[Bug c++/99700] [10/11 Regression] uninitialized variable accepted as a constant expression in C++ 20

[ppalka at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99700

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ppalka at gcc dot gnu.org
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot 
gnu.org
             Status|NEW                         |ASSIGNED

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
It seems reduced_constant_expression_p needs to check that a
CONSTRUCTOR_NO_CLEARING array ctor initializes all elements.

I'm testing:

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index c8d9dae36fb..6e530f9c88d 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -46,6 +46,7 @@ do {                                                         
        \

 static HOST_WIDE_INT find_array_ctor_elt (tree ary, tree dindex,
                                          bool insert = false);
+static int array_index_cmp (tree key, tree index);

 /* Returns true iff FUN is an instantiation of a constexpr function
    template or a defaulted constexpr function.  */
@@ -2912,7 +2913,25 @@ reduced_constant_expression_p (tree t)
          else if (cxx_dialect >= cxx20
                   /* An ARRAY_TYPE doesn't have any TYPE_FIELDS.  */
                   && TREE_CODE (TREE_TYPE (t)) == ARRAY_TYPE)
-           field = NULL_TREE;
+           {
+             tree min = TYPE_MIN_VALUE (TYPE_DOMAIN (TREE_TYPE (t)));
+             tree max = TYPE_MAX_VALUE (TYPE_DOMAIN (TREE_TYPE (t)));
+             if (find_array_ctor_elt (t, min) == -1
+                 || find_array_ctor_elt (t, max) == -1)
+               return false;
+             tree cursor = size_zero_node;
+             FOR_EACH_CONSTRUCTOR_ELT (CONSTRUCTOR_ELTS (t), i, idx, val)
+               {
+                 if (!reduced_constant_expression_p (val))
+                   return false;
+                 if (array_index_cmp (cursor, idx) != 0)
+                   return false;
+                 if (TREE_CODE (idx) == RANGE_EXPR)
+                   cursor = TREE_OPERAND (idx, 1);
+                 cursor = int_const_binop (PLUS_EXPR, cursor, size_one_node);
+               }
+             return true;
+           }
          else if (cxx_dialect >= cxx20
                   && TREE_CODE (TREE_TYPE (t)) == UNION_TYPE)
            {