Fix test failure with old Copy-On-Write std::string

	* testsuite/27_io/filesystem/filesystem_error/copy.cc: Fix static
	assertion failures with old std::string ABI.

From-SVN: r267577

libstdc++-v3/ChangeLog

 2019-01-04  Jonathan Wakely  <jwakely@redhat.com>
 
+	* testsuite/27_io/filesystem/filesystem_error/copy.cc: Fix static
+	assertion failures with old std::string ABI.
+
 	* include/bits/fs_path.h (path::_List::erase): Replace both overloads
 	with ...
 	(path::pop_back(), path::_M_erase_from(const_iterator)): New member

libstdc++-v3/testsuite/27_io/filesystem/filesystem_error/copy.cc

@@ -24,9 +24,19 @@
 
 using std::filesystem::filesystem_error;
 
+// The COW std::string does not have noexcept copies, because copying a
+// "leaked" string can throw (and for fully-dynamic strings, copying the
+// empty rep can also throw).
+// That's OK, because we know that the std::string in the std::runtime_error
+// or std::logic_error base class won't be leaked (and usually won't be empty).
+// The is_nothrow_xxx type traits don't know that though, so we can only
+// check them for the cxx11 ABI, which uses __cow_string, which has noexcept
+// copies.
+#if _GLIBCXX_USE_CXX11_ABI
 // PR libstdc++/83306
 static_assert(std::is_nothrow_copy_constructible_v<filesystem_error>);
 static_assert(std::is_nothrow_copy_assignable_v<filesystem_error>);
+#endif
 
 void
 test01()