[gcc r12-10231] libstdc++: Ensure active union member is correctly set

[gcc r12-10231] libstdc++: Ensure active union member is correctly set

[Jonathan Wakely]

https://gcc.gnu.org/g:bfe05fbb30594b9f1fdfba32f3fc7b317e5158e0

commit r12-10231-gbfe05fbb30594b9f1fdfba32f3fc7b317e5158e0
Author: Nathaniel Shead <nathanieloshead@gmail.com>
Date:   Fri Sep 29 10:30:41 2023 +0100

    libstdc++: Ensure active union member is correctly set
    
    This patch ensures that the union members for std::string and
    std::variant are always properly set when a change occurs.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/basic_string.h: (basic_string(basic_string&&)):
            Activate _M_local_buf when needed.
            (basic_string(basic_string&&, const _Alloc&)): Likewise.
            * include/bits/basic_string.tcc: (basic_string::swap): Likewise.
            * include/std/variant: (__detail::__variant::__construct_n): New.
            (__detail::__variant::__emplace): Use __construct_n.
    
    Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
    (cherry picked from commit 28adad7a32ed92a3c0906b38411c4ed686bbacc5)

Diff:
---
 libstdc++-v3/include/bits/basic_string.h   |  6 ++++--
 libstdc++-v3/include/bits/basic_string.tcc |  4 ++++
 libstdc++-v3/include/std/variant           | 32 ++++++++++++++++++++++++++++--
 3 files changed, 38 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 9a6fe883b6c..3f38f20dd18 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -680,6 +680,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       {
 	if (__str._M_is_local())
 	  {
+	    (void)_M_use_local_data();
 	    traits_type::copy(_M_local_buf, __str._M_local_buf,
 			      __str.length() + 1);
 	  }
@@ -693,7 +694,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	// basic_stringbuf relies on writing into unallocated capacity so
 	// we mess up the contents if we put a '\0' in the string.
 	_M_length(__str.length());
-	__str._M_data(__str._M_local_data());
+	__str._M_data(__str._M_use_local_data());
 	__str._M_set_length(0);
       }
 
@@ -719,6 +720,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       {
 	if (__str._M_is_local())
 	  {
+	    (void)_M_use_local_data();
 	    traits_type::copy(_M_local_buf, __str._M_local_buf,
 			      __str.length() + 1);
 	    _M_length(__str.length());
@@ -730,7 +732,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	    _M_data(__str._M_data());
 	    _M_length(__str.length());
 	    _M_capacity(__str._M_allocated_capacity);
-	    __str._M_data(__str._M_local_buf);
+	    __str._M_data(__str._M_use_local_data());
 	    __str._M_set_length(0);
 	  }
 	else
diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index 48fa28e6466..24465dab102 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -79,6 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      }
 	    else if (__s.length())
 	      {
+		(void)_M_use_local_data();
 		traits_type::copy(_M_local_buf, __s._M_local_buf,
 				  __s.length() + 1);
 		_M_length(__s.length());
@@ -87,6 +88,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      }
 	    else if (length())
 	      {
+		(void)__s._M_use_local_data();
 		traits_type::copy(__s._M_local_buf, _M_local_buf,
 				  length() + 1);
 		__s._M_length(length());
@@ -97,6 +99,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	else
 	  {
 	    const size_type __tmp_capacity = __s._M_allocated_capacity;
+	    (void)__s._M_use_local_data();
 	    traits_type::copy(__s._M_local_buf, _M_local_buf,
 			      length() + 1);
 	    _M_data(__s._M_data());
@@ -108,6 +111,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  const size_type __tmp_capacity = _M_allocated_capacity;
 	  if (__s._M_is_local())
 	    {
+	      (void)_M_use_local_data();
 	      traits_type::copy(_M_local_buf, __s._M_local_buf,
 				__s.length() + 1);
 	      __s._M_data(_M_data());
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index 2e784c744d3..64145da12bb 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -321,6 +321,33 @@ namespace __variant
     __get(_Variant&& __v) noexcept
     { return __variant::__get_n<_Np>(std::forward<_Variant>(__v)._M_u); }
 
+  // Gets the _Uninitialized to construct into for __u.
+  template<size_t _Np, typename _Union>
+    constexpr decltype(auto)
+    __construct_n(_Union& __u) noexcept
+    {
+      if constexpr (_Np == 0)
+	return &__u._M_first;
+      else if constexpr (_Np == 1)
+	{
+	  std::_Construct(&__u._M_rest);
+	  return &__u._M_rest._M_first;
+	}
+      else if constexpr (_Np == 2)
+	{
+	  std::_Construct(&__u._M_rest);
+	  std::_Construct(&__u._M_rest._M_rest);
+	  return &__u._M_rest._M_rest._M_first;
+	}
+      else
+	{
+	  std::_Construct(&__u._M_rest);
+	  std::_Construct(&__u._M_rest._M_rest);
+	  std::_Construct(&__u._M_rest._M_rest._M_rest);
+	  return __variant::__construct_n<_Np - 3>(__u._M_rest._M_rest._M_rest);
+	}
+    }
+
   template<typename... _Types>
     struct _Traits
     {
@@ -537,8 +564,9 @@ namespace __variant
     __emplace(_Variant_storage<_Triv, _Types...>& __v, _Args&&... __args)
     {
       __v._M_reset();
-      auto* __addr = std::__addressof(__variant::__get_n<_Np>(__v._M_u));
-      std::_Construct(__addr, std::forward<_Args>(__args)...);
+      auto* __addr = __variant::__construct_n<_Np>(__v._M_u);
+      std::_Construct(__addr, in_place_index<0>,
+		      std::forward<_Args>(__args)...);
       // Construction didn't throw, so can set the new index now:
       __v._M_index = _Np;
     }