From: Paul Keir <paul.keir@uws.ac.uk>
Date: Mon, 12 Feb 2024 18:15:49 +0000 (+0000)
Subject: libstdc++: Fix constexpr basic_string union member [PR113294]
X-Git-Tag: basepoints/gcc-15~1196
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=065dddc6e07a917c57c7955db13b1fe77abbcabc;p=thirdparty%2Fgcc.git

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>
---

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 43efc99bea9d..8a695a494efd 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -909,7 +909,7 @@ _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 0e28a6d44872..d7022af3cabe 100644
--- a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/constexpr.cc
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/constexpr.cc
@@ -50,3 +50,17 @@ 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() );