[Clang] Demote mixed enumeration arithmetic error to a warning

[cor3ntin]

In C++, defaulted to an error.

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

Fixes #92340

[llvmbot]

@llvm/pr-subscribers-clang

Author: cor3ntin (cor3ntin)

Changes

In C++, defaulted to an error.

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

Fixes #92340

---

Full diff: https://github.com/llvm/llvm-project/pull/131811.diff

4 Files Affected:

• (modified) clang/include/clang/Basic/DiagnosticSemaKinds.td (+5-1)
• (modified) clang/lib/Sema/SemaChecking.cpp (+1-1)
• (modified) clang/lib/Sema/SemaExpr.cpp (+1-1)
• (modified) clang/test/SemaCXX/cxx2c-enum-compare.cpp (+3-2)

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index a88af192fb2a4..6996e7ec1cb29 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/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>,
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index c6feb35efce85..fc8994c65659f 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/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;
   }
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index e19136b394800..3ff372bdc9d0c 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/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
diff --git a/clang/test/SemaCXX/cxx2c-enum-compare.cpp b/clang/test/SemaCXX/cxx2c-enum-compare.cpp
index f47278a60725e..1a7363ecf0be4 100644
--- a/clang/test/SemaCXX/cxx2c-enum-compare.cpp
+++ b/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,nowarn -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')}}
 }

[cor3ntin]

(this should be backported)

[AaronBallman]

nowarn is unused, but I think it should be silencing the invalid comparison diagnostic, shouldn't it?

[cor3ntin]

Comparing with double seems a lot weirder than comparing with another enum and no one complained about that; I think it should remain an error

[AaronBallman]

Then you're missing test coverage for when -Wno-enum-enum-conversion disables the diagnostic.

[AaronBallman]

Whoops, I was misreading the test, there is test coverage for this. Sorry!

[AaronBallman]

LGTM, but be sure to add a release note on the backport so users know about the change in behavior.

[cor3ntin]

/cherry-pick e4d910e

[llvmbot]

Failed to cherry-pick: e4d910e

https://github.com/llvm/llvm-project/actions/runs/13927528748

Please manually backport the fix and push it to your github fork. Once this is done, please create a pull request

[tstellar]

@cor3ntin Were you able to manually create a backport PR for this?

[cor3ntin]

@tstellar it slipped my mind, I can do that later today though

[cor3ntin]

@tstellar #139396