c++: __is_constructible ref binding [PR100667]

The requirement that a type argument be complete is excessive in the case of
direct reference binding to the same type, which does not rely on any
properties of the type.  This is LWG 2939.

	PR c++/100667

gcc/cp/ChangeLog:

	* semantics.c (same_type_ref_bind_p): New.
	(finish_trait_expr): Use it.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/is_constructible8.C: New test.

(cherry picked from commit 8bb3ef3f6e335e8794590fb712a2661d11d21973)

diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index 385e108a1c8d51c3a738c427eceadf596676d95b..57e71ec3a3c782c8ebc65232a573e4a3aee44ba9 100644 (file)
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -10570,6 +10570,38 @@ check_trait_type (tree type)
   return !!complete_type_or_else (strip_array_types (type), NULL_TREE);
 }
 
+/* True iff the conversion (if any) would be a direct reference
+   binding, not requiring complete types.  This is LWG2939.  */
+
+static bool
+same_type_ref_bind_p (cp_trait_kind kind, tree type1, tree type2)
+{
+  tree from, to;
+  switch (kind)
+    {
+      /* These put the target type first.  */
+    case CPTK_IS_CONSTRUCTIBLE:
+    case CPTK_IS_NOTHROW_CONSTRUCTIBLE:
+    case CPTK_IS_TRIVIALLY_CONSTRUCTIBLE:
+      to = type1;
+      from = type2;
+      break;
+
+    default:
+      gcc_unreachable ();
+    }
+
+  if (TREE_CODE (to) != REFERENCE_TYPE || !from)
+    return false;
+  if (TREE_CODE (from) == TREE_VEC && TREE_VEC_LENGTH (from) == 1)
+    from = TREE_VEC_ELT (from, 0);
+  else if (TREE_CODE (from) == TREE_LIST && !TREE_CHAIN (from))
+    from = TREE_VALUE (from);
+  return (TYPE_P (from)
+         && (same_type_ignoring_top_level_qualifiers_p
+             (non_reference (to), non_reference (from))));
+}
+
 /* Process a trait expression.  */
 
 tree
@@ -10619,10 +10651,15 @@ finish_trait_expr (location_t loc, cp_trait_kind kind, tree type1, tree type2)
     case CPTK_IS_CONSTRUCTIBLE:
       break;
 
-    case CPTK_IS_TRIVIALLY_ASSIGNABLE:
     case CPTK_IS_TRIVIALLY_CONSTRUCTIBLE:
-    case CPTK_IS_NOTHROW_ASSIGNABLE:
     case CPTK_IS_NOTHROW_CONSTRUCTIBLE:
+      /* Don't check completeness for direct reference binding.  */;
+      if (same_type_ref_bind_p (kind, type1, type2))
+       break;
+      gcc_fallthrough ();
+
+    case CPTK_IS_NOTHROW_ASSIGNABLE:
+    case CPTK_IS_TRIVIALLY_ASSIGNABLE:
       if (!check_trait_type (type1)
          || !check_trait_type (type2))
        return error_mark_node;

diff --git a/gcc/testsuite/g++.dg/ext/is_constructible8.C b/gcc/testsuite/g++.dg/ext/is_constructible8.C
new file mode 100644 (file)
index 0000000..4b8204c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/is_constructible8.C
@@ -0,0 +1,16 @@
+// PR c++/100667
+// { dg-do compile { target c++11 } }
+
+struct T;
+
+#define SA(X) static_assert ((X), #X);
+
+SA (__is_constructible(T&&, T));
+SA (__is_constructible(const T&, T));
+SA (!__is_constructible(T&, T));
+SA (__is_nothrow_constructible(T&&, T));
+SA (__is_nothrow_constructible(const T&, T));
+SA (!__is_nothrow_constructible(T&, T));
+SA (__is_trivially_constructible(T&&, T));
+SA (__is_trivially_constructible(const T&, T));
+SA (!__is_trivially_constructible(T&, T));