[clang] [clang-tools-extra] [Clang] [Sema] Diagnose unknown std::initializer_list layout in SemaInit (PR #95580)

[via cfe-commits]

================
@@ -9392,6 +9392,57 @@ ExprResult InitializationSequence::Perform(Sema &S,
       // Wrap it in a construction of a std::initializer_list<T>.
       CurInit = new (S.Context) CXXStdInitializerListExpr(Step->Type, MTE);
 
+      if (!Step->Type->isDependentType()) {
+        assert(S.isCompleteType(CurInit.get()->getExprLoc(), Step->Type,
+                                Sema::CompleteTypeKind::Normal) &&
+               "std::initializer_list<E> incomplete when used during "
+               "initialization");
+        QualType ElementType;
+        [[maybe_unused]] bool IsStdInitializerList =
+            S.isStdInitializerList(Step->Type, &ElementType);
+        assert(IsStdInitializerList &&
+               "StdInitializerList step to non-std::initializer_list");
+        RecordDecl *Record = Step->Type->castAs<RecordType>()->getDecl();
+
+        auto InvalidType = [&] {
+          S.Diag(Record->getLocation(),
+                 diag::err_std_initializer_list_malformed)
+              << Step->Type.getUnqualifiedType();
+          return ExprError();
+        };
+
+        // FIXME: What if the initializer_list type has base classes, etc?
+        if (Record->isUnion())
+          return InvalidType();
+
+        RecordDecl::field_iterator Field = Record->field_begin();
+        if (Field == Record->field_end())
+          return InvalidType();
+
+        // Start pointer
+        if (!Field->getType()->isPointerType() ||
+            !S.Context.hasSameType(Field->getType()->getPointeeType(),
+                                   ElementType.withConst()))
+          return InvalidType();
+
+        if (++Field == Record->field_end())
+          return InvalidType();
+
+        // Size or end pointer
+        if (Field->getType()->isPointerType()) {
+          if (!S.Context.hasSameType(Field->getType()->getPointeeType(),
+                                     ElementType.withConst()))
+            return InvalidType();
+        } else {
+          if (Field->isUnnamedBitField() ||
----------------
Sirraide wrote:

What about named bitfields? I think we should also be able to disallow bitfields outright here.

https://github.com/llvm/llvm-project/pull/95580