C++ : default template arguments for template specialization (was: Fix for bug 68)

["Artem Khodush" <kaa at comail dot ru>]

Hi all,

This is for gcc-20001225.

There is something wrong with this patch:

2000-12-14  Kriang Lerdsuwanakij  <lerdsuwa@users.sourceforge.net>

 * pt.c (check_explicit_specialization): Propagate default
 function arguments to explicit specializations.

 http://gcc.gnu.org/ml/gcc-patches/2000-12/msg00616.html

+    /* Inherit default function arguments from the template
+       DECL is specializing.  */
+    {
+      tree t1 = TYPE_ARG_TYPES (TREE_TYPE (decl));
+      tree t2 = TYPE_ARG_TYPES (TREE_TYPE (DECL_TEMPLATE_RESULT (tmpl)));
+ 
+      /* DECL may contain more parameters than TMPL due to the extra
+         in-charge parameter in constructors and destructors.  */
+      if (DECL_NONSTATIC_MEMBER_FUNCTION_P (decl))
+        t1 = TREE_CHAIN (t1), t2 = TREE_CHAIN (t2);
+      if (DECL_HAS_IN_CHARGE_PARM_P (decl))
+        t1 = TREE_CHAIN (t1);
+ 
+       /* Note that we do not need to reparse default arguments, 
+   since explicit specialization cannot be declared in 
+   class scope as in [temp.expl.spec].  */
+      for (; t1 && t2; t1 = TREE_CHAIN (t1), t2 = TREE_CHAIN (t2))
+        {
+        if (TREE_PURPOSE (t2))
+         TREE_PURPOSE (t1) = TREE_PURPOSE (t2);
+        }
+ 
+      my_friendly_assert (t1 == NULL_TREE && t2 == NULL_TREE, 20001211);
+    }
+ 

The testcase:

#include <vector>
#include <locale>

int main() {
 std::vector< int > v;
}

now gives me unexpected error:

vect.cxx: In function `int main()':
vect.cxx:5: call of overloaded `vector()' is ambiguous
/tmp/BUILD/gcc-20001225/libstdc++-v3/include/bits/stl_vector.h:252: candidates 
   are: std::vector<_Tp, _Alloc>::vector(unsigned int = 0) [with _Tp = int, 
   _Alloc = std::allocator<int>]
/tmp/BUILD/gcc-20001225/libstdc++-v3/include/bits/stl_vector.h:244:             
       std::vector<_Tp, _Alloc>::vector(typename std::_Vector_base<_Tp, 
   _Alloc>::allocator_type& = typename std::_Vector_base<_Tp, 
   _Alloc>::allocator_type()) [with _Tp = int, _Alloc = std::allocator<int>]

Looking at the tree dump for the testcase, I see that argument type nodes for an unsigned
argument are shared between vector::vector(unsigned) and explicit instantiation for
messages_byname<char>::messages_byname(char const*, unsigned). 

Apparently the code
+        if (TREE_PURPOSE (t2))
+         TREE_PURPOSE (t1) = TREE_PURPOSE (t2);

sets default argument value to all such shared tree nodes, so vector::vector(unsigned) now "inherits"
default value from the primary template for messages_byname::messages_byname(char const*, unsigned),
and becomes ambiguous with default vector constructor.

Hope this is useful,
Artem.