[committed v2] libstdc++: Add debug assertions to std::list and std::forward_list

[Jonathan Wakely]

While working on fancy pointer support for the linked lists I noticed
they didn't have any debug assertions. This adds the obvious non-empty
assertions to front() and back().

libstdc++-v3/ChangeLog:

        * include/bits/forward_list.h (forward_list::front): Add
        non-empty assertions.
        * include/bits/stl_list.h (list::front, list::back): Add
        non-empty assertions.
---

This v2 patch only adds the uncontroversial nonempty assertions, so
doesn't change _M_erase to assert on pop_back/pop_front/erase on empty
containers. I still plan to add those in some form, but not now.

Tested x86_64-linux. Pushed to trunk.

 libstdc++-v3/include/bits/forward_list.h |  3 +++
 libstdc++-v3/include/bits/stl_list.h     | 13 +++++++++++--
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/bits/forward_list.h 
b/libstdc++-v3/include/bits/forward_list.h
index ac1b3593c79..f6d4e6bd3d2 100644
--- a/libstdc++-v3/include/bits/forward_list.h
+++ b/libstdc++-v3/include/bits/forward_list.h
@@ -42,6 +42,7 @@
 #include <bits/allocator.h>
 #include <ext/alloc_traits.h>
 #include <ext/aligned_buffer.h>
+#include <debug/assertions.h>
 #if __glibcxx_ranges_to_container // C++ >= 23
 # include <bits/ranges_base.h> // ranges::begin, ranges::distance etc.
 # include <bits/ranges_util.h> // ranges::subrange
@@ -950,6 +951,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       reference
       front()
       {
+       __glibcxx_requires_nonempty();
        _Node* __front = static_cast<_Node*>(this->_M_impl._M_head._M_next);
        return *__front->_M_valptr();
       }
@@ -962,6 +964,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       const_reference
       front() const
       {
+       __glibcxx_requires_nonempty();
        _Node* __front = static_cast<_Node*>(this->_M_impl._M_head._M_next);
        return *__front->_M_valptr();
       }
diff --git a/libstdc++-v3/include/bits/stl_list.h 
b/libstdc++-v3/include/bits/stl_list.h
index 3f92de42996..df7f388ede5 100644
--- a/libstdc++-v3/include/bits/stl_list.h
+++ b/libstdc++-v3/include/bits/stl_list.h
@@ -59,6 +59,7 @@
 
 #include <bits/concept_check.h>
 #include <ext/alloc_traits.h>
+#include <debug/assertions.h>
 #if __cplusplus >= 201103L
 #include <initializer_list>
 #include <bits/allocated_ptr.h>
@@ -1271,7 +1272,10 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       _GLIBCXX_NODISCARD
       reference
       front() _GLIBCXX_NOEXCEPT
-      { return *begin(); }
+      {
+       __glibcxx_requires_nonempty();
+       return *begin();
+      }
 
       /**
        *  Returns a read-only (constant) reference to the data at the first
@@ -1280,7 +1284,10 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       _GLIBCXX_NODISCARD
       const_reference
       front() const _GLIBCXX_NOEXCEPT
-      { return *begin(); }
+      {
+       __glibcxx_requires_nonempty();
+       return *begin();
+      }
 
       /**
        *  Returns a read/write reference to the data at the last element
@@ -1290,6 +1297,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       reference
       back() _GLIBCXX_NOEXCEPT
       {
+       __glibcxx_requires_nonempty();
        iterator __tmp = end();
        --__tmp;
        return *__tmp;
@@ -1303,6 +1311,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       const_reference
       back() const _GLIBCXX_NOEXCEPT
       {
+       __glibcxx_requires_nonempty();
        const_iterator __tmp = end();
        --__tmp;
        return *__tmp;
-- 
2.47.0