libstdc++: Avoid aliasing violation in std::valarray [PR99117]

The call to __valarray_copy constructs an _Array object to refer to
this->_M_data but that means that accesses to this->_M_data are through
a restrict-qualified pointer. This leads to undefined behaviour when
copying from an _Expr object that actually aliases this->_M_data.

Replace the call to __valarray_copy with a plain loop. I think this
removes the only use of that overload of __valarray_copy, so it could
probably be removed. I haven't done that here.

libstdc++-v3/ChangeLog:

	PR libstdc++/99117
	* include/std/valarray (valarray::operator=(const _Expr&)):
	Use loop to copy instead of __valarray_copy with _Array.
	* testsuite/26_numerics/valarray/99117.cc: New test.

2 files changed, 24 insertions, 1 deletions

diff --git a/libstdc++-v3/include/std/valarray b/libstdc++-v3/include/std/valarray
index a4eecd8..46cd57e 100644
--- a/libstdc++-v3/include/std/valarray
+++ b/libstdc++-v3/include/std/valarray
@@ -840,7 +840,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       // _GLIBCXX_RESOLVE_LIB_DEFECTS
       // 630. arrays of valarray.
       if (_M_size == __e.size())
-	std::__valarray_copy(__e, _M_size, _Array<_Tp>(_M_data));
+	{
+	  // Copy manually instead of using __valarray_copy, because __e might
+	  // alias _M_data and the _Array param type of __valarray_copy uses
+	  // restrict which doesn't allow aliasing.
+	  for (size_t __i = 0; __i < _M_size; ++__i)
+	    _M_data[__i] = __e[__i];
+	}
       else
 	{
 	  if (_M_data)
diff --git a/libstdc++-v3/testsuite/26_numerics/valarray/99117.cc b/libstdc++-v3/testsuite/26_numerics/valarray/99117.cc
new file mode 100644
index 0000000..81621bd
--- /dev/null
+++ b/libstdc++-v3/testsuite/26_numerics/valarray/99117.cc
@@ -0,0 +1,17 @@
+// { dg-do run { target c++11 } }
+
+// PR libstdc++/99117 cannot accumulate std::valarray
+
+#include <valarray>
+#include <vector>
+#include <testsuite_hooks.h>
+
+int main()
+{
+    std::vector<std::valarray<int>> v = {{1,1}, {2,2}};
+    std::valarray<int> sum(2);
+    for (const auto& e : v)
+      sum = sum + e;
+    VERIFY(sum[0]==3);
+    VERIFY(sum[1]==3);
+}