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

[Samarth Narang via cfe-commits]

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@@ -2276,6 +2277,134 @@ static void DiagnoseNonTriviallyCopyableReason(Sema &SemaRef,
   SemaRef.Diag(D->getLocation(), diag::note_defined_here) << D;
 }
 
+static bool hasMixedAccessSpecifier(const CXXRecordDecl *D) {
+  AccessSpecifier FirstAccess = AS_none;
+  for (const FieldDecl *Field : D->fields()) {
+
+    if (Field->isUnnamedBitField())
+      continue;
+    AccessSpecifier FieldAccess = Field->getAccess();
+    if (FirstAccess == AS_none) {
+      FirstAccess = FieldAccess;
+    } else if (FieldAccess != FirstAccess) {
+      return true;
+    }
+  }
+  return false;
+}
+
+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()) {
+        ++NumBasesWithFields;
+        break; // Only count the base once.
+      }
+    }
+  }
+  return NumBasesWithFields > 1;
+}
+
+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::NonStdLayoutBase << B.getType()
+          << B.getSourceRange();
+    }
+  }
+  if (hasMixedAccessSpecifier(D)) {
+    SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+        << diag::TraitNotSatisfiedReason::MixedAccess;
+  }
+  if (hasMultipleDataBaseClassesWithFields(D)) {
+    SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+        << diag::TraitNotSatisfiedReason::MultipleDataBase;
+  }
+  if (D->isPolymorphic()) {
+    SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+        << diag::TraitNotSatisfiedReason::VirtualFunction;
+  }
+  for (const FieldDecl *Field : D->fields()) {
+    if (!Field->getType()->isStandardLayoutType()) {
+      SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+          << diag::TraitNotSatisfiedReason::NonStdLayoutMember << Field
+          << 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 =
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snarang181 wrote:

Done

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