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

[Tom Stellard 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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tstellar wrote:

The issue is that TableGen sorts the enums alphabetically, so adding a new value will change the name->integer mapping for approximately half of the enum.  Maybe ABI break is the wrong term here, but it means than an app built against 20.1.4, for example will stop working if 20.1.4 libraries are replaced by 20.1.5 libraries, which is something we want to avoid even if it's not technically an ABI break.

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