P0646R1 for Debug mode

[François Dumont <frs dot dumont at gmail dot com>]

This patch implements P0646R1 for Debug mode containers.

It fixes tests:

23_containers/forward_list/operations/remove_cxx20_return.cc
23_containers/forward_list/operations/unique_cxx20_return.cc
23_containers/list/operations/remove_cxx20_return.cc
23_containers/list/operations/unique_cxx20_return.cc

Note that it also adopt the Lib Defects 526 which had been implemented 
only for the normal mode.

I'll commit this tomorrow if not told otherwise.

    P0646R1 Improving the Return Value of Erase-Like Algorithms I
    * include/debug/forward_list (forward_list::__remove_return_type):
    Define typedef as size_type or void, according to __cplusplus value.
    (_GLIBCXX_FWDLIST_REMOVE_RETURN_TYPE_TAG): Define macro as abi-tag or
    empty, according to __cplusplus value.
    (_GLIBCXX20_ONLY): Define macro.
    (forward_list::remove, forward_list::unique): Use typedef and macro
    to change return type and add abi-tag for C++2a. Return number of
    removed elements for C++2a.
    (forward_list::remove_if<Pred>, forward_list::unique<BinPred>): Use
    typedef to change return type for C++2a. Return number of removed
    elements for C++2a.
    * include/debug/list (list::__remove_return_type): Define typedef as
    size_type or void, according to __cplusplus value.
    (_GLIBCXX_LIST_REMOVE_RETURN_TYPE_TAG): Define macro as abi-tag or
    empty, according to __cplusplus value.
    (_GLIBCXX20_ONLY): Define macro.
    (list::remove, list::unique): Use typedef and macro to change return
    type and add abi-tag for C++2a. Return number of removed elements for
    C++2a.
    (list::remove_if<Predicate>, list::unique<BinaryPredicate>): Use 
typedef
    to change return type for C++2a. Return number of removed elements for
    C++2a.

François

diff --git a/libstdc++-v3/include/debug/forward_list b/libstdc++-v3/include/debug/forward_list
index e5ac09e8..2340962 100644
--- a/libstdc++-v3/include/debug/forward_list
+++ b/libstdc++-v3/include/debug/forward_list
@@ -656,70 +656,114 @@ namespace __debug
 		   const_iterator __before, const_iterator __last)
       { splice_after(__pos, std::move(__list), __before, __last); }
 
-      void
+    private:
+#if __cplusplus > 201703L
+# define __cpp_lib_list_remove_return_type 201806L
+      using __remove_return_type = size_type;
+# define _GLIBCXX_FWDLIST_REMOVE_RETURN_TYPE_TAG \
+      __attribute__((__abi_tag__("__cxx20")))
+# define _GLIBCXX20_ONLY(__expr) __expr
+#else
+      using __remove_return_type = void;
+# define _GLIBCXX_FWDLIST_REMOVE_RETURN_TYPE_TAG
+# define _GLIBCXX20_ONLY(__expr)
+#endif
+
+    public:
+      _GLIBCXX_FWDLIST_REMOVE_RETURN_TYPE_TAG
+      __remove_return_type
       remove(const _Tp& __val)
       {
+	if (!this->_M_iterators && !this->_M_const_iterators)
+	  return _Base::remove(__val);
+
+	size_type __removed __attribute__((__unused__)) = 0;
 	_Base_iterator __x = _Base::before_begin();
 	_Base_iterator __old = __x++;
+	_Base_iterator __extra = _Base::end();
 	while (__x != _Base::end())
 	  {
 	    if (*__x == __val)
-	      __x = _M_erase_after(__old);
-	    else
-	      __old = __x++;
+	      {
+		if (std::__addressof(*__x) != std::__addressof(__val))
+		  {
+		    __x = _M_erase_after(__old);
+		    _GLIBCXX20_ONLY( __removed++ );
+		    continue;
+		  }
+		else
+		  __extra = __old;
+	      }
+	    __old = __x++;
 	  }
+
+	if (__extra != _Base::end())
+	  {
+	    this->_M_erase_after(__extra);
+	    _GLIBCXX20_ONLY( __removed++ );
+	  }
+
+	return _GLIBCXX20_ONLY( __removed );
       }
 
       template<typename _Pred>
-	void
+	__remove_return_type
 	remove_if(_Pred __pred)
 	{
+	  if (!this->_M_iterators && !this->_M_const_iterators)
+	    return _Base::remove_if(__pred);
+
+	  size_type __removed __attribute__((__unused__)) = 0;
 	  _Base_iterator __x = _Base::before_begin();
 	  _Base_iterator __old = __x++;
 	  while (__x != _Base::end())
-	    {
-	      if (__pred(*__x))
+	    if (__pred(*__x))
+	      {
 		__x = _M_erase_after(__old);
-	      else
-		__old = __x++;
-	    }
+		_GLIBCXX20_ONLY( __removed++ );
+	      }
+	    else
+	      __old = __x++;
+
+	  return _GLIBCXX20_ONLY( __removed );
 	}
 
-      void
+      _GLIBCXX_FWDLIST_REMOVE_RETURN_TYPE_TAG
+      __remove_return_type
       unique()
-      {
-	_Base_iterator __first = _Base::begin();
-	_Base_iterator __last = _Base::end();
-	if (__first == __last)
-	  return;
-	_Base_iterator __next = std::next(__first);
-	while (__next != __last)
-	  {
-	    if (*__first == *__next)
-	      __next = _M_erase_after(__first);
-	    else
-	      __first = __next++;
-	  }
-      }
+      { return unique(std::equal_to<_Tp>()); }
 
       template<typename _BinPred>
-	void
+	__remove_return_type
 	unique(_BinPred __binary_pred)
 	{
+	  if (!this->_M_iterators && !this->_M_const_iterators)
+	    return _Base::unique(__binary_pred);
+
 	  _Base_iterator __first = _Base::begin();
 	  _Base_iterator __last = _Base::end();
 	  if (__first == __last)
-	    return;
+	    return _GLIBCXX20_ONLY(0);
+
+	  size_type __removed __attribute__((__unused__)) = 0;
 	  _Base_iterator __next = std::next(__first);
 	  while (__next != __last)
 	    {
 	      if (__binary_pred(*__first, *__next))
-		__next = _M_erase_after(__first);
+		{
+		  __next = _M_erase_after(__first);
+		  _GLIBCXX20_ONLY( __removed++ );
+		}
 	      else
 		__first = __next++;
 	    }
+
+	  return _GLIBCXX20_ONLY( __removed );
 	}
 
+#undef _GLIBCXX_FWDLIST_REMOVE_RETURN_TYPE_TAG
+#undef _GLIBCXX20_ONLY
+
       void
       merge(forward_list&& __list)
       {
diff --git a/libstdc++-v3/include/debug/list b/libstdc++-v3/include/debug/list
index c6aea5d..5b3b88e 100644
--- a/libstdc++-v3/include/debug/list
+++ b/libstdc++-v3/include/debug/list
@@ -655,66 +655,135 @@ namespace __debug
       { splice(__position, std::move(__x), __first, __last); }
 #endif
 
-      void
+    private:
+#if __cplusplus > 201703L
+      typedef size_type __remove_return_type;
+# define _GLIBCXX_LIST_REMOVE_RETURN_TYPE_TAG \
+      __attribute__((__abi_tag__("__cxx20")))
+# define _GLIBCXX20_ONLY(__expr) __expr
+#else
+      typedef void __remove_return_type;
+# define _GLIBCXX_LIST_REMOVE_RETURN_TYPE_TAG
+# define _GLIBCXX20_ONLY(__expr)
+#endif
+
+    public:
+      _GLIBCXX_LIST_REMOVE_RETURN_TYPE_TAG
+      __remove_return_type
       remove(const _Tp& __value)
       {
-	for (_Base_iterator __x = _Base::begin(); __x != _Base::end(); )
+	if (!this->_M_iterators && !this->_M_const_iterators)
+	  return _Base::remove(__value);
+
+	size_type __removed __attribute__((__unused__)) = 0;
+	_Base_iterator __first = _Base::begin();
+	_Base_iterator __last = _Base::end();
+	_Base_iterator __extra = __last;
+	while (__first != __last)
 	  {
-	    if (*__x == __value)
-	      __x = _M_erase(__x);
+	    if (*__first == __value)
+	      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+	      // 526. Is it undefined if a function in the standard changes
+	      // in parameters?
+	      if (std::__addressof(*__first) != std::__addressof(__value))
+		{
+		  __first = _M_erase(__first);
+		  _GLIBCXX20_ONLY( __removed++ );
+		}
+	      else
+		{
+		  __extra = __first;
+		  ++__first;
+		}
 	    else
-	      ++__x;
+	      ++__first;
 	  }
+
+	if (__extra != __last)
+	  {
+	    _M_erase(__extra);
+	    _GLIBCXX20_ONLY( __removed++ );
+	  }
+	return _GLIBCXX20_ONLY( __removed );
       }
 
       template<class _Predicate>
-	void
+	__remove_return_type
 	remove_if(_Predicate __pred)
 	{
+	  if (!this->_M_iterators && !this->_M_const_iterators)
+	    return _Base::remove_if(__pred);
+
+	  size_type __removed __attribute__((__unused__)) = 0;
 	  for (_Base_iterator __x = _Base::begin(); __x != _Base::end(); )
-	    {
-	      if (__pred(*__x))
+	    if (__pred(*__x))
+	      {
 		__x = _M_erase(__x);
-	      else
-		++__x;
-	    }
+		_GLIBCXX20_ONLY( __removed++ );
+	      }
+	    else
+	      ++__x;
+
+	  return _GLIBCXX20_ONLY( __removed );
 	}
 
-      void
+      _GLIBCXX_LIST_REMOVE_RETURN_TYPE_TAG
+      __remove_return_type
       unique()
       {
+	if (!this->_M_iterators && !this->_M_const_iterators)
+	  return _Base::unique();
+
+	if (empty())
+	  return _GLIBCXX20_ONLY(0);
+
+        size_type __removed __attribute__((__unused__)) = 0;
 	_Base_iterator __first = _Base::begin();
 	_Base_iterator __last = _Base::end();
-	if (__first == __last)
-	  return;
-	_Base_iterator __next = __first; ++__next;
-	while (__next != __last)
-	  {
-	    if (*__first == *__next)
-	      __next = _M_erase(__next);
-	    else
-	      __first = __next++;
-	  }
+	_Base_iterator __next = __first;
+	while (++__next != __last)
+	  if (*__first == *__next)
+	    {
+	      _M_erase(__next);
+	      __next = __first;
+	      _GLIBCXX20_ONLY( __removed++ );
+	    }
+	  else
+	    __first = __next;
+
+	return _GLIBCXX20_ONLY( __removed );
       }
 
       template<class _BinaryPredicate>
-	void
+	__remove_return_type
 	unique(_BinaryPredicate __binary_pred)
 	{
+	  if (!this->_M_iterators && !this->_M_const_iterators)
+	    return _Base::unique(__binary_pred);
+
+	  if (empty())
+	    return _GLIBCXX20_ONLY(0);
+
+	  size_type __removed __attribute__((__unused__)) = 0;
 	  _Base_iterator __first = _Base::begin();
 	  _Base_iterator __last = _Base::end();
-	  if (__first == __last)
-	    return;
-	  _Base_iterator __next = __first; ++__next;
-	  while (__next != __last)
-	    {
-	      if (__binary_pred(*__first, *__next))
-		__next = _M_erase(__next);
-	      else
-		__first = __next++;
-	    }
+	  _Base_iterator __next = __first;;
+	  while (++__next != __last)
+	    if (__binary_pred(*__first, *__next))
+	      {
+		_M_erase(__next);
+		__next = __first;
+		_GLIBCXX20_ONLY( __removed++ );
+	      }
+	    else
+	      __first = __next;
+
+	  return _GLIBCXX20_ONLY( __removed );
 	}
 
+#undef _GLIBCXX_LIST_REMOVE_RETURN_TYPE_TAG
+#undef _GLIBCXX20_ONLY
+
       void
 #if __cplusplus >= 201103L
       merge(list&& __x)