[Clang] Implement diagnostics for why std::is_standard_layout is false

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -1767,7 +1767,8 @@ def note_unsatisfied_trait
     : Note<"%0 is not %enum_select<TraitName>{"
            "%TriviallyRelocatable{trivially relocatable}|"
            "%Replaceable{replaceable}|"
-           "%TriviallyCopyable{trivially copyable}"
+           "%TriviallyCopyable{trivially copyable}|"
+           "%StandardLayout{standard-layout}"
            "}1">;
 
 def note_unsatisfied_trait_reason
@@ -1787,6 +1788,13 @@ def note_unsatisfied_trait_reason
            "%NonReplaceableField{has a non-replaceable member %1 of type %2}|"
            "%NTCBase{has a non-trivially-copyable base %1}|"
            "%NTCField{has a non-trivially-copyable member %1 of type %2}|"
+           "%NonStandardLayoutBase{has a non-standard-layout base %1}|"
+           "%MixedAccess{has mixed access specifiers}|"
+           "%MixedAccessField{field %1 has a different access specifier than field %2}|"
+           "%MultipleDataBase{has multiple base classes with data members}|"
+           "%VirtualFunction{has a virtual function}|"
+           "%NonStandardLayoutMember{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

@@ -1956,6 +1956,7 @@ static std::optional<TypeTrait> StdNameToTypeTrait(StringRef Name) {
             TypeTrait::UTT_IsCppTriviallyRelocatable)
       .Case("is_replaceable", TypeTrait::UTT_IsReplaceable)
       .Case("is_trivially_copyable", TypeTrait::UTT_IsTriviallyCopyable)
+      .Case("is_standard_layout", TypeTrait::UTT_IsStandardLayout)
       .Default(std::nullopt);
 }
 
@@ -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;
+    }
+  }
+  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 CXXMethodDecl *Method : D->methods()) {
+      if (Method->isVirtual()) {
+        SemaRef.Diag(Method->getLocation(), diag::note_defined_here) << Method;
+        break;
+      }
+    }
+  }
+  for (const FieldDecl *Field : D->fields()) {
+    if (!Field->getType()->isStandardLayoutType()) {
+      SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+          << diag::TraitNotSatisfiedReason::NonStandardLayoutMember << Field
+          << Field->getType() << Field->getSourceRange();
+    }
+  }
+  // Find any indirect base classes that have fields.
+  if (D->hasDirectFields()) {
+    const CXXRecordDecl *Indirect = nullptr;
+    D->forallBases([&](const CXXRecordDecl *BaseDef) {
+      if (BaseDef->hasDirectFields()) {
+        Indirect = BaseDef;
+        return false; // stop traversal
+      }
+      return true; // continue to the next base
+    });
+    if (Indirect) {
+      SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+          << diag::TraitNotSatisfiedReason::IndirectBaseWithFields << Indirect
+          << Indirect->getSourceRange();
+    }
+  }
+}
+
+static void DiagnoseNonStandardLayoutReason(Sema &SemaRef, SourceLocation Loc,
+                                            QualType T) {
+  SemaRef.Diag(Loc, diag::note_unsatisfied_trait)
+      << T << diag::TraitName::StandardLayout;
+
+  // Check type-level exclusion first.
+  if (T->isVariablyModifiedType()) {
+    SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+        << diag::TraitNotSatisfiedReason::VLA;
+    return;
+  }
+
+  if (T->isReferenceType()) {
+    SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+        << diag::TraitNotSatisfiedReason::Ref;
+    return;
+  }
+  T = T.getNonReferenceType();
+  const CXXRecordDecl *D = T->getAsCXXRecordDecl();
+  if (!D || D->isInvalidDecl())
+    return;
+
+  if (D->hasDefinition())
+    DiagnoseNonStandardLayoutReason(SemaRef, Loc, D);
+
+  SemaRef.Diag(D->getLocation(), diag::note_defined_here) << D;
+}
+
 void Sema::DiagnoseTypeTraitDetails(const Expr *E) {
   E = E->IgnoreParenImpCasts();
   if (E->containsErrors())
@@ -2305,6 +2439,9 @@ void Sema::DiagnoseTypeTraitDetails(const Expr *E) {
   case UTT_IsTriviallyCopyable:
     DiagnoseNonTriviallyCopyableReason(*this, E->getBeginLoc(), Args[0]);
     break;
+  case UTT_IsStandardLayout:
+    DiagnoseNonStandardLayoutReason(*this, E->getBeginLoc(), Args[0]);
+    break;
   default:
     break;
   }

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

@@ -20,6 +20,13 @@ struct is_trivially_copyable {
 
 template <typename T>
 constexpr bool is_trivially_copyable_v = __is_trivially_copyable(T);
+
+template <typename T>
+struct is_standard_layout {
+static constexpr bool value = __is_standard_layout(T);
+};
+template <typename T>
+constexpr bool is_standard_layout_v = __is_standard_layout(T);
 #endif
 
 #ifdef STD2
@@ -44,6 +51,17 @@ using is_trivially_copyable  = __details_is_trivially_copyable<T>;
 
 template <typename T>
 constexpr bool is_trivially_copyable_v = __is_trivially_copyable(T);
+
+template <typename T>
+struct __details_is_standard_layout {
+static constexpr bool value = __is_standard_layout(T);
+
+
+};
+template <typename T>
+using is_standard_layout = __details_is_standard_layout<T>;
+template <typename T>
+constexpr bool is_standard_layout_v = __is_standard_layout(T);
 #endif
 
 
@@ -73,6 +91,13 @@ using is_trivially_copyable  = __details_is_trivially_copyable<T>;
 
 template <typename T>
 constexpr bool is_trivially_copyable_v = is_trivially_copyable<T>::value;
+
+template <typename T>
+struct __details_is_standard_layout : bool_constant<__is_standard_layout(T)> {};
+template <typename T>
+using is_standard_layout = __details_is_standard_layout<T>;
+template <typename T>
+constexpr bool is_standard_layout_v = is_standard_layout<T>::value;
 #endif
 
 }
@@ -100,6 +125,21 @@ static_assert(std::is_trivially_copyable_v<int&>);
 // expected-note@-1 {{because it is a reference type}}
 
 
+ // Direct tests
+ static_assert(std::is_standard_layout<int>::value);
+ static_assert(std::is_standard_layout_v<int>);
+
+ static_assert(std::is_standard_layout<int&>::value);
+ // expected-error-re@-1 {{static assertion failed due to requirement 'std::{{.*}}is_standard_layout<int &>::value'}} \
+ // expected-note@-1 {{'int &' is not standard-layout}} \
+ // expected-note@-1 {{because it is a reference type}}
+
+ static_assert(std::is_standard_layout_v<int&>);
+ // expected-error@-1 {{static assertion failed due to requirement 'std::is_standard_layout_v<int &>'}} \
+ // expected-note@-1 {{'int &' is not standard-layout}} \
+ // expected-note@-1 {{because it is a reference type}}
+
+
 namespace test_namespace {
     using namespace std;
     static_assert(is_trivially_relocatable<int&>::value);
@@ -119,6 +159,16 @@ namespace test_namespace {
     // expected-error@-1 {{static assertion failed due to requirement 'is_trivially_copyable_v<int &>'}} \
     // expected-note@-1 {{'int &' is not trivially copyable}} \
     // expected-note@-1 {{because it is a reference type}}
+
+    static_assert(is_standard_layout<int&>::value);
+     // expected-error-re@-1 {{static assertion failed due to requirement '{{.*}}is_standard_layout<int &>::value'}} \
+     // expected-note@-1 {{'int &' is not standard-layout}} \
+     // expected-note@-1 {{because it is a reference type}}
+
+     static_assert(is_standard_layout_v<int&>);
+     // expected-error@-1 {{static assertion failed due to requirement 'is_standard_layout_v<int &>'}} \
+     // expected-note@-1 {{'int &' is not standard-layout}} \
+     // expected-note@-1 {{because it is a reference type}}
 }