Add more descriptive messages for

indirect base class inheritance

Author: snarang181

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -1794,6 +1794,7 @@ def note_unsatisfied_trait_reason
            "%MultipleDataBase{has multiple base classes with data members}|"
            "%VirtualFunction{has virtual functions}|"
            "%NonStdLayoutMember{has a non-standard-layout member %1 of type %2}|"
+           "%IndirectBaseWithFields{has an indirect base %1 with data members}|"
            "%DeletedDtr{has a %select{deleted|user-provided}1 destructor}|"
            "%UserProvidedCtr{has a user provided %select{copy|move}1 "
            "constructor}|"

clang/lib/Sema/SemaTypeTraits.cpp

@@ -2407,6 +2407,37 @@ static void DiagnoseNonStandardLayoutReason(Sema &SemaRef, SourceLocation Loc,
           << Field->getType() << Field->getSourceRange();
     }
   }
+
+  //  if this class and an indirect base
+  // both have non-static data members, grab the first such base.
+  if (D->hasDirectFields()) {
+    SmallVector<const CXXRecordDecl *, 4> Records;
+
+    // Recursive lambda to collect all bases that declare fields
+    std::function<void(const CXXRecordDecl *)> collect =
+        [&](const CXXRecordDecl *R) {
+          for (const CXXBaseSpecifier &B : R->bases()) {
+            const auto *BR = B.getType()->getAsCXXRecordDecl();
+            if (!BR || !BR->hasDefinition())
+              continue;
+            if (BR->hasDirectFields())
+              Records.push_back(BR);
+            // Recurse into the base class.
+            collect(BR);
+          }
+        };
+
+    // Collect all bases that declare fields.
+    collect(D);
+
+    // If more than one record has fields, then the layout is non-standard.
+    if (!Records.empty()) {
+      const CXXRecordDecl *Indirect = Records.front();
+      SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+          << diag::TraitNotSatisfiedReason::IndirectBaseWithFields << Indirect
+          << Indirect->getSourceRange();
+    }
+  }
 }
 
 static void DiagnoseNonStandardLayoutReason(Sema &SemaRef, SourceLocation Loc,

clang/test/SemaCXX/type-traits-unsatisfied-diags-std.cpp

@@ -217,7 +217,6 @@ namespace test_namespace {
 }
 
 
-
 namespace concepts {
 template <typename T>
 requires std::is_trivially_relocatable<T>::value void f();  // #cand1
@@ -288,5 +287,3 @@ static_assert(std::is_replaceable_v<int&>);
 // expected-error@-1 {{static assertion failed due to requirement 'std::is_replaceable_v<int &>'}} \
 // expected-note@-1 {{'int &' is not replaceable}} \
 // expected-note@-1 {{because it is a reference type}}
-
-

clang/test/SemaCXX/type-traits-unsatisfied-diags.cpp

@@ -621,6 +621,7 @@ struct BaseC: BaseB { int c; };                               // #sl-BaseC
 static_assert(__is_standard_layout(BaseC));
 // expected-error@-1 {{static assertion failed due to requirement '__is_standard_layout(standard_layout_tests::BaseC)'}} \
 // expected-note@-1 {{'BaseC' is not standard-layout}} \
+// expected-note@-1 {{because it has an indirect base 'BaseA' with data members}} \
 // expected-note@#sl-BaseC {{'BaseC' defined here}} \
 
 // Multiple direct base classes with no data members --> standard layout