libstdc++: check length in string append [PR103534]

In the testcase for 103534 we get a warning about append leading to memcpy
of a very large number of bytes overflowing the buffer.  This turns out to
be because we weren't calling _M_check_length for string append.  Rather
than do that directly, let's go through the public pointer append that calls
it.

	PR c++/103534

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (append (basic_string)): Call pointer
	append instead of _M_append directly.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wstringop-overflow-8.C: New test.

1 files changed, 10 insertions, 0 deletions

diff --git a/gcc/testsuite/g++.dg/warn/Wstringop-overflow-8.C b/gcc/testsuite/g++.dg/warn/Wstringop-overflow-8.C
new file mode 100644
index 0000000..d0ef5e7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wstringop-overflow-8.C
@@ -0,0 +1,10 @@
+// PR c++/103534
+// { dg-additional-options "-O -Wall" }
+
+#include <string>
+
+std::string foo(std::string x)
+{
+  // This used to get a bogus -Wstringop-overflow warning.
+  return std::string("1234567890123456") + x;
+}