Fix PR27684 - std::tuple no longer accepts reference to incomplete type in some cases.

Libc++ has to deduce the 'allocator_arg_t' parameter as 'AllocArgT' for the
following constructor:

  template <class Alloc> tuple(allocator_arg_t, Alloc const&)

Previously libc++ has tried to support tags derived from 'allocator_arg_t' by
using 'is_base_of<AllocArgT, allocator_arg_t>'. However this breaks whenever a
2-tuple contains a reference to an incomplete type as its first parameter.

See https://llvm.org/bugs/show_bug.cgi?id=27684

llvm-svn: 273334

1 files changed, 5 insertions, 20 deletions

diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc.pass.cpp
index c69163de1fd..4da5fc7f839 100644
--- a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc.pass.cpp
@@ -7,6 +7,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: c++98, c++03
+
 // <tuple>
 
 // template <class... Types> class tuple;
@@ -14,7 +16,9 @@
 // template <class Alloc>
 //   tuple(allocator_arg_t, const Alloc& a);
 
-// UNSUPPORTED: c++98, c++03
+// NOTE: this constructor does not currently support tags derived from
+// allocator_arg_t because libc++ has to deduce the parameter as a template
+// argument. See PR27684 (https://llvm.org/bugs/show_bug.cgi?id=27684)
 
 #include <tuple>
 #include <cassert>
@@ -42,10 +46,6 @@ int main()
         std::tuple<> t(std::allocator_arg, A1<int>());
     }
     {
-        DerivedFromAllocArgT tag;
-        std::tuple<> t(tag, A1<int>());
-    }
-    {
         std::tuple<int> t(std::allocator_arg, A1<int>());
         assert(std::get<0>(t) == 0);
     }
@@ -95,21 +95,6 @@ int main()
         assert(std::get<2>(t) == alloc_last());
     }
     {
-        // Test that allocator construction is selected when the user provides
-        // a custom tag type which derives from allocator_arg_t.
-        DerivedFromAllocArgT tag;
-        alloc_first::allocator_constructed = false;
-        alloc_last::allocator_constructed = false;
-
-        std::tuple<DefaultOnly, alloc_first, alloc_last> t(tag, A1<int>(5));
-
-        assert(std::get<0>(t) == DefaultOnly());
-        assert(alloc_first::allocator_constructed);
-        assert(std::get<1>(t) == alloc_first());
-        assert(alloc_last::allocator_constructed);
-        assert(std::get<2>(t) == alloc_last());
-    }
-    {
         // Test that the uses-allocator default constructor does not evaluate
         // it's SFINAE when it otherwise shouldn't be selected. Do this by
         // using 'NonDefaultConstructible' which will cause a compile error