libstdc++: Fix constexpr basic_string union member [PR113294]

A call to `basic_string::clear()` in the std::string move assignment
operator leads to a constexpr error from an access of inactive union
member `_M_local_buf` in the added test (`test_move()`). Changing
`__str._M_local_buf` to `__str._M_use_local_data()` in
`operator=(basic_string&& __str)` fixes this.

	PR libstdc++/113294

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (basic_string::operator=): Use
	_M_use_local_data() instead of _M_local_buf on the moved-from
	string.
	* testsuite/21_strings/basic_string/modifiers/constexpr.cc
	(test_move): New test.

Signed-off-by: Paul Keir <paul.keir@uws.ac.uk>
Reviewed-by: Patrick Palka <ppalka@redhat.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
(cherry picked from commit 065dddc6e07a917c57c7955db13b1fe77abbcabc)

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index b73991d96da1dfb39235ae07b4d09f77bb7826d3..36598803f00c9062b3add9571ead56678a9cf1af 100644 (file)
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -904,7 +904,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
                __str._M_capacity(__capacity);
              }
            else
-             __str._M_data(__str._M_local_buf);
+             __str._M_data(__str._M_use_local_data());
          }
        else // Need to do a deep copy
          assign(__str);

diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/constexpr.cc b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/constexpr.cc
index 77fe59bb785860fdea3afbc7b50857fb78dd81f6..2c12fdf42409492ada1a99fad29bdc85882ffc58 100644 (file)
--- a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/constexpr.cc
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/constexpr.cc
@@ -51,3 +51,17 @@ test_erasure()
 }
 
 static_assert( test_erasure() );
+
+constexpr bool
+test_move()
+{
+  // PR libstdc++/113294
+  std::string s1;
+  std::string s2 = "1234567890123456"; // 16 chars: more than _S_local_capacity
+  s1 = std::move(s2);
+  VERIFY( s1 == "1234567890123456" );
+
+  return true;
+}
+
+static_assert( test_move() );