fixup: verify span-like struct/class bases

If present, they must be empty.

Author: a-nogikh

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3465,6 +3465,8 @@ def note_returned_not_integer_field
 def note_returned_not_wide_enough_field
     : Note<"integer field #%0 of span-like type is not wide enough (minimum "
            "width: %1)">;
+def note_inherits_not_empty_base
+    : Note<"returned struct/class inherits from a non-empty base">;
 def warn_attribute_return_pointers_refs_only : Warning<
   "%0 attribute only applies to return values that are pointers or references">,
   InGroup<IgnoredAttributes>;

clang/lib/Sema/SemaDeclAttr.cpp

@@ -1857,6 +1857,14 @@ bool Sema::CheckSpanLikeType(const AttributeCommonInfo &CI,
   const RecordDecl *RD = Ty->getAsRecordDecl();
   if (!RD || RD->isUnion())
     return emitWarning(diag::note_returned_not_struct);
+  if (const auto *CXXRD = dyn_cast<CXXRecordDecl>(RD)) {
+    for (const auto &Base : CXXRD->bases()) {
+      const CXXRecordDecl *BaseDecl = Base.getType()->getAsCXXRecordDecl();
+      if (!BaseDecl || !BaseDecl->isEmpty()) {
+        return emitWarning(diag::note_inherits_not_empty_base);
+      }
+    }
+  }
   auto FieldsBegin = RD->field_begin();
   auto FieldsCount = std::distance(FieldsBegin, RD->field_end());
   if (FieldsCount != 2)

clang/test/SemaCXX/attr-malloc_span.cpp

@@ -108,3 +108,26 @@ __attribute((malloc_span)) auto trailing_return_temmplate_good(void) -> Pair<int
 __attribute((malloc_span)) auto trailing_return_temmplate_bad(void) -> Pair<int, int> {
   return Pair<int, int>{};
 }
+
+struct EmptyBase {
+};
+
+struct GoodChildSpan : EmptyBase {
+  void *p;
+  int n;
+};
+
+__attribute((malloc_span)) GoodChildSpan return_span_with_good_base(void); // no-warning
+
+struct NonEmptyBase {
+  void *other_p;
+};
+
+struct BadChildSpan : EmptyBase, NonEmptyBase {
+  void *p;
+  int n;
+};
+
+// expected-warning@+2 {{attribute only applies to functions that return span-like structures}}
+// expected-note@+1 {{returned struct/class inherits from a non-empty base}}
+__attribute((malloc_span)) BadChildSpan return_span_with_bad_base(void);