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.

7 files changed, 67 insertions, 0 deletions

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index 568318f..5839bfb 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -3697,6 +3697,12 @@ diagnose_trait_expr (tree expr, tree args)
     case CPTK_HAS_UNIQUE_OBJ_REPRESENTATIONS:
       inform (loc, "  %qT does not have unique object representations", t1);
       break;
+    case CPTK_IS_CONVERTIBLE:
+      inform (loc, "  %qT is not convertible from %qE", t2, t1);
+      break;
+    case CPTK_IS_NOTHROW_CONVERTIBLE:
+	inform (loc, "  %qT is not %<nothrow%> convertible from %qE", t2, t1);
+      break;
     case CPTK_REF_CONSTRUCTS_FROM_TEMPORARY:
       inform (loc, "  %qT is not a reference that binds to a temporary "
 	      "object of type %qT (direct-initialization)", t1, t2);
diff --git a/gcc/cp/cp-objcp-common.cc b/gcc/cp/cp-objcp-common.cc
index 1ffac08..6497569 100644
--- a/gcc/cp/cp-objcp-common.cc
+++ b/gcc/cp/cp-objcp-common.cc
@@ -463,6 +463,8 @@ names_builtin_p (const char *name)
     case RID_IS_NOTHROW_ASSIGNABLE:
     case RID_IS_NOTHROW_CONSTRUCTIBLE:
     case RID_UNDERLYING_TYPE:
+    case RID_IS_CONVERTIBLE:
+    case RID_IS_NOTHROW_CONVERTIBLE:
     case RID_REF_CONSTRUCTS_FROM_TEMPORARY:
     case RID_REF_CONVERTS_FROM_TEMPORARY:
       return true;
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index f19ecaf..e4d8920 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -1407,6 +1407,8 @@ enum cp_trait_kind
   CPTK_IS_CONSTRUCTIBLE,
   CPTK_IS_NOTHROW_ASSIGNABLE,
   CPTK_IS_NOTHROW_CONSTRUCTIBLE,
+  CPTK_IS_CONVERTIBLE,
+  CPTK_IS_NOTHROW_CONVERTIBLE,
   CPTK_REF_CONSTRUCTS_FROM_TEMPORARY,
   CPTK_REF_CONVERTS_FROM_TEMPORARY
 };
@@ -7116,6 +7118,8 @@ extern tree forward_parm			(tree);
 extern bool is_trivially_xible			(enum tree_code, tree, tree);
 extern bool is_nothrow_xible			(enum tree_code, tree, tree);
 extern bool is_xible				(enum tree_code, tree, tree);
+extern bool is_convertible			(tree, tree);
+extern bool is_nothrow_convertible		(tree, tree);
 extern bool ref_xes_from_temporary		(tree, tree, bool);
 extern tree get_defaulted_eh_spec		(tree, tsubst_flags_t = tf_warning_or_error);
 extern bool maybe_explain_implicit_delete	(tree);
diff --git a/gcc/cp/cxx-pretty-print.cc b/gcc/cp/cxx-pretty-print.cc
index 4459083..e18143e3 100644
--- a/gcc/cp/cxx-pretty-print.cc
+++ b/gcc/cp/cxx-pretty-print.cc
@@ -2696,6 +2696,12 @@ pp_cxx_trait_expression (cxx_pretty_printer *pp, tree t)
     case CPTK_IS_NOTHROW_CONSTRUCTIBLE:
       pp_cxx_ws_string (pp, "__is_nothrow_constructible");
       break;
+    case CPTK_IS_CONVERTIBLE:
+      pp_cxx_ws_string (pp, "__is_convertible");
+      break;
+    case CPTK_IS_NOTHROW_CONVERTIBLE:
+      pp_cxx_ws_string (pp, "__is_nothrow_convertible");
+      break;
     case CPTK_REF_CONSTRUCTS_FROM_TEMPORARY:
       pp_cxx_ws_string (pp, "__reference_constructs_from_temporary");
       break;
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)
diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 3cbe0d6..bb83d1c 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -5922,6 +5922,8 @@ cp_parser_primary_expression (cp_parser *parser,
 	case RID_IS_CONSTRUCTIBLE:
 	case RID_IS_NOTHROW_ASSIGNABLE:
 	case RID_IS_NOTHROW_CONSTRUCTIBLE:
+	case RID_IS_CONVERTIBLE:
+	case RID_IS_NOTHROW_CONVERTIBLE:
 	case RID_REF_CONSTRUCTS_FROM_TEMPORARY:
 	case RID_REF_CONVERTS_FROM_TEMPORARY:
 	  return cp_parser_trait_expr (parser, token->keyword);
@@ -11008,6 +11010,14 @@ cp_parser_trait_expr (cp_parser* parser, enum rid keyword)
       kind = CPTK_IS_NOTHROW_CONSTRUCTIBLE;
       variadic = true;
       break;
+    case RID_IS_CONVERTIBLE:
+      kind = CPTK_IS_CONVERTIBLE;
+      binary = true;
+      break;
+    case RID_IS_NOTHROW_CONVERTIBLE:
+      kind = CPTK_IS_NOTHROW_CONVERTIBLE;
+      binary = true;
+      break;
     case RID_REF_CONSTRUCTS_FROM_TEMPORARY:
       kind = CPTK_REF_CONSTRUCTS_FROM_TEMPORARY;
       binary = true;
diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index 8656207..92fc795 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -12044,6 +12044,12 @@ trait_expr_value (cp_trait_kind kind, tree type1, tree type2)
     case CPTK_IS_NOTHROW_CONSTRUCTIBLE:
       return is_nothrow_xible (INIT_EXPR, type1, type2);
 
+    case CPTK_IS_CONVERTIBLE:
+      return is_convertible (type1, type2);
+
+    case CPTK_IS_NOTHROW_CONVERTIBLE:
+      return is_nothrow_convertible (type1, type2);
+
     case CPTK_REF_CONSTRUCTS_FROM_TEMPORARY:
       return ref_xes_from_temporary (type1, type2, /*direct_init=*/true);
 
@@ -12165,6 +12171,8 @@ finish_trait_expr (location_t loc, cp_trait_kind kind, tree type1, tree type2)
     case CPTK_IS_TRIVIALLY_CONSTRUCTIBLE:
     case CPTK_IS_NOTHROW_ASSIGNABLE:
     case CPTK_IS_NOTHROW_CONSTRUCTIBLE:
+    case CPTK_IS_CONVERTIBLE:
+    case CPTK_IS_NOTHROW_CONVERTIBLE:
     case CPTK_REF_CONSTRUCTS_FROM_TEMPORARY:
     case CPTK_REF_CONVERTS_FROM_TEMPORARY:
       if (!check_trait_type (type1)