c++: Implement __is_const built-in trait

This patch implements built-in trait for std::is_const.

gcc/cp/ChangeLog:

	* cp-trait.def: Define __is_const.
	* constraint.cc (diagnose_trait_expr): Handle CPTK_IS_CONST.
	* semantics.cc (trait_expr_value): Likewise.
	(finish_trait_expr): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/has-builtin-1.C: Test existence of __is_const.
	* g++.dg/ext/is_const.C: New test.

Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>
Reviewed-by: Jason Merrill <jason@redhat.com>

5 files changed, 31 insertions, 0 deletions

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index 8a3b5d8..eaf17a5 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -3768,6 +3768,9 @@ diagnose_trait_expr (tree expr, tree args)
     case CPTK_IS_CLASS:
       inform (loc, "  %qT is not a class", t1);
       break;
+    case CPTK_IS_CONST:
+      inform (loc, "  %qT is not a const type", t1);
+      break;
     case CPTK_IS_CONSTRUCTIBLE:
       if (!t2)
     inform (loc, "  %qT is not default constructible", t1);
diff --git a/gcc/cp/cp-trait.def b/gcc/cp/cp-trait.def
index 394f006..36faed9 100644
--- a/gcc/cp/cp-trait.def
+++ b/gcc/cp/cp-trait.def
@@ -64,6 +64,7 @@ DEFTRAIT_EXPR (IS_ASSIGNABLE, "__is_assignable", 2)
 DEFTRAIT_EXPR (IS_BASE_OF, "__is_base_of", 2)
 DEFTRAIT_EXPR (IS_BOUNDED_ARRAY, "__is_bounded_array", 1)
 DEFTRAIT_EXPR (IS_CLASS, "__is_class", 1)
+DEFTRAIT_EXPR (IS_CONST, "__is_const", 1)
 DEFTRAIT_EXPR (IS_CONSTRUCTIBLE, "__is_constructible", -1)
 DEFTRAIT_EXPR (IS_CONVERTIBLE, "__is_convertible", 2)
 DEFTRAIT_EXPR (IS_EMPTY, "__is_empty", 1)
diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index d0fa324..b4cda21 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -12532,6 +12532,9 @@ trait_expr_value (cp_trait_kind kind, tree type1, tree type2)
     case CPTK_IS_CLASS:
       return NON_UNION_CLASS_TYPE_P (type1);
 
+    case CPTK_IS_CONST:
+      return CP_TYPE_CONST_P (type1);
+
     case CPTK_IS_CONSTRUCTIBLE:
       return is_xible (INIT_EXPR, type1, type2);
 
@@ -12814,6 +12817,7 @@ finish_trait_expr (location_t loc, cp_trait_kind kind, tree type1, tree type2)
     case CPTK_IS_ARRAY:
     case CPTK_IS_BOUNDED_ARRAY:
     case CPTK_IS_CLASS:
+    case CPTK_IS_CONST:
     case CPTK_IS_ENUM:
     case CPTK_IS_FUNCTION:
     case CPTK_IS_MEMBER_FUNCTION_POINTER:
diff --git a/gcc/testsuite/g++.dg/ext/has-builtin-1.C b/gcc/testsuite/g++.dg/ext/has-builtin-1.C
index 02b4b4d..e3640fa 100644
--- a/gcc/testsuite/g++.dg/ext/has-builtin-1.C
+++ b/gcc/testsuite/g++.dg/ext/has-builtin-1.C
@@ -71,6 +71,9 @@
 #if !__has_builtin (__is_class)
 # error "__has_builtin (__is_class) failed"
 #endif
+#if !__has_builtin (__is_const)
+# error "__has_builtin (__is_const) failed"
+#endif
 #if !__has_builtin (__is_constructible)
 # error "__has_builtin (__is_constructible) failed"
 #endif
diff --git a/gcc/testsuite/g++.dg/ext/is_const.C b/gcc/testsuite/g++.dg/ext/is_const.C
new file mode 100644
index 0000000..8a0e8df
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/is_const.C
@@ -0,0 +1,20 @@
+// { dg-do compile { target c++11 } }
+
+#define SA(X) static_assert((X),#X)
+
+class ClassType { };
+using cClassType = const ClassType;
+using vClassType = volatile ClassType;
+using cvClassType = const volatile ClassType;
+
+// Positive tests.
+SA(__is_const(const int));
+SA(__is_const(const volatile int));
+SA(__is_const(cClassType));
+SA(__is_const(cvClassType));
+
+// Negative tests.
+SA(!__is_const(int));
+SA(!__is_const(volatile int));
+SA(!__is_const(ClassType));
+SA(!__is_const(vClassType));