Enforce LWG DR 2566 requirement for container adaptors

Although there is no good use for stack<int, deque<double>> or similar
types with a mismatched value_type, it's possible somebody is doing that
and getting away with it currently. This patch only enforces the new
requirement for C++17 and later. During stage 1 we should consider
enforcing it for C++11 and C++14.

	* doc/xml/manual/intro.xml: Document LWG 2566 status.
	* include/bits/stl_queue.h (queue, priority_queue): Add static
	assertions to enforce LWG 2566 requirement on value_type.
	* include/bits/stl_stack.h (stack): Likewise.

From-SVN: r268877

libstdc++-v3/ChangeLog

 2019-02-14  Jonathan Wakely  <jwakely@redhat.com>
 
+	* doc/xml/manual/intro.xml: Document LWG 2566 status.
+	* include/bits/stl_queue.h (queue, priority_queue): Add static
+	assertions to enforce LWG 2566 requirement on value_type.
+	* include/bits/stl_stack.h (stack): Likewise.
+
 	PR middle-end/89303
 	* testsuite/20_util/enable_shared_from_this/89303.cc: New test.
 

libstdc++-v3/doc/xml/manual/intro.xml

@@ -1120,11 +1120,18 @@ requirements of the license of GCC.
     ill-formed.
     </para></listitem></varlistentry>
 
+    <varlistentry xml:id="manual.bugs.dr2537"><term><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="&DR;#2537">2537</link>:
+       <emphasis>Requirements on the first template parameter of container adaptors
+       </emphasis>
+    </term>
+    <listitem><para>Add static assertions to enforce the requirement.
+    </para></listitem></varlistentry>
+
     <varlistentry xml:id="manual.bugs.dr2583"><term><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="&DR;#2583">2583</link>:
        <emphasis>There is no way to supply an allocator for <code>basic_string(str, pos)</code>
        </emphasis>
     </term>
-    <listitem><para>Add new constructor
+    <listitem><para>Add new constructor.
     </para></listitem></varlistentry>
 
     <varlistentry xml:id="manual.bugs.dr2684"><term><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="&DR;#2684">2684</link>:

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

@@ -118,7 +118,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       template<typename _Alloc>
 	using _Uses = typename
 	  enable_if<uses_allocator<_Sequence, _Alloc>::value>::type;
-#endif
+
+#if __cplusplus >= 201703L
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 2566. Requirements on the first template parameter of container
+      // adaptors
+      static_assert(is_same<_Tp, typename _Sequence::value_type>::value,
+	  "value_type must be the same as the underlying container");
+#endif // C++17
+#endif // C++11
 
     public:
       typedef typename	_Sequence::value_type		value_type;
@@ -451,17 +459,25 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       template<typename _Alloc>
 	using _Uses = typename
 	  enable_if<uses_allocator<_Sequence, _Alloc>::value>::type;
-#endif
+
+#if __cplusplus >= 201703L
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 2566. Requirements on the first template parameter of container
+      // adaptors
+      static_assert(is_same<_Tp, typename _Sequence::value_type>::value,
+	  "value_type must be the same as the underlying container");
+#endif // C++17
+#endif // C++11
 
     public:
       typedef typename	_Sequence::value_type		value_type;
-      typedef typename	_Sequence::reference		 reference;
-      typedef typename	_Sequence::const_reference	   const_reference;
-      typedef typename	_Sequence::size_type		 size_type;
-      typedef		_Sequence			    container_type;
+      typedef typename	_Sequence::reference		reference;
+      typedef typename	_Sequence::const_reference	const_reference;
+      typedef typename	_Sequence::size_type		size_type;
+      typedef		_Sequence			container_type;
       // _GLIBCXX_RESOLVE_LIB_DEFECTS
       // DR 2684. priority_queue lacking comparator typedef
-      typedef	       _Compare				    value_compare;
+      typedef	       _Compare				value_compare;
 
     protected:
       //  See queue::c for notes on these names.

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

@@ -120,7 +120,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       template<typename _Alloc>
 	using _Uses = typename
 	  enable_if<uses_allocator<_Sequence, _Alloc>::value>::type;
-#endif
+
+#if __cplusplus >= 201703L
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 2566. Requirements on the first template parameter of container
+      // adaptors
+      static_assert(is_same<_Tp, typename _Sequence::value_type>::value,
+	  "value_type must be the same as the underlying container");
+#endif // C++17
+#endif // C++11
 
     public:
       typedef typename _Sequence::value_type		value_type;