c++: Implement __is_{nothrow_,}convertible [PR106784]

To improve compile times, the C++ library could use compiler built-ins
rather than implementing std::is_convertible (and _nothrow) as class
templates.  This patch adds the built-ins.  We already have
__is_constructible and __is_assignable, and the nothrow forms of those.

Microsoft (and clang, for compatibility) also provide an alias called
__is_convertible_to.  I did not add it, but it would be trivial to do
so.

I noticed that our __is_assignable doesn't implement the "Access checks
are performed as if from a context unrelated to either type" requirement,
therefore std::is_assignable / __is_assignable give two different results
here:

  class S {
    operator int();
    friend void g(); // #1
  };

  void
  g ()
  {
    // #1 doesn't matter
    static_assert(std::is_assignable<int&, S>::value, "");
    static_assert(__is_assignable(int&, S), "");
  }

This is not a problem if __is_assignable is not meant to be used by
the users.

This patch doesn't make libstdc++ use the new built-ins, but I had to
rename a class otherwise its name would clash with the new built-in.

	PR c++/106784

gcc/c-family/ChangeLog:

	* c-common.cc (c_common_reswords): Add __is_convertible and
	__is_nothrow_convertible.
	* c-common.h (enum rid): Add RID_IS_CONVERTIBLE and
	RID_IS_NOTHROW_CONVERTIBLE.

gcc/cp/ChangeLog:

	* constraint.cc (diagnose_trait_expr): Handle CPTK_IS_CONVERTIBLE
	and CPTK_IS_NOTHROW_CONVERTIBLE.
	* cp-objcp-common.cc (names_builtin_p): Handle RID_IS_CONVERTIBLE
	RID_IS_NOTHROW_CONVERTIBLE.
	* cp-tree.h (enum cp_trait_kind): Add CPTK_IS_CONVERTIBLE and
	CPTK_IS_NOTHROW_CONVERTIBLE.
	(is_convertible): Declare.
	(is_nothrow_convertible): Likewise.
	* cxx-pretty-print.cc (pp_cxx_trait_expression): Handle
	CPTK_IS_CONVERTIBLE and CPTK_IS_NOTHROW_CONVERTIBLE.
	* method.cc (is_convertible): New.
	(is_nothrow_convertible): Likewise.
	* parser.cc (cp_parser_primary_expression): Handle RID_IS_CONVERTIBLE
	and RID_IS_NOTHROW_CONVERTIBLE.
	(cp_parser_trait_expr): Likewise.
	* semantics.cc (trait_expr_value): Handle CPTK_IS_CONVERTIBLE and
	CPTK_IS_NOTHROW_CONVERTIBLE.
	(finish_trait_expr): Likewise.

libstdc++-v3/ChangeLog:

	* include/std/type_traits: Rename __is_nothrow_convertible to
	__is_nothrow_convertible_lib.
	* testsuite/20_util/is_nothrow_convertible/value_ext.cc: Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/has-builtin-1.C: Enhance to test __is_convertible and
	__is_nothrow_convertible.
	* g++.dg/ext/is_convertible1.C: New test.
	* g++.dg/ext/is_convertible2.C: New test.
	* g++.dg/ext/is_nothrow_convertible1.C: New test.
	* g++.dg/ext/is_nothrow_convertible2.C: New test.

1 files changed, 31 insertions, 0 deletions

diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc
index 573ef01..c35a59f 100644
--- a/gcc/cp/method.cc
+++ b/gcc/cp/method.cc
@@ -2236,6 +2236,37 @@ ref_xes_from_temporary (tree to, tree from, bool direct_init_p)
   return ref_conv_binds_directly (to, val, direct_init_p).is_false ();
 }
 
+/* Return true if FROM can be converted to TO using implicit conversions,
+   or both FROM and TO are possibly cv-qualified void.  NB: This doesn't
+   implement the "Access checks are performed as if from a context unrelated
+   to either type" restriction.  */
+
+bool
+is_convertible (tree from, tree to)
+{
+  if (VOID_TYPE_P (from) && VOID_TYPE_P (to))
+    return true;
+  tree expr = build_stub_object (from);
+  expr = perform_implicit_conversion (to, expr, tf_none);
+  if (expr == error_mark_node)
+    return false;
+  return !!expr;
+}
+
+/* Like is_convertible, but the conversion is also noexcept.  */
+
+bool
+is_nothrow_convertible (tree from, tree to)
+{
+  if (VOID_TYPE_P (from) && VOID_TYPE_P (to))
+    return true;
+  tree expr = build_stub_object (from);
+  expr = perform_implicit_conversion (to, expr, tf_none);
+  if (expr == NULL_TREE || expr == error_mark_node)
+    return false;
+  return expr_noexcept_p (expr, tf_none);
+}
+
 /* Categorize various special_function_kinds.  */
 #define SFK_CTOR_P(sfk) \
   ((sfk) >= sfk_constructor && (sfk) <= sfk_move_constructor)