https://gcc.gnu.org/PR87822

From 581b5447f18f4758a55b1fda4f8bf597e9466d40 Mon Sep 17 00:00:00 2001
From: redi <redi@138bc75d-0d04-0410-961f-82ee72b054a4>
Date: Wed, 31 Oct 2018 12:29:02 +0000
Subject: [PATCH] PR libstdc++/87822 fix layout change for nested std::pair

The introduction of the empty __pair_base base class for PR 86751
changed the layout of std::pair<std::pair<...>, ...>. The outer pair and
its first member both have a base class of the same type, which cannot
exist at the same address. This causes the first member to be at a
non-zero offset.

The solution is to make the base class depend on the template
parameters, so that each pair type has a different base class type,
which allows the base classes of the outer pair and its first member to
have the same address.

	PR libstdc++/87822
	* include/bits/stl_pair.h (__pair_base): Change to class template.
	(pair): Make base class type depend on template parameters.
	* testsuite/20_util/pair/87822.cc: New test.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@265678 138bc75d-0d04-0410-961f-82ee72b054a4
---
 libstdc++-v3/include/bits/stl_pair.h         |  4 +-

--- a/libstdc++-v3/include/bits/stl_pair.h
+++ b/libstdc++-v3/include/bits/stl_pair.h
@@ -187,7 +187,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   };
 #endif // C++11
 
-  class __pair_base
+  template<typename _U1, typename _U2> class __pair_base
   {
 #if __cplusplus >= 201103L
     template<typename _T1, typename _T2> friend struct pair;
@@ -206,7 +206,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    */
   template<typename _T1, typename _T2>
     struct pair
-    : private __pair_base
+    : private __pair_base<_T1, _T2>
     {
       typedef _T1 first_type;    /// @c first_type is the first bound type
       typedef _T2 second_type;   /// @c second_type is the second bound type