[Clang] Demote mixed enumeration arithmetic error to a warning

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -7639,9 +7639,13 @@ 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/SemaChecking.cpp

@@ -15323,7 +15323,7 @@ static bool checkBuiltinVectorMathMixedEnums(Sema &S, Expr *LHS, Expr *RHS,
            R = RHS->getEnumCoercedType(S.Context);
   if (L->isUnscopedEnumerationType() && R->isUnscopedEnumerationType() &&
       !S.Context.hasSameUnqualifiedType(L, R)) {
-    return S.Diag(Loc, diag::err_conv_mixed_enum_types_cxx26)
+    return S.Diag(Loc, diag::err_conv_mixed_enum_types)
            << LHS->getSourceRange() << RHS->getSourceRange()
            << /*Arithmetic Between*/ 0 << L << R;
   }

clang/lib/Sema/SemaExpr.cpp

@@ -1533,7 +1533,7 @@ void Sema::checkEnumArithmeticConversions(Expr *LHS, Expr *RHS,
     // In C++ 26, usual arithmetic conversions between 2 different enum types
     // are ill-formed.
     if (getLangOpts().CPlusPlus26)
-      DiagID = diag::err_conv_mixed_enum_types_cxx26;
+      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')}}
 }