libstdc++: Add hint to compiler about vector invariants [PR106434]

The PR shows a bogus warning where jump threading generates code for the
undefined case that the insertion point is a value-initialized iterator
but _M_finish and _M_end_of_storage are unequal (so at least one must be
non-null). Using __builtin_unreachable() removes the bogus warning. Also
add an assertion to diagnose undefined misuses of a null iterator here,
so we don't just silently optimize that undefined code to something
unsafe.

libstdc++-v3/ChangeLog:

	PR c++/106434
	* include/bits/vector.tcc (insert(const_iterator, const T&)):
	Add assertion and optimization hint that the iterator for the
	insertion point must be non-null.

1 files changed, 23 insertions, 17 deletions

diff --git a/libstdc++-v3/include/bits/vector.tcc b/libstdc++-v3/include/bits/vector.tcc
index 27ef1a4..8ae79ff 100644
--- a/libstdc++-v3/include/bits/vector.tcc
+++ b/libstdc++-v3/include/bits/vector.tcc
@@ -139,26 +139,32 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
     {
       const size_type __n = __position - begin();
       if (this->_M_impl._M_finish != this->_M_impl._M_end_of_storage)
-	if (__position == end())
-	  {
-	    _GLIBCXX_ASAN_ANNOTATE_GROW(1);
-	    _Alloc_traits::construct(this->_M_impl, this->_M_impl._M_finish,
-				     __x);
-	    ++this->_M_impl._M_finish;
-	    _GLIBCXX_ASAN_ANNOTATE_GREW(1);
-	  }
-	else
-	  {
+	{
+	  __glibcxx_assert(__position != const_iterator());
+	  if (!(__position != const_iterator()))
+	    __builtin_unreachable(); // PR 106434
+
+	  if (__position == end())
+	    {
+	      _GLIBCXX_ASAN_ANNOTATE_GROW(1);
+	      _Alloc_traits::construct(this->_M_impl, this->_M_impl._M_finish,
+				       __x);
+	      ++this->_M_impl._M_finish;
+	      _GLIBCXX_ASAN_ANNOTATE_GREW(1);
+	    }
+	  else
+	    {
 #if __cplusplus >= 201103L
-	    const auto __pos = begin() + (__position - cbegin());
-	    // __x could be an existing element of this vector, so make a
-	    // copy of it before _M_insert_aux moves elements around.
-	    _Temporary_value __x_copy(this, __x);
-	    _M_insert_aux(__pos, std::move(__x_copy._M_val()));
+	      const auto __pos = begin() + (__position - cbegin());
+	      // __x could be an existing element of this vector, so make a
+	      // copy of it before _M_insert_aux moves elements around.
+	      _Temporary_value __x_copy(this, __x);
+	      _M_insert_aux(__pos, std::move(__x_copy._M_val()));
 #else
-	    _M_insert_aux(__position, __x);
+	      _M_insert_aux(__position, __x);
 #endif
-	  }
+	    }
+	}
       else
 #if __cplusplus >= 201103L
 	_M_realloc_insert(begin() + (__position - cbegin()), __x);