Make filesystem::path safe for self assignment

The standard says "If *this and p are the same object, has no effect."
Previously we ended up clearing the path.

	* include/bits/fs_path.h (path::operator=(path&&)): Check for self
	assignment.
	* src/c++17/fs_path.cc (path::operator=(const path&)): Likewise.
	* testsuite/27_io/filesystem/path/assign/copy.cc: Test self
	assignment.

From-SVN: r270171

libstdc++-v3/ChangeLog

 2019-04-05  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/bits/fs_path.h (path::operator=(path&&)): Check for self
+	assignment.
+	* src/c++17/fs_path.cc (path::operator=(const path&)): Likewise.
+	* testsuite/27_io/filesystem/path/assign/copy.cc: Test self
+	assignment.
+
 	PR libstdc++/87431 (again)
 	* include/bits/basic_string.h (__variant::_Never_valueless_alt):
 	Define partial specialization for basic_string.

libstdc++-v3/include/bits/fs_path.h

@@ -877,6 +877,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
   inline path&
   path::operator=(path&& __p) noexcept
   {
+    if (&__p == this) [[__unlikely__]]
+      return *this;
+
     _M_pathname = std::move(__p._M_pathname);
     _M_cmpts = std::move(__p._M_cmpts);
     __p.clear();

libstdc++-v3/src/c++17/fs_path.cc

@@ -444,6 +444,9 @@ path::_List::reserve(int newcap, bool exact = false)
 path&
 path::operator=(const path& p)
 {
+  if (&p == this) [[__unlikely__]]
+    return *this;
+
   _M_pathname.reserve(p._M_pathname.length());
   _M_cmpts = p._M_cmpts;	// might throw
   _M_pathname = p._M_pathname;	// won't throw because we reserved enough space

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

@@ -20,6 +20,7 @@
 
 #include <filesystem>
 #include <testsuite_fs.h>
+#include <testsuite_hooks.h>
 
 using std::filesystem::path;
 using __gnu_test::compare_paths;
@@ -47,9 +48,26 @@ test02()
   }
 }
 
+void
+test03()
+{
+  // self assignment should have no effect
+  const path orig = "foo/bar/baz";
+  path p = orig;
+  const auto ptr1 = p.c_str();
+  const auto ptr2 = p.begin()->c_str();
+  p = std::move(p);
+  __gnu_test::compare_paths(p, orig);
+  p = p;
+  __gnu_test::compare_paths(p, orig);
+  VERIFY( ptr1 == p.c_str() );
+  VERIFY( ptr2 == p.begin()->c_str() );
+}
+
 int
 main()
 {
   test01();
   test02();
+  test03();
 }