[clang] Implement __is_pointer_interconvertible_base_of() (#88473)

This patch implements intrinsic that supports
`std::is_pointer_interconvertible_base_of` type trait from
[P0466R5](https://wg21.link/p0466r5) "Layout-compatibility and
Pointer-interconvertibility Traits".

Normative wording:
> Comment: If `Base` and Derived are non-union class types and are not
(possibly _cv_-qualified) versions of the same type, `Derived` is a
complete type.
> Condition: `Derived` is unambiguously derived from `Base` without
regard to _cv_-qualifiers, and each object of type `Derived` is
pointer-interconvertible (6.7.2 [basic.compound]) with its `Base`
subobject, or `Base` and `Derived` are not unions and name the same
class type without regard to _cv_-qualifiers.

The paper also express the following intent:
> Note that `is_pointer_interconvertible_base_of_v<T,T>` is always true
under this wording, even though `T` is not derived from itself.

I find the treatment of unions in the wording contradictory to this
intent, and I'm not able to find anything relevant in minutes or on the
reflector. That said, this patch implements what the wording says, since
it's very explicit about unions.

clang/docs/ReleaseNotes.rst

@@ -98,7 +98,8 @@ C++20 Feature Support
   behavior can use the flag '-Xclang -fno-skip-odr-check-in-gmf'.
   (#GH79240).
 
-- Implemented the `__is_layout_compatible` intrinsic to support
+- Implemented the `__is_layout_compatible` and `__is_pointer_interconvertible_base_of`
+  intrinsics to support
   `P0466R5: Layout-compatibility and Pointer-interconvertibility Traits <https://wg21.link/P0466R5>`_.
 
 - Clang now implements [module.import]p7 fully. Clang now will import module

clang/include/clang/Basic/TokenKinds.def

@@ -521,6 +521,7 @@ TYPE_TRAIT_1(__is_union, IsUnion, KEYCXX)
 TYPE_TRAIT_1(__has_unique_object_representations,
              HasUniqueObjectRepresentations, KEYCXX)
 TYPE_TRAIT_2(__is_layout_compatible, IsLayoutCompatible, KEYCXX)
+TYPE_TRAIT_2(__is_pointer_interconvertible_base_of, IsPointerInterconvertibleBaseOf, KEYCXX)
 
 #define TRANSFORM_TYPE_TRAIT_DEF(_, Trait) KEYWORD(__##Trait, KEYCXX)
 #include "clang/Basic/TransformTypeTraits.def"

clang/include/clang/Sema/Sema.h

@@ -1999,6 +1999,8 @@ class Sema final : public SemaBase {
   };
 
   bool IsLayoutCompatible(QualType T1, QualType T2) const;
+  bool IsPointerInterconvertibleBaseOf(const TypeSourceInfo *Base,
+                                       const TypeSourceInfo *Derived);
 
   bool CheckFunctionCall(FunctionDecl *FDecl, CallExpr *TheCall,
                          const FunctionProtoType *Proto);

clang/lib/Sema/SemaChecking.cpp

@@ -19710,6 +19710,27 @@ bool Sema::IsLayoutCompatible(QualType T1, QualType T2) const {
   return isLayoutCompatible(getASTContext(), T1, T2);
 }
 
+//===-------------- Pointer interconvertibility ----------------------------//
+
+bool Sema::IsPointerInterconvertibleBaseOf(const TypeSourceInfo *Base,
+                                           const TypeSourceInfo *Derived) {
+  QualType BaseT = Base->getType()->getCanonicalTypeUnqualified();
+  QualType DerivedT = Derived->getType()->getCanonicalTypeUnqualified();
+
+  if (BaseT->isStructureOrClassType() && DerivedT->isStructureOrClassType() &&
+      getASTContext().hasSameType(BaseT, DerivedT))
+    return true;
+
+  if (!IsDerivedFrom(Derived->getTypeLoc().getBeginLoc(), DerivedT, BaseT))
+    return false;
+
+  // Per [basic.compound]/4.3, containing object has to be standard-layout.
+  if (DerivedT->getAsCXXRecordDecl()->isStandardLayout())
+    return true;
+
+  return false;
+}
+
 //===--- CHECK: pointer_with_type_tag attribute: datatypes should match ----//
 
 /// Given a type tag expression find the type tag itself.

clang/lib/Sema/SemaExprCXX.cpp

@@ -6100,6 +6100,22 @@ static bool EvaluateBinaryTypeTrait(Sema &Self, TypeTrait BTT, const TypeSourceI
       Self.Diag(Rhs->getTypeLoc().getBeginLoc(), diag::err_vla_unsupported)
           << 1 << tok::kw___is_layout_compatible;
     return Self.IsLayoutCompatible(LhsT, RhsT);
+  }
+  case BTT_IsPointerInterconvertibleBaseOf: {
+    if (LhsT->isStructureOrClassType() && RhsT->isStructureOrClassType() &&
+        !Self.getASTContext().hasSameUnqualifiedType(LhsT, RhsT)) {
+      Self.RequireCompleteType(Rhs->getTypeLoc().getBeginLoc(), RhsT,
+                               diag::err_incomplete_type);
+    }
+
+    if (LhsT->isVariableArrayType())
+      Self.Diag(Lhs->getTypeLoc().getBeginLoc(), diag::err_vla_unsupported)
+          << 1 << tok::kw___is_pointer_interconvertible_base_of;
+    if (RhsT->isVariableArrayType())
+      Self.Diag(Rhs->getTypeLoc().getBeginLoc(), diag::err_vla_unsupported)
+          << 1 << tok::kw___is_pointer_interconvertible_base_of;
+
+    return Self.IsPointerInterconvertibleBaseOf(Lhs, Rhs);
   }
     default: llvm_unreachable("not a BTT");
   }

clang/test/SemaCXX/type-traits.cpp

@@ -1740,7 +1740,7 @@ void is_layout_compatible(int n)
   static_assert(!__is_layout_compatible(void, int));
   static_assert(__is_layout_compatible(void, const void));
   static_assert(__is_layout_compatible(void, volatile void));
-  static_assert(__is_layout_compatible(const int, volatile int));
+  static_assert(__is_layout_compatible(const void, volatile void));
   static_assert(__is_layout_compatible(int, int));
   static_assert(__is_layout_compatible(int, const int));
   static_assert(__is_layout_compatible(int, volatile int));
@@ -1839,6 +1839,95 @@ void is_layout_compatible(int n)
   static_assert(!__is_layout_compatible(EnumClassForward, int));
 }
 
+namespace IPIBO {
+struct Base {};
+struct Base2 {};
+struct Base3 : Base {};
+struct Base3Virtual : virtual Base {};
+struct Derived : Base {};
+struct DerivedIndirect : Base3 {};
+struct DerivedMultiple : Base, Base2 {};
+struct DerivedAmbiguous : Base, Base3 {};
+/* expected-warning@-1 {{direct base 'Base' is inaccessible due to ambiguity:
+    struct IPIBO::DerivedAmbiguous -> Base
+    struct IPIBO::DerivedAmbiguous -> Base3 -> Base}} */
+struct DerivedPrivate : private Base {};
+struct DerivedVirtual : virtual Base {};
+
+union Union {};
+union UnionIncomplete;
+struct StructIncomplete; // #StructIncomplete
+
+void is_pointer_interconvertible_base_of(int n)
+{
+  static_assert(__is_pointer_interconvertible_base_of(Base, Derived));
+  static_assert(!__is_pointer_interconvertible_base_of(Base2, Derived));
+  static_assert(__is_pointer_interconvertible_base_of(Base, DerivedIndirect));
+  static_assert(__is_pointer_interconvertible_base_of(Base, DerivedMultiple));
+  static_assert(!__is_pointer_interconvertible_base_of(Base3, DerivedMultiple));
+  static_assert(!__is_pointer_interconvertible_base_of(Base, DerivedAmbiguous));
+  static_assert(__is_pointer_interconvertible_base_of(Base, DerivedPrivate));
+  static_assert(!__is_pointer_interconvertible_base_of(Base, DerivedVirtual));
+  static_assert(!__is_pointer_interconvertible_base_of(Union, Union));
+  static_assert(!__is_pointer_interconvertible_base_of(UnionIncomplete, UnionIncomplete));
+  static_assert(__is_pointer_interconvertible_base_of(StructIncomplete, StructIncomplete));
+  static_assert(__is_pointer_interconvertible_base_of(StructIncomplete, const StructIncomplete));
+  static_assert(__is_pointer_interconvertible_base_of(StructIncomplete, volatile StructIncomplete));
+  static_assert(__is_pointer_interconvertible_base_of(const StructIncomplete, volatile StructIncomplete));
+  static_assert(!__is_pointer_interconvertible_base_of(StructIncomplete, Derived));
+  static_assert(!__is_pointer_interconvertible_base_of(Base, StructIncomplete));
+  // expected-error@-1 {{incomplete type 'StructIncomplete' where a complete type is required}}
+  //   expected-note@#StructIncomplete {{forward declaration of 'IPIBO::StructIncomplete'}}
+  static_assert(!__is_pointer_interconvertible_base_of(CStruct2, CppStructNonStandardByBase2));
+  static_assert(!__is_pointer_interconvertible_base_of(void, void));
+  static_assert(!__is_pointer_interconvertible_base_of(void, int));
+  static_assert(!__is_pointer_interconvertible_base_of(void, const void));
+  static_assert(!__is_pointer_interconvertible_base_of(void, volatile void));
+  static_assert(!__is_pointer_interconvertible_base_of(const void, volatile void));
+  static_assert(!__is_pointer_interconvertible_base_of(int, int));
+  static_assert(!__is_pointer_interconvertible_base_of(int, const int));
+  static_assert(!__is_pointer_interconvertible_base_of(int, volatile int));
+  static_assert(!__is_pointer_interconvertible_base_of(const int, volatile int));
+  static_assert(!__is_pointer_interconvertible_base_of(int *, int * __restrict));
+  static_assert(!__is_pointer_interconvertible_base_of(int, _Atomic int));
+  static_assert(!__is_pointer_interconvertible_base_of(_Atomic(int), _Atomic int));
+  static_assert(!__is_pointer_interconvertible_base_of(int, unsigned int));
+  static_assert(!__is_pointer_interconvertible_base_of(char, unsigned char));
+  static_assert(!__is_pointer_interconvertible_base_of(char, signed char));
+  static_assert(!__is_pointer_interconvertible_base_of(unsigned char, signed char));
+  using function_type = void();
+  using function_type2 = void(char);
+  static_assert(!__is_pointer_interconvertible_base_of(const function_type, const function_type));
+  // expected-warning@-1 {{'const' qualifier on function type 'function_type' (aka 'void ()') has no effect}}
+  // expected-warning@-2 {{'const' qualifier on function type 'function_type' (aka 'void ()') has no effect}}
+  static_assert(!__is_pointer_interconvertible_base_of(function_type, const function_type));
+  // expected-warning@-1 {{'const' qualifier on function type 'function_type' (aka 'void ()') has no effect}}
+  static_assert(!__is_pointer_interconvertible_base_of(const function_type, const function_type2));
+  // expected-warning@-1 {{'const' qualifier on function type 'function_type' (aka 'void ()') has no effect}}
+  // expected-warning@-2 {{'const' qualifier on function type 'function_type2' (aka 'void (char)') has no effect}}
+  static_assert(!__is_pointer_interconvertible_base_of(int CStruct2::*, int CStruct2::*));
+  static_assert(!__is_pointer_interconvertible_base_of(int CStruct2::*, char CStruct2::*));
+  static_assert(!__is_pointer_interconvertible_base_of(void(CStruct2::*)(int), void(CStruct2::*)(int)));
+  static_assert(!__is_pointer_interconvertible_base_of(void(CStruct2::*)(int), void(CStruct2::*)(char)));
+  static_assert(!__is_pointer_interconvertible_base_of(int[], int[]));
+  static_assert(!__is_pointer_interconvertible_base_of(int[], double[]));
+  static_assert(!__is_pointer_interconvertible_base_of(int[2], int[2]));
+  static_assert(!__is_pointer_interconvertible_base_of(int[n], int[2]));
+  // expected-error@-1 {{variable length arrays are not supported in '__is_pointer_interconvertible_base_of'}}
+  static_assert(!__is_pointer_interconvertible_base_of(int[n], int[n]));
+  // expected-error@-1 {{variable length arrays are not supported in '__is_pointer_interconvertible_base_of'}}
+  // expected-error@-2 {{variable length arrays are not supported in '__is_pointer_interconvertible_base_of'}}
+  static_assert(!__is_pointer_interconvertible_base_of(int&, int&));
+  static_assert(!__is_pointer_interconvertible_base_of(int&, char&));
+  static_assert(!__is_pointer_interconvertible_base_of(void(int), void(int)));
+  static_assert(!__is_pointer_interconvertible_base_of(void(int), void(char)));
+  static_assert(!__is_pointer_interconvertible_base_of(void(&)(int), void(&)(int)));
+  static_assert(!__is_pointer_interconvertible_base_of(void(&)(int), void(&)(char)));
+  static_assert(!__is_pointer_interconvertible_base_of(void(*)(int), void(*)(int)));
+  static_assert(!__is_pointer_interconvertible_base_of(void(*)(int), void(*)(char)));
+}
+}
+
 void is_signed()
 {
   //static_assert(__is_signed(char));