[PATCH v4 11/12] c++: Implement __decay built-in trait

[Ken Matsui <kmatsui@gcc.gnu.org>]

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

gcc/cp/ChangeLog:

	* cp-trait.def: Define __decay.
	* semantics.cc (finish_trait_type): Handle CPTK_DECAY.

gcc/testsuite/ChangeLog:

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

Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>
---
 gcc/cp/cp-trait.def                      |  1 +
 gcc/cp/semantics.cc                      | 12 ++++++++
 gcc/testsuite/g++.dg/ext/decay.C         | 39 ++++++++++++++++++++++++
 gcc/testsuite/g++.dg/ext/has-builtin-1.C |  3 ++
 4 files changed, 55 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/ext/decay.C

diff --git a/gcc/cp/cp-trait.def b/gcc/cp/cp-trait.def
index 9e8f9eb38b8..11270f3ae6b 100644
--- a/gcc/cp/cp-trait.def
+++ b/gcc/cp/cp-trait.def
@@ -51,6 +51,7 @@
 DEFTRAIT_TYPE (ADD_LVALUE_REFERENCE, "__add_lvalue_reference", 1)
 DEFTRAIT_TYPE (ADD_POINTER, "__add_pointer", 1)
 DEFTRAIT_TYPE (ADD_RVALUE_REFERENCE, "__add_rvalue_reference", 1)
+DEFTRAIT_TYPE (DECAY, "__decay", 1)
 DEFTRAIT_EXPR (HAS_NOTHROW_ASSIGN, "__has_nothrow_assign", 1)
 DEFTRAIT_EXPR (HAS_NOTHROW_CONSTRUCTOR, "__has_nothrow_constructor", 1)
 DEFTRAIT_EXPR (HAS_NOTHROW_COPY, "__has_nothrow_copy", 1)
diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index f437e272ea6..256e7ef8166 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -12785,6 +12785,18 @@ finish_trait_type (cp_trait_kind kind, tree type1, tree type2,
 	return type1;
       return cp_build_reference_type (type1, /*rval=*/true);
 
+    case CPTK_DECAY:
+      if (TYPE_REF_P (type1))
+	type1 = TREE_TYPE (type1);
+
+      if (TREE_CODE (type1) == ARRAY_TYPE)
+	return finish_trait_type (CPTK_ADD_POINTER, TREE_TYPE (type1), type2,
+				  complain);
+      else if (TREE_CODE (type1) == FUNCTION_TYPE)
+	return finish_trait_type (CPTK_ADD_POINTER, type1, type2, complain);
+      else
+	return cv_unqualified (type1);
+
     case CPTK_REMOVE_ALL_EXTENTS:
       return strip_array_types (type1);
 
diff --git a/gcc/testsuite/g++.dg/ext/decay.C b/gcc/testsuite/g++.dg/ext/decay.C
new file mode 100644
index 00000000000..cf224b7452c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/decay.C
@@ -0,0 +1,39 @@
+// { dg-do compile { target c++11 } }
+
+#define SA(X) static_assert((X),#X)
+
+// class ClassType { };
+
+// Positive tests.
+using test1_type = __decay(bool);
+SA(__is_same(test1_type, bool));
+
+// NB: DR 705.
+using test2_type = __decay(const int);
+SA(__is_same(test2_type, int));
+
+using test3_type = __decay(int[4]);
+SA(__is_same(test3_type, __remove_extent(int[4])*));
+
+using fn_type = void ();
+using test4_type = __decay(fn_type);
+SA(__is_same(test4_type, __add_pointer(fn_type)));
+
+using cfn_type = void () const;
+using test5_type = __decay(cfn_type);
+SA(__is_same(test5_type, cfn_type));
+
+// SA(__is_same(__add_rvalue_reference(int), int&&));
+// SA(__is_same(__add_rvalue_reference(int&&), int&&));
+// SA(__is_same(__add_rvalue_reference(int&), int&));
+// SA(__is_same(__add_rvalue_reference(const int), const int&&));
+// SA(__is_same(__add_rvalue_reference(int*), int*&&));
+// SA(__is_same(__add_rvalue_reference(ClassType&&), ClassType&&));
+// SA(__is_same(__add_rvalue_reference(ClassType), ClassType&&));
+// SA(__is_same(__add_rvalue_reference(int(int)), int(&&)(int)));
+// SA(__is_same(__add_rvalue_reference(void), void));
+// SA(__is_same(__add_rvalue_reference(const void), const void));
+// SA(__is_same(__add_rvalue_reference(bool(int) const), bool(int) const));
+// SA(__is_same(__add_rvalue_reference(bool(int) &), bool(int) &));
+// SA(__is_same(__add_rvalue_reference(bool(int) const &&), bool(int) const &&));
+// SA(__is_same(__add_rvalue_reference(bool(int)), bool(&&)(int)));
diff --git a/gcc/testsuite/g++.dg/ext/has-builtin-1.C b/gcc/testsuite/g++.dg/ext/has-builtin-1.C
index 9d7e59b47fb..5b590db1cf6 100644
--- a/gcc/testsuite/g++.dg/ext/has-builtin-1.C
+++ b/gcc/testsuite/g++.dg/ext/has-builtin-1.C
@@ -11,6 +11,9 @@
 #if !__has_builtin (__add_rvalue_reference)
 # error "__has_builtin (__add_rvalue_reference) failed"
 #endif
+#if !__has_builtin (__decay)
+# error "__has_builtin (__decay) failed"
+#endif
 #if !__has_builtin (__builtin_addressof)
 # error "__has_builtin (__builtin_addressof) failed"
 #endif
-- 
2.43.0