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Here, during cxx_eval_increment_expression (with lval=false) of ++__first where __first is &"mystr"[0], we correctly update __first to &"mystr"[1] but we end up returning &"mystr"[0] + 1 instead of &"mystr"[1]. This unreduced return value inhibits other pointer arithmetic folding during later constexpr evaluation, which ultimately causes the constexpr evaluation to fail. It turns out the simplification of &"mystr"[0] + 1 to &"mystr"[1] is performed by cxx_fold_pointer_plus_expression, not by fold_build2. So we perform this simplification during constexpr evaluation of the temporary MODIFY_EXPR (during which we assign to __first the simplified value), but then we return 'mod' which has only been folded via fold_build2 and hasn't gone through cxx_fold_pointer_plus_expression. This patch fixes this by updating 'mod' with the result of the MODIFY_EXPR evaluation appropriately, so that it captures any additional folding of the expression when !lval. We now need to be wary of this evaluation failing and returning e.g. the MODIFY_EXPR or NULL_TREE; it seems checking *non_constant_p should cover our bases here and is generally prudent. gcc/cp/ChangeLog: PR c++/99287 * constexpr.c (cxx_eval_increment_expression): Pass lval when evaluating the MODIFY_EXPR, and update 'mod' with the result of this evaluation. Check *non_constant_p afterwards. For prefix ops, just return 'mod'. gcc/testsuite/ChangeLog: PR c++/99287 * g++.dg/cpp2a/constexpr-99287.C: New test. Co-authored-by: Jakub Jelinek <jakub@redhat.com>
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