c++: Fix error recovery for enums with bad underlying type [PR122540]

The r16-4698 patch to use fold_convert in finish_enum_value_list instead
of copy_node + set TREE_TYPE caused UB in the compiler as detected e.g.
by valgrind on the g++.dg/parse/pr96442.C g++.dg/cpp0x/auto9.C testcases.
If underlying type is valid, start_enum does:
      else if (CP_INTEGRAL_TYPE_P (underlying_type))
        {
          copy_type_enum (enumtype, underlying_type);
          ENUM_UNDERLYING_TYPE (enumtype) = underlying_type;
        }
and the copy_type_enum ensures the ENUMERAL_TYPE has the same
TYPE_PRECISION, TYPE_SIZE etc.  But if the underlying type is erroneous,
it errors and for error recovery sets
ENUM_UNDERLYING_TYPE (enumtype) = integer_type_node;
This means TYPE_PRECISION on the ENUMERAL_TYPE remains 0, TYPE_SIZE NULL
etc. and then later on when we try to fold_convert the enumerator values
to that type, we invoke UB in the wide_int code because it really doesn't
like casts to 0 precision integral types.

The following patch fixes it by calling also copy_type_enum from
integer_type_node when we set such underlying type.

2025-11-20  Jakub Jelinek  <jakub@redhat.com>

	PR c++/122540
	* decl.cc (start_enum): When setting ENUM_UNDERLYING_TYPE
	to integer_type_node during error recovery, also call copy_type_enum.

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 29165e447b38d80afd1ac141abe3b4222ab0e138..c62f0777f5b2922995e34a66c32b44abeebf1131 100644 (file)
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -18723,6 +18723,7 @@ start_enum (tree name, tree enumtype, tree underlying_type,
        {
          error ("underlying type %qT of %qT must be an integral type",
                 underlying_type, enumtype);
+         copy_type_enum (enumtype, integer_type_node);
          ENUM_UNDERLYING_TYPE (enumtype) = integer_type_node;
        }
     }