[gcc r13-3458] c++: Fix up constexpr handling of char/signed char/short pre/post inc/decrement [PR105774]

[Jakub Jelinek <jakub@gcc.gnu.org>]

https://gcc.gnu.org/g:da8c362c4c18cff2f2dfd5c4706bdda7576899a4

commit r13-3458-gda8c362c4c18cff2f2dfd5c4706bdda7576899a4
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Mon Oct 24 16:25:29 2022 +0200

    c++: Fix up constexpr handling of char/signed char/short pre/post inc/decrement [PR105774]
    
    signed char, char or short int pre/post inc/decrement are represented by
    normal {PRE,POST}_{INC,DEC}REMENT_EXPRs in the FE and only gimplification
    ensures that the {PLUS,MINUS}_EXPR is done in unsigned version of those
    types:
        case PREINCREMENT_EXPR:
        case PREDECREMENT_EXPR:
        case POSTINCREMENT_EXPR:
        case POSTDECREMENT_EXPR:
          {
            tree type = TREE_TYPE (TREE_OPERAND (*expr_p, 0));
            if (INTEGRAL_TYPE_P (type) && c_promoting_integer_type_p (type))
              {
                if (!TYPE_OVERFLOW_WRAPS (type))
                  type = unsigned_type_for (type);
                return gimplify_self_mod_expr (expr_p, pre_p, post_p, 1, type);
              }
            break;
          }
    This means during constant evaluation we need to do it similarly (either
    using unsigned_type_for or using widening to integer_type_node).
    The following patch does the latter.
    
    2022-10-24  Jakub Jelinek  <jakub@redhat.com>
    
            PR c++/105774
            * constexpr.cc (cxx_eval_increment_expression): For signed types
            that promote to int, evaluate PLUS_EXPR or MINUS_EXPR in int type.
    
            * g++.dg/cpp1y/constexpr-105774.C: New test.

Diff:
---
 gcc/cp/constexpr.cc                           | 12 ++++++++++++
 gcc/testsuite/g++.dg/cpp1y/constexpr-105774.C | 15 +++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index c3ee970a724..94b54fc71dc 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -6234,6 +6234,18 @@ cxx_eval_increment_expression (const constexpr_ctx *ctx, tree t,
 	offset = fold_build1 (NEGATE_EXPR, TREE_TYPE (offset), offset);
       mod = fold_build2 (POINTER_PLUS_EXPR, type, val, offset);
     }
+  else if (c_promoting_integer_type_p (type)
+	   && !TYPE_UNSIGNED (type)
+	   && TYPE_PRECISION (type) < TYPE_PRECISION (integer_type_node))
+    {
+      offset = fold_convert (integer_type_node, offset);
+      mod = fold_convert (integer_type_node, val);
+      tree t = fold_build2 (inc ? PLUS_EXPR : MINUS_EXPR, integer_type_node,
+			    mod, offset);
+      mod = fold_convert (type, t);
+      if (TREE_OVERFLOW_P (mod) && !TREE_OVERFLOW_P (t))
+	TREE_OVERFLOW (mod) = false;
+    }
   else
     mod = fold_build2 (inc ? PLUS_EXPR : MINUS_EXPR, type, val, offset);
   if (!ptr)
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-105774.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-105774.C
new file mode 100644
index 00000000000..8ca6fce7570
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-105774.C
@@ -0,0 +1,15 @@
+// PR c++/105774
+// { dg-do compile { target c++14 } }
+
+constexpr signed char
+foo ()
+{
+#if __SCHAR_MAX__ < __INT_MAX__
+  signed char x = __SCHAR_MAX__;
+#else
+  signed char x = 0;
+#endif
+  return ++x;
+}
+
+constexpr auto a = foo ();