_order memory_order_acq_rel = memory_order::acq_rel;
   inline constexpr memory_order memory_order_seq_cst = memory_order::seq_cst;
 #else
-  typedef enum memory_order
+  enum memory_order : int
     {
       memory_order_relaxed,
       memory_order_consume,
@@ -86,7 +86,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       memory_order_release,
       memory_order_acq_rel,
       memory_order_seq_cst
-    } memory_order;
+    };
 #endif
 
   /// @cond undocumented
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic/89624.cc b/libstdc++-v3/testsuite/29_atomics/atomic/89624.cc
new file mode 100644
index 0000000..480f7c6
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic/89624.cc
@@ -0,0 +1,9 @@
+// { dg-options "-fshort-enums" }
+// { dg-do compile { target c++11 } }
+
+// Bug 89624 HLE bits don't work with -fshort-enums or -fstrict-enums
+
+#include <atomic>
+
+static_assert((std::memory_order_acquire | std::__memory_order_hle_acquire)
+    != std::memory_order_acquire, "HLE acquire sets a bit");