[clang] [Clang] Add diagnostic when scoped enumeration requires an explicit conversion for binary operations (PR #152698)

[Timothy Choi via cfe-commits]

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@@ -10742,9 +10742,53 @@ static void DiagnoseBadDivideOrRemainderValues(Sema& S, ExprResult &LHS,
                             << IsDiv << RHS.get()->getSourceRange());
 }
 
+static void diagnoseScopedEnums(Sema &S, const SourceLocation Loc,
+                                const ExprResult &LHS, const ExprResult &RHS,
+                                BinaryOperatorKind Opc) {
+  const Expr *LHSExpr = LHS.get();
+  const Expr *RHSExpr = RHS.get();
+  if (!LHSExpr || !RHSExpr)
+    return;
+  const QualType LHSType = LHSExpr->getType();
+  const QualType RHSType = RHSExpr->getType();
+  const bool LHSIsScoped = LHSType->isScopedEnumeralType();
+  const bool RHSIsScoped = RHSType->isScopedEnumeralType();
+  if (!LHSIsScoped && !RHSIsScoped)
+    return;
+  if (!LHSIsScoped && !LHSType->isIntegralOrUnscopedEnumerationType())
+    return;
+  if (!RHSIsScoped && !RHSType->isIntegralOrUnscopedEnumerationType())
+    return;
+  if (BinaryOperator::isAssignmentOp(Opc) && LHSIsScoped)
+    return;
+  bool isCxx23 = S.getLangOpts().CPlusPlus23;
+  unsigned diagID =
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tinnamchoi wrote:

Would something like `no implicit conversion for scoped enum; consider casting to underlying type, or using std::to_underlying in standards >= C++23` regardless of language mode be good? It's more verbose than I'd like but feels better than suggesting something outdated.

Also, I recently found out `std::underlying_type` is a thing. I don't think it's useful enough to add and feels a bit cluttering, but I'd like to hear some feedback on it as it feels more semantically correct. 

https://github.com/llvm/llvm-project/pull/152698