[Bug c++/108158] [11/12/13 Regression] modification of '...' is not a constant expression since r12-2304

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108158

--- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Candidate fix:

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index be99bec17e7..6718c29b0cd 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -4301,9 +4301,13 @@ cxx_eval_array_reference (const constexpr_ctx *ctx, tree
t,
   if (!SCALAR_TYPE_P (elem_type))
     {
       new_ctx = *ctx;
-      if (ctx->object)
+      if (ctx->object
+         && !same_type_ignoring_top_level_qualifiers_p
+             (TREE_TYPE (t), TREE_TYPE (ctx->object)))
        /* If there was no object, don't add one: it could confuse us
-          into thinking we're modifying a const object.  */
+          into thinking we're modifying a const object.  Similarly, if
+          the types are the same, replacing .object could lead to a
+          failure to evaluate it (c++/108158).  */
        new_ctx.object = t;
       new_ctx.ctor = build_constructor (elem_type, NULL);
       ctx = &new_ctx;