[gcc r15-374] c++: Implement __decay built-in trait

[gcc r15-374] c++: Implement __decay built-in trait

[Ken Matsui]

https://gcc.gnu.org/g:142d1d8e057c0e6da7f4e6eba4cc265c556d2dd8

commit r15-374-g142d1d8e057c0e6da7f4e6eba4cc265c556d2dd8
Author: Ken Matsui <kmatsui@gcc.gnu.org>
Date:   Thu Feb 15 04:43:41 2024 -0800

    c++: Implement __decay built-in trait
    
    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>
    Reviewed-by: Jason Merrill <jason@redhat.com>

Diff:
---
 gcc/cp/cp-trait.def                      |  1 +
 gcc/cp/semantics.cc                      | 12 ++++++++++++
 gcc/testsuite/g++.dg/ext/decay.C         | 22 ++++++++++++++++++++++
 gcc/testsuite/g++.dg/ext/has-builtin-1.C |  3 +++
 4 files changed, 38 insertions(+)

diff --git a/gcc/cp/cp-trait.def b/gcc/cp/cp-trait.def
index 173818adf792..2d1cb7c227c0 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 f6338f77d239..d499b858057c 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -12946,6 +12946,18 @@ finish_trait_type (cp_trait_kind kind, tree type1, tree type2,
 	return cp_build_reference_type (type1, /*rval=*/true);
       return type1;
 
+    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 000000000000..8adedfeefe6c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/decay.C
@@ -0,0 +1,22 @@
+// { dg-do compile { target c++11 } }
+
+#define SA(X) static_assert((X),#X)
+
+// 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));
diff --git a/gcc/testsuite/g++.dg/ext/has-builtin-1.C b/gcc/testsuite/g++.dg/ext/has-builtin-1.C
index 266cb35a3684..d2e6a5b4dcb3 100644
--- a/gcc/testsuite/g++.dg/ext/has-builtin-1.C
+++ b/gcc/testsuite/g++.dg/ext/has-builtin-1.C
@@ -32,6 +32,9 @@
 #if !__has_builtin (__builtin_source_location)
 # error "__has_builtin (__builtin_source_location) failed"
 #endif
+#if !__has_builtin (__decay)
+# error "__has_builtin (__decay) failed"
+#endif
 #if !__has_builtin (__has_nothrow_assign)
 # error "__has_builtin (__has_nothrow_assign) failed"
 #endif