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.cc (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.cc b/gcc/cp/semantics.cc
index 3bf478fd68cd3d7a6669f1ff5e85a0cd9fbbcb9f..079ad5c93bf1436b1f241c2bbcbf9a6d35ef7e27 100644 (file)
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -12176,6 +12176,47 @@ check_trait_type (tree type, int kind = 1)
   return true;
 }
 
+/* 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:
+    case CPTK_REF_CONSTRUCTS_FROM_TEMPORARY:
+    case CPTK_REF_CONVERTS_FROM_TEMPORARY:
+      to = type1;
+      from = type2;
+      break;
+
+      /* These put it second.  */
+    case CPTK_IS_CONVERTIBLE:
+    case CPTK_IS_NOTHROW_CONVERTIBLE:
+      to = type2;
+      from = type1;
+      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
@@ -12241,14 +12282,19 @@ finish_trait_expr (location_t loc, cp_trait_kind kind, tree type1, tree type2)
        return error_mark_node;
       break;
 
-    case CPTK_IS_TRIVIALLY_ASSIGNABLE:
     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:
+      /* 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..a27ec6e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/is_constructible8.C
@@ -0,0 +1,31 @@
+// 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));
+
+SA (__is_convertible(T, T&&));
+SA (__is_convertible(T, const T&));
+SA (!__is_convertible(T, T&));
+SA (__is_nothrow_convertible(T, T&&));
+SA (__is_nothrow_convertible(T, const T&));
+SA (!__is_nothrow_convertible(T, T&));
+
+// All false because either the conversion fails or it doesn't bind a temporary
+SA (!__reference_constructs_from_temporary (T&&, T));
+SA (!__reference_constructs_from_temporary (const T&, T));
+SA (!__reference_constructs_from_temporary (T&, T));
+SA (!__reference_converts_from_temporary (T&&, T));
+SA (!__reference_converts_from_temporary (const T&, T));
+SA (!__reference_converts_from_temporary (T&, T));