[clang] [Clang] Implement diagnostics for why `std::is_standard_layout` is false (PR #144161)

[Samarth Narang via cfe-commits]

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@@ -2285,6 +2286,139 @@ static void DiagnoseNonTriviallyCopyableReason(Sema &SemaRef,
   SemaRef.Diag(D->getLocation(), diag::note_defined_here) << D;
 }
 
+static bool hasMultipleDataBaseClassesWithFields(const CXXRecordDecl *D) {
+  int NumBasesWithFields = 0;
+  for (const CXXBaseSpecifier &Base : D->bases()) {
+    const CXXRecordDecl *BaseRD = Base.getType()->getAsCXXRecordDecl();
+    if (!BaseRD || BaseRD->isInvalidDecl())
+      continue;
+
+    for (const FieldDecl *Field : BaseRD->fields()) {
+      if (!Field->isUnnamedBitField()) {
+        if (++NumBasesWithFields > 1)
+          return true; // found more than one base class with fields
+        break;         // no need to check further fields in this base class
+      }
+    }
+  }
+  return false;
+}
+
+static void DiagnoseNonStandardLayoutReason(Sema &SemaRef, SourceLocation Loc,
+                                            const CXXRecordDecl *D) {
+  for (const CXXBaseSpecifier &B : D->bases()) {
+    assert(B.getType()->getAsCXXRecordDecl() && "invalid base?");
+    if (B.isVirtual()) {
+      SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+          << diag::TraitNotSatisfiedReason::VBase << B.getType()
+          << B.getSourceRange();
+    }
+    if (!B.getType()->isStandardLayoutType()) {
+      SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+          << diag::TraitNotSatisfiedReason::NonStandardLayoutBase << B.getType()
+          << B.getSourceRange();
+    }
+  }
+  // Check for mixed access specifiers in fields.
+  const FieldDecl *FirstField = nullptr;
+  AccessSpecifier FirstAccess = AS_none;
+
+  for (const FieldDecl *Field : D->fields()) {
+    if (Field->isUnnamedBitField())
+      continue;
+
+    // Record the first field we see
+    if (!FirstField) {
+      FirstField = Field;
+      FirstAccess = Field->getAccess();
+      continue;
+    }
+
+    // Check if the field has a different access specifier than the first one.
+    if (Field->getAccess() != FirstAccess) {
+      // Emit a diagnostic about mixed access specifiers.
+      SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+          << diag::TraitNotSatisfiedReason::MixedAccess;
+
+      SemaRef.Diag(FirstField->getLocation(), diag::note_defined_here)
+          << FirstField;
+
+      SemaRef.Diag(Field->getLocation(), diag::note_unsatisfied_trait_reason)
+          << diag::TraitNotSatisfiedReason::MixedAccessField << Field
+          << FirstField;
+
+      // No need to check further fields, as we already found mixed access.
+      return;
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snarang181 wrote:

You are correct, this should be a `break`. 

https://github.com/llvm/llvm-project/pull/144161