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.

1 files changed, 4 insertions, 0 deletions

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;