[clang] [Clang] Implement CWG2598: Union of non-literal types (PR #78195)

[Mital Ashok via cfe-commits]

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@@ -1383,6 +1383,34 @@ void CXXRecordDecl::addedMember(Decl *D) {
   }
 }
 
+bool CXXRecordDecl::isLiteral() const {
+  const LangOptions &LangOpts = getLangOpts();
+  if (!(LangOpts.CPlusPlus20 ? hasConstexprDestructor()
+                             : hasTrivialDestructor()))
+    return false;
+
+  if (isLambda() && !LangOpts.CPlusPlus17)
+    return false;
+
+  if (hasNonLiteralTypeFieldsOrBases()) {
+    // CWG2598
+    // is an aggregate union type that has either no variant
+    // members or at least one variant member of non-volatile literal type,
+    if (!isUnion())
+      return false;
+    bool HasAtLeastOneTrivialMember =
+        fields().empty() || any_of(fields(), [this](const FieldDecl *D) {
+          return !D->getType().isVolatileQualified() &&
+                 D->getType().isTrivialType(getASTContext());
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MitalAshok wrote:

What about literal non-trivial types (e.g. `struct Literal { constexpr Literal() {} }; union union6 { NonLiteral NL; Literal L; };`?

`.isLiteralType` should work here

https://github.com/llvm/llvm-project/pull/78195