[Clang][Backport] Demote mixed enumeration arithmetic error to a warning (llvm#131811)

In C++, defaulted to an error.

C++ removed these features but the removal negatively impacts users.

Fixes llvm#92340

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -299,6 +299,9 @@ C++ Language Changes
 - The builtin type alias ``__builtin_common_type`` has been added to improve the
   performance of ``std::common_type``.
 
+- In C++26 mode, arithmetic conversion errors between mixed enumeration types can
+  be disabled with ``-Wno-enum-enum-conversion`` (#GH92340).
+
 C++2c Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -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;
 
 def warn_arith_conv_mixed_anon_enum_types : Warning<
   warn_arith_conv_mixed_enum_types.Summary>,

clang/lib/Sema/SemaExpr.cpp

@@ -1518,8 +1518,8 @@ static void checkEnumArithmeticConversions(Sema &S, Expr *LHS, Expr *RHS,
     unsigned DiagID;
     // In C++ 26, usual arithmetic conversions between 2 different enum types
     // are ill-formed.
-    if (S.getLangOpts().CPlusPlus26)
-      DiagID = diag::err_conv_mixed_enum_types_cxx26;
+    if (getLangOpts().CPlusPlus26)
+      DiagID = diag::warn_conv_mixed_enum_types_cxx26;
     else if (!L->castAs<EnumType>()->getDecl()->hasNameForLinkage() ||
              !R->castAs<EnumType>()->getDecl()->hasNameForLinkage()) {
       // If either enumeration type is unnamed, it's less likely that the

clang/test/SemaCXX/cxx2c-enum-compare.cpp

@@ -1,9 +1,10 @@
-// RUN: %clang_cc1 %s -std=c++2c -fsyntax-only -verify -triple %itanium_abi_triple
+// RUN: %clang_cc1 %s -std=c++2c -fsyntax-only -verify=both,expected
+// RUN: %clang_cc1 %s -std=c++2c -fsyntax-only -verify=both -Wno-enum-enum-conversion
 
 enum E1 { e };
 enum E2 { f };
 void test() {
-    int b = e <= 3.7; // expected-error {{invalid comparison of enumeration type 'E1' with floating-point type 'double'}}
+    int b = e <= 3.7; // both-error {{invalid comparison of enumeration type 'E1' with floating-point type 'double'}}
     int k = f - e; // expected-error {{invalid arithmetic between different enumeration types ('E2' and 'E1')}}
     int x = 1 ? e : f; // expected-error {{invalid conditional expression between different enumeration types ('E1' and 'E2')}}
 }