c++: Implement P0466R5 __cpp_lib_is_pointer_interconvertible compiler helpers [PR101539]

The following patch attempts to implement the compiler helpers for
libstdc++ std::is_pointer_interconvertible_base_of trait and
std::is_pointer_interconvertible_with_class template function.

For the former __is_pointer_interconvertible_base_of trait that checks first
whether base and derived aren't non-union class types that are the same
ignoring toplevel cv-qualifiers, otherwise if derived is unambiguously
derived from base without cv-qualifiers, derived being a complete type,
and if so, my limited understanding of any derived object being
pointer-interconvertible with base subobject IMHO implies (because one can't
inherit from unions or unions can't inherit) that we check if derived is
standard layout type and we walk bases of derived
recursively, stopping on a class that has any non-static data members and
check if any of the bases is base.  On class with non-static data members
no bases are compared already.
Upon discussions, this is something that maybe should have been changed
in the standard with CWG 2254 and the patch no longer performs this and
assumes all base subobjects of standard-layout class types are
pointer-interconvertible with the whole class objects.

The latter is implemented using a FE
__builtin_is_pointer_interconvertible_with_class, but because on the library
side it will be a template function, the builtin takes ... arguments and
only during folding verifies it has a single argument with pointer to member
type.  The initial errors IMHO can only happen if one uses the builtin
incorrectly by hand, the template function should ensure that it has
exactly a single argument that has pointer to member type.
Otherwise, again with my limited understanding of what
the template function should do and pointer-interconvertibility,
it folds to false for pointer-to-member-function, errors if
basetype of the OFFSET_TYPE is incomplete, folds to false
for non-std-layout non-union basetype, then finds the first non-static
data member in the basetype or its bases (by ignoring
DECL_FIELD_IS_BASE FIELD_DECLs that are empty, recursing into
DECL_FIELD_IS_BASE FIELD_DECLs type that are non-empty (I think
std layout should ensure there is at most one), for unions
checks if membertype is same type as any of the union FIELD_DECLs,
for non-unions the first other FIELD_DECL only, and for anonymous
aggregates similarly (union vs. non-union) but recurses into the
anon aggr types with std layout check for anon structures.  If
membertype doesn't match the type of first non-static data member
(or for unions any of the members), then the builtin folds to false,
otherwise the built folds to a check whether the argument is equal
to OFFSET_TYPE of 0 or not, either at compile time if it is constant
(e.g. for constexpr folding) or at runtime otherwise.

As I wrote in the PR, I've tried my testcases with MSVC on godbolt
that claims to implement it, and https://godbolt.org/z/3PnjM33vM
for the first testcase shows it disagrees with my expectations on
static_assert (std::is_pointer_interconvertible_base_of_v<D, F>);
static_assert (std::is_pointer_interconvertible_base_of_v<E, F>);
static_assert (!std::is_pointer_interconvertible_base_of_v<D, G>);
static_assert (!std::is_pointer_interconvertible_base_of_v<D, I>);
static_assert (std::is_pointer_interconvertible_base_of_v<H, volatile I>);
Is that a bug in my patch or is MSVC buggy on these (or mix thereof)?
https://godbolt.org/z/aYeYnne9d
shows the second testcase, here it differs on:
static_assert (std::is_pointer_interconvertible_with_class<F, int> (&F::b));
static_assert (std::is_pointer_interconvertible_with_class<I, int> (&I::g));
static_assert (std::is_pointer_interconvertible_with_class<L, int> (&L::b));
static_assert (std::is_pointer_interconvertible_with_class (&V::a));
static_assert (std::is_pointer_interconvertible_with_class (&V::b));
Again, my bug, MSVC bug, mix thereof?
According to Jason the <D, G>, <D, I> case are the subject of the
CWG 2254 above discussed change and the rest are likely MSVC bugs.

Oh, and there is another thing, the standard has an example:
struct A { int a; };                    // a standard-layout class
struct B { int b; };                    // a standard-layout class
struct C: public A, public B { };       // not a standard-layout class

static_assert( is_pointer_interconvertible_with_class( &C::b ) );
  // Succeeds because, despite its appearance, &C::b has type
  // “pointer to member of B of type int”.
static_assert( is_pointer_interconvertible_with_class<C>( &C::b ) );
  // Forces the use of class C, and fails.
It seems to work as written with MSVC (second assertion fails),
but fails with GCC with the patch:
/tmp/1.C:22:57: error: no matching function for call to ‘is_pointer_interconvertible_with_class<C>(int B::*)’
   22 | static_assert( is_pointer_interconvertible_with_class<C>( &C::b ) );
      |                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~
/tmp/1.C:8:1: note: candidate: ‘template<class S, class M> constexpr bool std::is_pointer_interconvertible_with_class(M S::*)’
    8 | is_pointer_interconvertible_with_class (M S::*m) noexcept
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/tmp/1.C:8:1: note:   template argument deduction/substitution failed:
/tmp/1.C:22:57: note:   mismatched types ‘C’ and ‘B’
   22 | static_assert( is_pointer_interconvertible_with_class<C>( &C::b ) );
      |                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~
the second int argument isn't deduced.

This boils down to:
template <class S, class M>
bool foo (M S::*m) noexcept;
struct A { int a; };
struct B { int b; };
struct C : public A, public B {};
bool a = foo (&C::b);
bool b = foo<C, int> (&C::b);
bool c = foo<C> (&C::b);
which with /std:c++20 or -std=c++20 is accepted by latest MSVC and ICC but
rejected by GCC and clang (in both cases on the last line).
Is this a GCC/clang bug in argument deduction (in that case I think we want
a separate PR), or a bug in ICC/MSVC and the standard itself that should
specify in the examples both template arguments instead of just the first?
And this has been raised with the CWG.

2021-07-30  Jakub Jelinek  <jakub@redhat.com>

	PR c++/101539
gcc/c-family/
	* c-common.h (enum rid): Add RID_IS_POINTER_INTERCONVERTIBLE_BASE_OF.
	* c-common.c (c_common_reswords): Add
	__is_pointer_interconvertible_base_of.
gcc/cp/
	* cp-tree.h (enum cp_trait_kind): Add
	CPTK_IS_POINTER_INTERCONVERTIBLE_BASE_OF.
	(enum cp_built_in_function): Add
	CP_BUILT_IN_IS_POINTER_INTERCONVERTIBLE_WITH_CLASS.
	(fold_builtin_is_pointer_inverconvertible_with_class): Declare.
	* parser.c (cp_parser_primary_expression): Handle
	RID_IS_POINTER_INTERCONVERTIBLE_BASE_OF.
	(cp_parser_trait_expr): Likewise.
	* cp-objcp-common.c (names_builtin_p): Likewise.
	* constraint.cc (diagnose_trait_expr): Handle
	CPTK_IS_POINTER_INTERCONVERTIBLE_BASE_OF.
	* decl.c (cxx_init_decl_processing): Register
	__builtin_is_pointer_interconvertible_with_class builtin.
	* constexpr.c (cxx_eval_builtin_function_call): Handle
	CP_BUILT_IN_IS_POINTER_INTERCONVERTIBLE_WITH_CLASS builtin.
	* semantics.c (pointer_interconvertible_base_of_p,
	first_nonstatic_data_member_p,
	fold_builtin_is_pointer_inverconvertible_with_class): New functions.
	(trait_expr_value): Handle CPTK_IS_POINTER_INTERCONVERTIBLE_BASE_OF.
	(finish_trait_expr): Likewise.  Formatting fix.
	* cp-gimplify.c (cp_gimplify_expr): Fold
	CP_BUILT_IN_IS_POINTER_INTERCONVERTIBLE_WITH_CLASS.  Call
	fndecl_built_in_p just once.
	(cp_fold): Likewise.
	* tree.c (builtin_valid_in_constant_expr_p): Handle
	CP_BUILT_IN_IS_POINTER_INTERCONVERTIBLE_WITH_CLASS.  Call
	fndecl_built_in_p just once.
	* cxx-pretty-print.c (pp_cxx_trait_expression): Handle
	CPTK_IS_POINTER_INTERCONVERTIBLE_BASE_OF.
gcc/testsuite/
	* g++.dg/cpp2a/is-pointer-interconvertible-base-of1.C: New test.
	* g++.dg/cpp2a/is-pointer-interconvertible-with-class1.C: New test.
	* g++.dg/cpp2a/is-pointer-interconvertible-with-class2.C: New test.
	* g++.dg/cpp2a/is-pointer-interconvertible-with-class3.C: New test.
	* g++.dg/cpp2a/is-pointer-interconvertible-with-class4.C: New test.
	* g++.dg/cpp2a/is-pointer-interconvertible-with-class5.C: New test.
	* g++.dg/cpp2a/is-pointer-interconvertible-with-class6.C: New test.

19 files changed, 578 insertions, 24 deletions

diff --git a/gcc/c-family/c-common.c b/gcc/c-family/c-common.c
index 21da679..00ac3c5 100644
--- a/gcc/c-family/c-common.c
+++ b/gcc/c-family/c-common.c
@@ -421,6 +421,8 @@ const struct c_common_resword c_common_reswords[] =
   { "__is_enum",	RID_IS_ENUM,	D_CXXONLY },
   { "__is_final",	RID_IS_FINAL,	D_CXXONLY },
   { "__is_literal_type", RID_IS_LITERAL_TYPE, D_CXXONLY },
+  { "__is_pointer_interconvertible_base_of",
+			RID_IS_POINTER_INTERCONVERTIBLE_BASE_OF, D_CXXONLY },
   { "__is_pod",		RID_IS_POD,	D_CXXONLY },
   { "__is_polymorphic",	RID_IS_POLYMORPHIC, D_CXXONLY },
   { "__is_same",     RID_IS_SAME_AS, D_CXXONLY },
diff --git a/gcc/c-family/c-common.h b/gcc/c-family/c-common.h
index c4b2789e..65d8c1c 100644
--- a/gcc/c-family/c-common.h
+++ b/gcc/c-family/c-common.h
@@ -174,6 +174,7 @@ enum rid
   RID_IS_BASE_OF,              RID_IS_CLASS,
   RID_IS_EMPTY,                RID_IS_ENUM,
   RID_IS_FINAL,                RID_IS_LITERAL_TYPE,
+  RID_IS_POINTER_INTERCONVERTIBLE_BASE_OF,
   RID_IS_POD,                  RID_IS_POLYMORPHIC,
   RID_IS_SAME_AS,
   RID_IS_STD_LAYOUT,           RID_IS_TRIVIAL,
diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 31fa5b6..1af365d 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -1427,8 +1427,20 @@ cxx_eval_builtin_function_call (const constexpr_ctx *ctx, tree t, tree fun,
       && ctx->call
       && ctx->call->fundef)
     current_function_decl = ctx->call->fundef->decl;
-  new_call = fold_builtin_call_array (EXPR_LOCATION (t), TREE_TYPE (t),
-				      CALL_EXPR_FN (t), nargs, args);
+  if (fndecl_built_in_p (fun,
+			 CP_BUILT_IN_IS_POINTER_INTERCONVERTIBLE_WITH_CLASS,
+			 BUILT_IN_FRONTEND))
+    {
+      location_t loc = EXPR_LOCATION (t);
+      if (nargs >= 1)
+	VERIFY_CONSTANT (args[0]);
+      new_call
+	= fold_builtin_is_pointer_inverconvertible_with_class (loc, nargs,
+							       args);
+    }
+  else
+    new_call = fold_builtin_call_array (EXPR_LOCATION (t), TREE_TYPE (t),
+					CALL_EXPR_FN (t), nargs, args);
   current_function_decl = save_cur_fn;
   force_folding_builtin_constant_p = save_ffbcp;
   if (new_call == NULL)
diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index 4ee5215..e608c5a 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -3631,6 +3631,10 @@ diagnose_trait_expr (tree expr, tree args)
     case CPTK_IS_LITERAL_TYPE:
       inform (loc, "  %qT is not a literal type", t1);
       break;
+    case CPTK_IS_POINTER_INTERCONVERTIBLE_BASE_OF:
+      inform (loc, "  %qT is not pointer-interconvertible base of %qT",
+	      t1, t2);
+      break;
     case CPTK_IS_POD:
       inform (loc, "  %qT is not a POD type", t1);
       break;
diff --git a/gcc/cp/cp-gimplify.c b/gcc/cp/cp-gimplify.c
index 0520fa4..6e274ac 100644
--- a/gcc/cp/cp-gimplify.c
+++ b/gcc/cp/cp-gimplify.c
@@ -648,14 +648,23 @@ cp_gimplify_expr (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p)
       if (ret != GS_ERROR)
 	{
 	  tree decl = cp_get_callee_fndecl_nofold (*expr_p);
-	  if (decl
-	      && fndecl_built_in_p (decl, CP_BUILT_IN_IS_CONSTANT_EVALUATED,
-				    BUILT_IN_FRONTEND))
-	    *expr_p = boolean_false_node;
-	  else if (decl
-		   && fndecl_built_in_p (decl, CP_BUILT_IN_SOURCE_LOCATION,
-					 BUILT_IN_FRONTEND))
-	    *expr_p = fold_builtin_source_location (EXPR_LOCATION (*expr_p));
+	  if (decl && fndecl_built_in_p (decl, BUILT_IN_FRONTEND))
+	    switch (DECL_FE_FUNCTION_CODE (decl))
+	      {
+	      case CP_BUILT_IN_IS_CONSTANT_EVALUATED:
+		*expr_p = boolean_false_node;
+		break;
+	      case CP_BUILT_IN_SOURCE_LOCATION:
+		*expr_p
+		  = fold_builtin_source_location (EXPR_LOCATION (*expr_p));
+		break;
+	      case CP_BUILT_IN_IS_POINTER_INTERCONVERTIBLE_WITH_CLASS:
+		*expr_p
+		  = fold_builtin_is_pointer_inverconvertible_with_class
+			(EXPR_LOCATION (*expr_p), call_expr_nargs (*expr_p),
+			 &CALL_EXPR_ARG (*expr_p, 0));
+		break;
+	      }
 	}
       break;
 
@@ -2560,11 +2569,26 @@ cp_fold (tree x)
 	    && DECL_DECLARED_CONSTEXPR_P (current_function_decl))
 	  nw = 1;
 
-	/* Defer folding __builtin_is_constant_evaluated.  */
-	if (callee
-	    && fndecl_built_in_p (callee, CP_BUILT_IN_IS_CONSTANT_EVALUATED,
-				  BUILT_IN_FRONTEND))
-	  break;
+	if (callee && fndecl_built_in_p (callee, BUILT_IN_FRONTEND))
+	  {
+	    switch (DECL_FE_FUNCTION_CODE (callee))
+	      {
+		/* Defer folding __builtin_is_constant_evaluated.  */
+	      case CP_BUILT_IN_IS_CONSTANT_EVALUATED:
+		break;
+	      case CP_BUILT_IN_SOURCE_LOCATION:
+		x = fold_builtin_source_location (EXPR_LOCATION (x));
+		break;
+	      case CP_BUILT_IN_IS_POINTER_INTERCONVERTIBLE_WITH_CLASS:
+                x = fold_builtin_is_pointer_inverconvertible_with_class
+			(EXPR_LOCATION (x), call_expr_nargs (x),
+			 &CALL_EXPR_ARG (x, 0));
+		break;
+	      default:
+		break;
+	      }
+	    break;
+	  }
 
 	if (callee
 	    && fndecl_built_in_p (callee, CP_BUILT_IN_SOURCE_LOCATION,
diff --git a/gcc/cp/cp-objcp-common.c b/gcc/cp/cp-objcp-common.c
index ee25573..beef012 100644
--- a/gcc/cp/cp-objcp-common.c
+++ b/gcc/cp/cp-objcp-common.c
@@ -414,6 +414,7 @@ names_builtin_p (const char *name)
     case RID_IS_ENUM:
     case RID_IS_FINAL:
     case RID_IS_LITERAL_TYPE:
+    case RID_IS_POINTER_INTERCONVERTIBLE_BASE_OF:
     case RID_IS_POD:
     case RID_IS_POLYMORPHIC:
     case RID_IS_SAME_AS:
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index ddf8f43..9a47a87 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -1366,6 +1366,7 @@ enum cp_trait_kind
   CPTK_IS_ENUM,
   CPTK_IS_FINAL,
   CPTK_IS_LITERAL_TYPE,
+  CPTK_IS_POINTER_INTERCONVERTIBLE_BASE_OF,
   CPTK_IS_POD,
   CPTK_IS_POLYMORPHIC,
   CPTK_IS_SAME_AS,
@@ -6355,6 +6356,7 @@ struct GTY((chain_next ("%h.next"))) tinst_level {
 enum cp_built_in_function {
   CP_BUILT_IN_IS_CONSTANT_EVALUATED,
   CP_BUILT_IN_INTEGER_PACK,
+  CP_BUILT_IN_IS_POINTER_INTERCONVERTIBLE_WITH_CLASS,
   CP_BUILT_IN_SOURCE_LOCATION,
   CP_BUILT_IN_LAST
 };
@@ -7570,6 +7572,7 @@ extern tree baselink_for_fns                    (tree);
 extern void finish_static_assert                (tree, tree, location_t,
 						 bool, bool);
 extern tree finish_decltype_type                (tree, bool, tsubst_flags_t);
+extern tree fold_builtin_is_pointer_inverconvertible_with_class (location_t, int, tree *);
 extern tree finish_trait_expr			(location_t, enum cp_trait_kind, tree, tree);
 extern tree build_lambda_expr                   (void);
 extern tree build_lambda_object			(tree);
diff --git a/gcc/cp/cxx-pretty-print.c b/gcc/cp/cxx-pretty-print.c
index 3709d0f..b899162 100644
--- a/gcc/cp/cxx-pretty-print.c
+++ b/gcc/cp/cxx-pretty-print.c
@@ -2645,6 +2645,9 @@ pp_cxx_trait_expression (cxx_pretty_printer *pp, tree t)
     case CPTK_IS_FINAL:
       pp_cxx_ws_string (pp, "__is_final");
       break;
+    case CPTK_IS_POINTER_INTERCONVERTIBLE_BASE_OF:
+      pp_cxx_ws_string (pp, "__is_pointer_interconvertible_base_of");
+      break;
     case CPTK_IS_POD:
       pp_cxx_ws_string (pp, "__is_pod");
       break;
@@ -2695,7 +2698,9 @@ pp_cxx_trait_expression (cxx_pretty_printer *pp, tree t)
   pp_cxx_left_paren (pp);
   pp->type_id (TRAIT_EXPR_TYPE1 (t));
 
-  if (kind == CPTK_IS_BASE_OF || kind == CPTK_IS_SAME_AS)
+  if (kind == CPTK_IS_BASE_OF
+      || kind == CPTK_IS_SAME_AS
+      || kind == CPTK_IS_POINTER_INTERCONVERTIBLE_BASE_OF)
     {
       pp_cxx_separate_with (pp, ',');
       pp->type_id (TRAIT_EXPR_TYPE2 (t));
diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 71308a0..e4be6be 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -4467,6 +4467,15 @@ cxx_init_decl_processing (void)
 			       BUILT_IN_FRONTEND, NULL, NULL_TREE);
   set_call_expr_flags (decl, ECF_CONST | ECF_NOTHROW | ECF_LEAF);
 
+  tree bool_vaftype = build_varargs_function_type_list (boolean_type_node,
+							NULL_TREE);
+  decl
+    = add_builtin_function ("__builtin_is_pointer_interconvertible_with_class",
+			    bool_vaftype,
+			    CP_BUILT_IN_IS_POINTER_INTERCONVERTIBLE_WITH_CLASS,
+			    BUILT_IN_FRONTEND, NULL, NULL_TREE);
+  set_call_expr_flags (decl, ECF_CONST | ECF_NOTHROW | ECF_LEAF);
+
   integer_two_node = build_int_cst (NULL_TREE, 2);
 
   /* Guess at the initial static decls size.  */
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 97078f9..ab74e9d 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -5799,6 +5799,7 @@ cp_parser_primary_expression (cp_parser *parser,
 	case RID_IS_ENUM:
 	case RID_IS_FINAL:
 	case RID_IS_LITERAL_TYPE:
+	case RID_IS_POINTER_INTERCONVERTIBLE_BASE_OF:
 	case RID_IS_POD:
 	case RID_IS_POLYMORPHIC:
 	case RID_IS_SAME_AS:
@@ -10688,6 +10689,10 @@ cp_parser_trait_expr (cp_parser* parser, enum rid keyword)
     case RID_IS_LITERAL_TYPE:
       kind = CPTK_IS_LITERAL_TYPE;
       break;
+    case RID_IS_POINTER_INTERCONVERTIBLE_BASE_OF:
+      kind = CPTK_IS_POINTER_INTERCONVERTIBLE_BASE_OF;
+      binary = true;
+      break;
     case RID_IS_POD:
       kind = CPTK_IS_POD;
       break;
diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index f64b084..34e5d76 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -10566,6 +10566,110 @@ classtype_has_nothrow_assign_or_copy_p (tree type, bool assign_p)
   return saw_copy;
 }
 
+/* Return true if DERIVED is pointer interconvertible base of BASE.  */
+
+static bool
+pointer_interconvertible_base_of_p (tree base, tree derived)
+{
+  if (base == error_mark_node || derived == error_mark_node)
+    return false;
+  base = TYPE_MAIN_VARIANT (base);
+  derived = TYPE_MAIN_VARIANT (derived);
+  if (!NON_UNION_CLASS_TYPE_P (base)
+      || !NON_UNION_CLASS_TYPE_P (derived))
+    return false;
+    
+  if (same_type_p (base, derived))
+    return true;
+
+  if (!std_layout_type_p (derived))
+    return false;
+
+  return uniquely_derived_from_p (base, derived);
+}
+
+/* Helper function for fold_builtin_is_pointer_inverconvertible_with_class,
+   return true if MEMBERTYPE is the type of the first non-static data member
+   of TYPE or for unions of any members.  */
+static bool
+first_nonstatic_data_member_p (tree type, tree membertype)
+{
+  for (tree field = TYPE_FIELDS (type); field; field = DECL_CHAIN (field))
+    {
+      if (TREE_CODE (field) != FIELD_DECL)
+	continue;
+      if (DECL_FIELD_IS_BASE (field) && is_empty_field (field))
+	continue;
+      if (DECL_FIELD_IS_BASE (field))
+	return first_nonstatic_data_member_p (TREE_TYPE (field), membertype);
+      if (ANON_AGGR_TYPE_P (TREE_TYPE (field)))
+	{
+	  if ((TREE_CODE (TREE_TYPE (field)) == UNION_TYPE
+	       || std_layout_type_p (TREE_TYPE (field)))
+	      && first_nonstatic_data_member_p (TREE_TYPE (field), membertype))
+	    return true;
+	}
+      else if (same_type_ignoring_top_level_qualifiers_p (TREE_TYPE (field),
+							  membertype))
+	return true;
+      if (TREE_CODE (type) != UNION_TYPE)
+	return false;
+    }
+  return false;
+}
+
+/* Fold __builtin_is_pointer_interconvertible_with_class call.  */
+
+tree
+fold_builtin_is_pointer_inverconvertible_with_class (location_t loc, int nargs,
+						     tree *args)
+{
+  /* Unless users call the builtin directly, the following 3 checks should be
+     ensured from std::is_pointer_interconvertible_with_class function
+     template.  */
+  if (nargs != 1)
+    {
+      error_at (loc, "%<__builtin_is_pointer_interconvertible_with_class%> "
+		     "needs a single argument");
+      return boolean_false_node;
+    }
+  tree arg = args[0];
+  if (error_operand_p (arg))
+    return boolean_false_node;
+  if (!TYPE_PTRMEM_P (TREE_TYPE (arg)))
+    {
+      error_at (loc, "%<__builtin_is_pointer_interconvertible_with_class%> "
+		     "argument is not pointer to member");
+      return boolean_false_node;
+    }
+
+  if (!TYPE_PTRDATAMEM_P (TREE_TYPE (arg)))
+    return boolean_false_node;
+
+  tree membertype = TREE_TYPE (TREE_TYPE (arg));
+  tree basetype = TYPE_OFFSET_BASETYPE (TREE_TYPE (arg));
+  if (!complete_type_or_else (basetype, NULL_TREE))
+    return boolean_false_node;
+
+  if (TREE_CODE (basetype) != UNION_TYPE
+      && !std_layout_type_p (basetype))
+    return boolean_false_node;
+
+  if (!first_nonstatic_data_member_p (basetype, membertype))
+    return boolean_false_node;
+
+  if (TREE_CODE (arg) == PTRMEM_CST)
+    arg = cplus_expand_constant (arg);
+
+  if (integer_nonzerop (arg))
+    return boolean_false_node;
+  if (integer_zerop (arg))
+    return boolean_true_node;
+
+  return fold_build2 (EQ_EXPR, boolean_type_node, arg,
+		      build_zero_cst (TREE_TYPE (arg)));
+}
+
 /* Actually evaluates the trait.  */
 
 static bool
@@ -10659,6 +10763,9 @@ trait_expr_value (cp_trait_kind kind, tree type1, tree type2)
     case CPTK_IS_LITERAL_TYPE:
       return literal_type_p (type1);
 
+    case CPTK_IS_POINTER_INTERCONVERTIBLE_BASE_OF:
+      return pointer_interconvertible_base_of_p (type1, type2);
+
     case CPTK_IS_POD:
       return pod_type_p (type1);
 
@@ -10786,6 +10893,7 @@ finish_trait_expr (location_t loc, cp_trait_kind kind, tree type1, tree type2)
       break;
 
     case CPTK_IS_BASE_OF:
+    case CPTK_IS_POINTER_INTERCONVERTIBLE_BASE_OF:
       if (NON_UNION_CLASS_TYPE_P (type1) && NON_UNION_CLASS_TYPE_P (type2)
 	  && !same_type_ignoring_top_level_qualifiers_p (type1, type2)
 	  && !complete_type_or_else (type2, NULL_TREE))
@@ -10803,9 +10911,9 @@ finish_trait_expr (location_t loc, cp_trait_kind kind, tree type1, tree type2)
       gcc_unreachable ();
     }
 
-tree val = (trait_expr_value (kind, type1, type2)
-	    ? boolean_true_node : boolean_false_node);
- return maybe_wrap_with_location (val, loc);
+  tree val = (trait_expr_value (kind, type1, type2)
+	      ? boolean_true_node : boolean_false_node);
+  return maybe_wrap_with_location (val, loc);
 }
 
 /* Do-nothing variants of functions to handle pragma FLOAT_CONST_DECIMAL64,
diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
index 2a14fa9..8345396 100644
--- a/gcc/cp/tree.c
+++ b/gcc/cp/tree.c
@@ -450,11 +450,16 @@ builtin_valid_in_constant_expr_p (const_tree decl)
     return false;
   if (DECL_BUILT_IN_CLASS (decl) != BUILT_IN_NORMAL)
     {
-      if (fndecl_built_in_p (decl, CP_BUILT_IN_IS_CONSTANT_EVALUATED,
-			     BUILT_IN_FRONTEND)
-	  || fndecl_built_in_p (decl, CP_BUILT_IN_SOURCE_LOCATION,
-				BUILT_IN_FRONTEND))
-	return true;
+      if (fndecl_built_in_p (decl, BUILT_IN_FRONTEND))
+	switch (DECL_FE_FUNCTION_CODE (decl))
+	  {
+	  case CP_BUILT_IN_IS_CONSTANT_EVALUATED:
+	  case CP_BUILT_IN_SOURCE_LOCATION:
+	  case CP_BUILT_IN_IS_POINTER_INTERCONVERTIBLE_WITH_CLASS:
+	    return true;
+	  default:
+	    break;
+	  }
       /* Not a built-in.  */
       return false;
     }
diff --git a/gcc/testsuite/g++.dg/cpp2a/is-pointer-interconvertible-base-of1.C b/gcc/testsuite/g++.dg/cpp2a/is-pointer-interconvertible-base-of1.C
new file mode 100644
index 0000000..d94f18f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/is-pointer-interconvertible-base-of1.C
@@ -0,0 +1,55 @@
+// P0466R5
+// { dg-do compile { target c++20 } }
+
+namespace std
+{
+template <typename T, T v>
+struct integral_constant
+{
+  static constexpr T value = v;
+};
+
+template <typename, typename>
+struct is_pointer_interconvertible_base_of;
+
+template<typename T, typename U>
+struct is_pointer_interconvertible_base_of
+  : public integral_constant <bool, __is_pointer_interconvertible_base_of (T, U)>
+{
+};
+
+template <typename T, typename U>
+inline constexpr bool is_pointer_interconvertible_base_of_v = __is_pointer_interconvertible_base_of (T, U);
+}
+
+struct A;
+struct B { int b; };
+struct C : virtual B { int c; };
+struct D {};
+struct E {};
+struct F : public B, D, E {};
+struct G : public D, E { int g; };
+struct H {};
+struct I : public G, H {};
+struct J { int j1; private: int j2; };
+struct K : public J {};
+union U { int a; };
+
+static_assert (std::is_pointer_interconvertible_base_of<A, A>::value);
+static_assert (std::is_pointer_interconvertible_base_of_v<A, A>);
+static_assert (std::is_pointer_interconvertible_base_of_v<const A, volatile A>);
+static_assert (std::is_pointer_interconvertible_base_of_v<B, const B>);
+static_assert (std::is_pointer_interconvertible_base_of_v<C, const volatile C>);
+static_assert (!std::is_pointer_interconvertible_base_of_v<D, E>);
+static_assert (!std::is_pointer_interconvertible_base_of_v<D, const B>);
+static_assert (std::is_pointer_interconvertible_base_of_v<const B, F>);
+static_assert (std::is_pointer_interconvertible_base_of_v<D, const F>);
+static_assert (std::is_pointer_interconvertible_base_of_v<E, F>);
+static_assert (std::is_pointer_interconvertible_base_of_v<D, volatile G>);
+static_assert (std::is_pointer_interconvertible_base_of_v<const E, volatile G>);
+static_assert (std::is_pointer_interconvertible_base_of_v<D, I>);
+static_assert (std::is_pointer_interconvertible_base_of_v<const E, const I>);
+static_assert (std::is_pointer_interconvertible_base_of_v<G, I>);
+static_assert (std::is_pointer_interconvertible_base_of_v<H, volatile I>);
+static_assert (!std::is_pointer_interconvertible_base_of_v<volatile J, const K>);
+static_assert (!std::is_pointer_interconvertible_base_of_v<U, U>);
diff --git a/gcc/testsuite/g++.dg/cpp2a/is-pointer-interconvertible-with-class1.C b/gcc/testsuite/g++.dg/cpp2a/is-pointer-interconvertible-with-class1.C
new file mode 100644
index 0000000..efce355
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/is-pointer-interconvertible-with-class1.C
@@ -0,0 +1,65 @@
+// P0466R5
+// { dg-do compile { target c++20 } }
+
+namespace std
+{
+template <class S, class M>
+constexpr bool
+is_pointer_interconvertible_with_class (M S::*m) noexcept
+{
+  return __builtin_is_pointer_interconvertible_with_class (m);
+}
+}
+
+struct A;
+struct B { int b; double b2; };
+struct C : virtual B { int c; };
+struct D {};
+struct E {};
+struct F : public B, D, E {};
+struct G : public D, E { int g; };
+struct H {};
+struct I : public G, H {};
+struct J { int j1; private: int j2; public: int j3; };
+struct K : public J {};
+struct L : public B, D, E {};
+struct M { D d [[no_unique_address]]; E e [[no_unique_address]]; int f; };
+union U { int a; double b; long long c; };
+struct V { union { int a; long b; }; int c; };
+union X { int a; union { short b; long c; }; long long d; };
+struct Y { void foo () {} };
+union Z { int a; private: int b; protected: int c; public: int d; };
+
+static_assert (std::is_pointer_interconvertible_with_class (&B::b));
+static_assert (!std::is_pointer_interconvertible_with_class (&B::b2));
+static_assert (std::is_pointer_interconvertible_with_class (&C::b));
+static_assert (std::is_pointer_interconvertible_with_class (&F::b));
+static_assert (std::is_pointer_interconvertible_with_class<F, int> (&F::b));
+static_assert (std::is_pointer_interconvertible_with_class (&G::g));
+static_assert (std::is_pointer_interconvertible_with_class<G, int> (&G::g));
+static_assert (std::is_pointer_interconvertible_with_class (&I::g));
+static_assert (std::is_pointer_interconvertible_with_class<I, int> (&I::g));
+static_assert (!std::is_pointer_interconvertible_with_class (&J::j1));
+static_assert (!std::is_pointer_interconvertible_with_class (&J::j3));
+static_assert (!std::is_pointer_interconvertible_with_class (&K::j1));
+static_assert (!std::is_pointer_interconvertible_with_class (&K::j3));
+static_assert (std::is_pointer_interconvertible_with_class (&L::b));
+static_assert (std::is_pointer_interconvertible_with_class<L, int> (&L::b));
+static_assert (std::is_pointer_interconvertible_with_class (&L::b));
+static_assert (std::is_pointer_interconvertible_with_class (&M::d));
+static_assert (!std::is_pointer_interconvertible_with_class (&M::e));
+static_assert (!std::is_pointer_interconvertible_with_class (&M::f));
+static_assert (std::is_pointer_interconvertible_with_class (&U::a));
+static_assert (std::is_pointer_interconvertible_with_class (&U::b));
+static_assert (std::is_pointer_interconvertible_with_class (&U::c));
+static_assert (std::is_pointer_interconvertible_with_class (&V::a));
+static_assert (std::is_pointer_interconvertible_with_class (&V::b));
+static_assert (!std::is_pointer_interconvertible_with_class (&V::c));
+static_assert (std::is_pointer_interconvertible_with_class (&X::a));
+static_assert (std::is_pointer_interconvertible_with_class (&X::b));
+static_assert (std::is_pointer_interconvertible_with_class (&X::c));
+static_assert (std::is_pointer_interconvertible_with_class (&X::d));
+static_assert (!std::is_pointer_interconvertible_with_class ((int B::*) nullptr));
+static_assert (!std::is_pointer_interconvertible_with_class (&Y::foo));
+static_assert (std::is_pointer_interconvertible_with_class (&Z::a));
+static_assert (std::is_pointer_interconvertible_with_class (&Z::d));
diff --git a/gcc/testsuite/g++.dg/cpp2a/is-pointer-interconvertible-with-class2.C b/gcc/testsuite/g++.dg/cpp2a/is-pointer-interconvertible-with-class2.C
new file mode 100644
index 0000000..b663a39
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/is-pointer-interconvertible-with-class2.C
@@ -0,0 +1,135 @@
+// P0466R5
+// { dg-do run { target c++20 } }
+
+namespace std
+{
+template <class S, class M>
+constexpr bool
+is_pointer_interconvertible_with_class (M S::*m) noexcept
+{
+  return __builtin_is_pointer_interconvertible_with_class (m);
+}
+}
+
+struct A;
+struct B { int b; double b2; };
+struct C : virtual B { int c; };
+struct D {};
+struct E {};
+struct F : public B, D, E {};
+struct G : public D, E { int g; };
+struct H {};
+struct I : public G, H {};
+struct J { int j1; private: int j2; public: int j3; };
+struct K : public J {};
+struct L : public B, D, E {};
+struct M { D d [[no_unique_address]]; E e [[no_unique_address]]; int f; };
+union U { int a; double b; long long c; };
+struct V { union { int a; long b; }; int c; };
+union X { int a; union { short b; long c; }; long long d; };
+struct Y { void foo () {} };
+union Z { int a; private: int b; protected: int c; public: int d; };
+
+int
+main ()
+{
+  auto t1 = &B::b;
+  if (!std::is_pointer_interconvertible_with_class (t1))
+    __builtin_abort ();
+  auto t2 = &B::b2;
+  if (std::is_pointer_interconvertible_with_class (t2))
+    __builtin_abort ();
+  auto t3 = &C::b;
+  if (!std::is_pointer_interconvertible_with_class (t3))
+    __builtin_abort ();
+  auto t4 = &F::b;
+  if (!std::is_pointer_interconvertible_with_class (t4))
+    __builtin_abort ();
+  int F::*t5 = &F::b;
+  if (!std::is_pointer_interconvertible_with_class (t5))
+    __builtin_abort ();
+  auto t6 = &G::g;
+  if (!std::is_pointer_interconvertible_with_class (t6))
+    __builtin_abort ();
+  int G::*t7 = &G::g;
+  if (!std::is_pointer_interconvertible_with_class (t7))
+    __builtin_abort ();
+  auto t8 = &I::g;
+  if (!std::is_pointer_interconvertible_with_class (t8))
+    __builtin_abort ();
+  int I::*t9 = &I::g;
+  if (!std::is_pointer_interconvertible_with_class (t9))
+    __builtin_abort ();
+  auto t10 = &J::j1;
+  if (std::is_pointer_interconvertible_with_class (t10))
+    __builtin_abort ();
+  auto t11 = &J::j3;
+  if (std::is_pointer_interconvertible_with_class (t11))
+    __builtin_abort ();
+  auto t12 = &K::j1;
+  if (std::is_pointer_interconvertible_with_class (t12))
+    __builtin_abort ();
+  auto t13 = &K::j3;
+  if (std::is_pointer_interconvertible_with_class (t13))
+    __builtin_abort ();
+  auto t14 = &L::b;
+  if (!std::is_pointer_interconvertible_with_class (t14))
+    __builtin_abort ();
+  int L::*t15 = &L::b;
+  if (!std::is_pointer_interconvertible_with_class (t15))
+    __builtin_abort ();
+  auto t16 = &L::b;
+  if (!std::is_pointer_interconvertible_with_class (t16))
+    __builtin_abort ();
+  auto t17 = &M::d;
+  if (!std::is_pointer_interconvertible_with_class (t17))
+    __builtin_abort ();
+  auto t18 = &M::e;
+  if (std::is_pointer_interconvertible_with_class (t18))
+    __builtin_abort ();
+  auto t19 = &M::f;
+  if (std::is_pointer_interconvertible_with_class (t19))
+    __builtin_abort ();
+  auto t20 = &U::a;
+  if (!std::is_pointer_interconvertible_with_class (t20))
+    __builtin_abort ();
+  auto t21 = &U::b;
+  if (!std::is_pointer_interconvertible_with_class (t21))
+    __builtin_abort ();
+  auto t22 = &U::c;
+  if (!std::is_pointer_interconvertible_with_class (t22))
+    __builtin_abort ();
+  auto t23 = &V::a;
+  if (!std::is_pointer_interconvertible_with_class (t23))
+    __builtin_abort ();
+  auto t24 = &V::b;
+  if (!std::is_pointer_interconvertible_with_class (t24))
+    __builtin_abort ();
+  auto t25 = &V::c;
+  if (std::is_pointer_interconvertible_with_class (t25))
+    __builtin_abort ();
+  auto t26 = &X::a;
+  if (!std::is_pointer_interconvertible_with_class (t26))
+    __builtin_abort ();
+  auto t27 = &X::b;
+  if (!std::is_pointer_interconvertible_with_class (t27))
+    __builtin_abort ();
+  auto t28 = &X::c;
+  if (!std::is_pointer_interconvertible_with_class (t28))
+    __builtin_abort ();
+  auto t29 = &X::d;
+  if (!std::is_pointer_interconvertible_with_class (t29))
+    __builtin_abort ();
+  auto t30 = (int B::*) nullptr;
+  if (std::is_pointer_interconvertible_with_class (t30))
+    __builtin_abort ();
+  auto t31 = &Y::foo;
+  if (std::is_pointer_interconvertible_with_class (t31))
+    __builtin_abort ();
+  auto t32 = &Z::a;
+  if (!std::is_pointer_interconvertible_with_class (t32))
+    __builtin_abort ();
+  auto t33 = &Z::d;
+  if (!std::is_pointer_interconvertible_with_class (t33))
+    __builtin_abort ();
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/is-pointer-interconvertible-with-class3.C b/gcc/testsuite/g++.dg/cpp2a/is-pointer-interconvertible-with-class3.C
new file mode 100644
index 0000000..b02f1b9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/is-pointer-interconvertible-with-class3.C
@@ -0,0 +1,11 @@
+// P0466R5
+// { dg-do compile { target c++20 } }
+
+struct A { int a; };
+struct B;
+
+bool a = __builtin_is_pointer_interconvertible_with_class ();			// { dg-error "needs a single argument" }
+bool b = __builtin_is_pointer_interconvertible_with_class (&A::a, &A::a);	// { dg-error "needs a single argument" }
+bool c = __builtin_is_pointer_interconvertible_with_class (1);			// { dg-error "argument is not pointer to member" }
+bool d = __builtin_is_pointer_interconvertible_with_class (1.0);		// { dg-error "argument is not pointer to member" }
+bool e = __builtin_is_pointer_interconvertible_with_class ((int B::*) nullptr);	// { dg-error "invalid use of incomplete type" }
diff --git a/gcc/testsuite/g++.dg/cpp2a/is-pointer-interconvertible-with-class4.C b/gcc/testsuite/g++.dg/cpp2a/is-pointer-interconvertible-with-class4.C
new file mode 100644
index 0000000..e5e2451
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/is-pointer-interconvertible-with-class4.C
@@ -0,0 +1,31 @@
+// P0466R5
+// { dg-do compile { target c++20 } }
+// { dg-options "" }
+
+namespace std
+{
+template <class S, class M>
+constexpr bool
+is_pointer_interconvertible_with_class (M S::*m) noexcept
+{
+  return __builtin_is_pointer_interconvertible_with_class (m);
+}
+}
+
+struct W { struct { int a; long b; }; int c; };
+union X { int a; struct { short b; long c; }; long long d; };
+struct D { int x; private: int y; };                                                                                                                                                  
+union Y { int a; struct { short b; long c; D z; }; long long d; };                                                                                                                    
+
+static_assert (std::is_pointer_interconvertible_with_class (&W::a));
+static_assert (!std::is_pointer_interconvertible_with_class (&W::b));
+static_assert (!std::is_pointer_interconvertible_with_class (&W::c));
+static_assert (std::is_pointer_interconvertible_with_class (&X::a));
+static_assert (std::is_pointer_interconvertible_with_class (&X::b));
+static_assert (!std::is_pointer_interconvertible_with_class (&X::c));
+static_assert (std::is_pointer_interconvertible_with_class (&X::d));
+static_assert (std::is_pointer_interconvertible_with_class (&Y::a));
+static_assert (!std::is_pointer_interconvertible_with_class (&Y::b));
+static_assert (!std::is_pointer_interconvertible_with_class (&Y::c));
+static_assert (!std::is_pointer_interconvertible_with_class (&Y::z));
+static_assert (std::is_pointer_interconvertible_with_class (&Y::d));
diff --git a/gcc/testsuite/g++.dg/cpp2a/is-pointer-interconvertible-with-class5.C b/gcc/testsuite/g++.dg/cpp2a/is-pointer-interconvertible-with-class5.C
new file mode 100644
index 0000000..246ff92
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/is-pointer-interconvertible-with-class5.C
@@ -0,0 +1,59 @@
+// P0466R5
+// { dg-do run { target c++20 } }
+// { dg-options "" }
+
+namespace std
+{
+template <class S, class M>
+constexpr bool
+is_pointer_interconvertible_with_class (M S::*m) noexcept
+{
+  return __builtin_is_pointer_interconvertible_with_class (m);
+}
+}
+
+struct W { struct { int a; long b; }; int c; };
+union X { int a; struct { short b; long c; }; long long d; };
+struct D { int x; private: int y; };                                                                                                                                                  
+union Y { int a; struct { short b; long c; D z; }; long long d; };                                                                                                                    
+
+int
+main ()
+{
+  auto t1 = &W::a;
+  if (!std::is_pointer_interconvertible_with_class (t1))
+    __builtin_abort ();
+  auto t2 = &W::b;
+  if (std::is_pointer_interconvertible_with_class (t2))
+    __builtin_abort ();
+  auto t3 = &W::c;
+  if (std::is_pointer_interconvertible_with_class (t3))
+    __builtin_abort ();
+  auto t4 = &X::a;
+  if (!std::is_pointer_interconvertible_with_class (t4))
+    __builtin_abort ();
+  auto t5 = &X::b;
+  if (!std::is_pointer_interconvertible_with_class (t5))
+    __builtin_abort ();
+  auto t6 = &X::c;
+  if (std::is_pointer_interconvertible_with_class (t6))
+    __builtin_abort ();
+  auto t7 = &X::d;
+  if (!std::is_pointer_interconvertible_with_class (t7))
+    __builtin_abort ();
+  auto t8 = &Y::a;
+  if (!std::is_pointer_interconvertible_with_class (t8))
+    __builtin_abort ();
+  auto t9 = &Y::b;
+  if (std::is_pointer_interconvertible_with_class (t9))
+    __builtin_abort ();
+  auto t10 = &Y::c;
+  if (std::is_pointer_interconvertible_with_class (t10))
+    __builtin_abort ();
+  auto t11 = &Y::z;
+  if (std::is_pointer_interconvertible_with_class (t11))
+    __builtin_abort ();
+  auto t12 = &Y::d;
+  if (!std::is_pointer_interconvertible_with_class (t12))
+    __builtin_abort ();
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/is-pointer-interconvertible-with-class6.C b/gcc/testsuite/g++.dg/cpp2a/is-pointer-interconvertible-with-class6.C
new file mode 100644
index 0000000..4b6801c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/is-pointer-interconvertible-with-class6.C
@@ -0,0 +1,19 @@
+// P0466R5
+// { dg-do compile { target c++20 } }
+
+namespace std
+{
+template <class S, class M>
+constexpr bool
+is_pointer_interconvertible_with_class (M S::*m) noexcept
+{
+  return __builtin_is_pointer_interconvertible_with_class (m);
+}
+}
+
+struct A { int a; };
+
+double A::*a = nullptr;
+constexpr double A::*b = nullptr;
+constexpr auto c = std::is_pointer_interconvertible_with_class (a);	// { dg-error "is not usable in a constant expression" }
+constexpr auto d = std::is_pointer_interconvertible_with_class (b);