[llvm-branch-commits] [clang] [Clang][Backport] Demote mixed enumeration arithmetic error to a warning (#131811) (PR #139396)

[Aaron Ballman via llvm-branch-commits]

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@@ -7567,9 +7567,13 @@ def warn_arith_conv_mixed_enum_types_cxx20 : Warning<
   "%sub{select_arith_conv_kind}0 "
   "different enumeration types%diff{ ($ and $)|}1,2 is deprecated">,
   InGroup<DeprecatedEnumEnumConversion>;
-def err_conv_mixed_enum_types_cxx26 : Error<
+
+def err_conv_mixed_enum_types: Error <
   "invalid %sub{select_arith_conv_kind}0 "
   "different enumeration types%diff{ ($ and $)|}1,2">;
+def warn_conv_mixed_enum_types_cxx26 : Warning <
+  err_conv_mixed_enum_types.Summary>,
+  InGroup<EnumEnumConversion>, DefaultError;
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AaronBallman wrote:

I don't disagree (it does add a new enum value), but I am starting to think this ABI requirement is onerous enough to be worth rethinking the scope of it. The point to the ABI requirement is that you should be able to use Clang as a library interchangeably between the dot releases. The platform calling convention ABI does not change when an enumeration gains a new enumerator, nor does name mangling behavior change. In fact, adding an enumerator can only increase the number of valid integer values that can be represented by the enumeration, which means the addition won't introduce new UB but could remove UB by widening the range of valid values. So I think this kind of change isn't introducing a kind of ABI break we need to avoid.

Or am I missing something?

https://github.com/llvm/llvm-project/pull/139396