[patch] libstdc++/69293 Use static assertion for uses-allocator construction

[Jonathan Wakely <jwakely at redhat dot com>]

The PR is actually due to a defect in the standard, which I reported
today. The reporter said we're missing a check for is_constructible<T,
Args..., inner_allocator_type> that would ensure we go to bullet (9.4)
and make the example in the PR ill-formed. I didn't add that check
because it's redundant, we don't need to check is_constructible we can
just try the construction and if it fails the program is ill-formed
anyway.

However, due to the  defect in the standard the example in the PR
*should* be ill-formed, but isn't.  This patch adds a static assertion
making it ill-formed. It would be ill-formed anyway once the defect is
resolved and we implement the resolution, but with the static
assertion we give a better diagnostic, both for
scoped_allocator_adaptor and for ill-formed uses of allocators with
std::tuple.

Tested powerpc64le-linux, committed to trunk.


commit ccd75a6477a0be335d8d38fcbcd6047cebaee096
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Jan 15 19:48:25 2016 +0000

    Use static assertion for uses-allocator construction
    
    	PR libstdc++/69293
    	* include/bits/uses_allocator.h (__uses_alloc<true, ...>): Add
    	static assertion that type is constructible from the arguments.
    	* testsuite/20_util/scoped_allocator/69293_neg.cc: New.
    	* testsuite/20_util/uses_allocator/69293_neg.cc: New.
    	* testsuite/20_util/uses_allocator/cons_neg.cc: Adjust dg-error.

diff --git a/libstdc++-v3/include/bits/uses_allocator.h b/libstdc++-v3/include/bits/uses_allocator.h
index 70ba007..b1ff58a 100644
--- a/libstdc++-v3/include/bits/uses_allocator.h
+++ b/libstdc++-v3/include/bits/uses_allocator.h
@@ -85,7 +85,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
         is_constructible<_Tp, allocator_arg_t, _Alloc, _Args...>::value,
         __uses_alloc1<_Alloc>,
        	__uses_alloc2<_Alloc>>::type
-    { };
+    {
+      static_assert(__or_<
+	  is_constructible<_Tp, allocator_arg_t, _Alloc, _Args...>,
+	  is_constructible<_Tp, _Args..., _Alloc>>::value, "construction with"
+	  " an allocator must be possible if uses_allocator is true");
+    };
 
   template<typename _Tp, typename _Alloc, typename... _Args>
     struct __uses_alloc<false, _Tp, _Alloc, _Args...>
diff --git a/libstdc++-v3/testsuite/20_util/scoped_allocator/69293_neg.cc b/libstdc++-v3/testsuite/20_util/scoped_allocator/69293_neg.cc
new file mode 100644
index 0000000..f3b2d87
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/scoped_allocator/69293_neg.cc
@@ -0,0 +1,51 @@
+// Copyright (C) 2016 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++11" }
+// { dg-do compile }
+
+// PR libstdc++/69293
+
+#include <scoped_allocator>
+#include <memory>
+
+using std::allocator;
+using std::allocator_arg_t;
+using std::uses_allocator;
+using std::scoped_allocator_adaptor;
+using std::is_constructible;
+
+struct X
+{
+  using allocator_type = allocator<int>;
+};
+
+using scoped_alloc = scoped_allocator_adaptor<allocator<X>, X::allocator_type>;
+using inner_alloc_type = scoped_alloc::inner_allocator_type;
+
+static_assert(uses_allocator<X, inner_alloc_type>{}, "");
+static_assert(!is_constructible<X, allocator_arg_t, inner_alloc_type>{}, "");
+static_assert(!is_constructible<X, inner_alloc_type>{}, "");
+
+void
+test01()
+{
+  scoped_alloc sa;
+  auto p = sa.allocate(1);
+  sa.construct(p);  // this is required to be ill-formed
+  // { dg-error "static assertion failed" "" { target *-*-* } 89 }
+}
diff --git a/libstdc++-v3/testsuite/20_util/uses_allocator/69293_neg.cc b/libstdc++-v3/testsuite/20_util/uses_allocator/69293_neg.cc
new file mode 100644
index 0000000..19417fc
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/uses_allocator/69293_neg.cc
@@ -0,0 +1,49 @@
+// Copyright (C) 2016 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++11" }
+// { dg-do compile }
+
+// PR libstdc++/69293
+
+#include <tuple>
+#include <memory>
+
+using std::allocator;
+using std::allocator_arg_t;
+using std::uses_allocator;
+using std::tuple;
+using std::is_constructible;
+
+struct X
+{
+  using allocator_type = allocator<int>;
+};
+
+using alloc_type = X::allocator_type;
+
+static_assert(uses_allocator<X, alloc_type>{}, "");
+static_assert(!is_constructible<X, allocator_arg_t, alloc_type>{}, "");
+static_assert(!is_constructible<X, alloc_type>{}, "");
+
+void
+test01()
+{
+  alloc_type a;
+  std::tuple<X> t(std::allocator_arg, a); // this is required to be ill-formed
+  // { dg-error "static assertion failed" "" { target *-*-* } 89 }
+}
diff --git a/libstdc++-v3/testsuite/20_util/uses_allocator/cons_neg.cc b/libstdc++-v3/testsuite/20_util/uses_allocator/cons_neg.cc
index 00f96d6..b3df4ae 100644
--- a/libstdc++-v3/testsuite/20_util/uses_allocator/cons_neg.cc
+++ b/libstdc++-v3/testsuite/20_util/uses_allocator/cons_neg.cc
@@ -44,4 +44,4 @@ void test01()
 
   tuple<Type> t(allocator_arg, a, 1);
 }
-// { dg-error "no matching function" "" { target *-*-* } 92 }
+// { dg-error "static assertion failed" "" { target *-*-* } 89 }