c++/1551: explicit specialization of constructor causes dementia

[fsm@robots.ox.ac.uk]

>Number:         1551
>Category:       c++
>Synopsis:       explicit specialization of constructor causes dementia
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          rejects-legal
>Submitter-Id:   net
>Arrival-Date:   Thu Jan 04 13:06:00 PST 2001
>Closed-Date:
>Last-Modified:
>Originator:     Frederik Schaffalitzky
>Release:        CVS gcc version 2.97 20010104 (experimental)
>Organization:
>Environment:
Linux 2.2.17 #3 SMP i686 unknown
>Description:
Explicit specialization of a templated constructor
causes the compiler to reject legal code later. There
is such a specialization in <locale> and hence in 
<fstream>.

Symptom:
f.cxx: In constructor `derived<T>::derived() [with T = double]':
f.cxx:27:   instantiated from here
f.cxx:11: call of overloaded `base()' is ambiguous
f.cxx:5: candidates are: base<T>::base(unsigned int = 0) [with T = double]
f.cxx:4:                 base<T>::base() [with T = double]
>How-To-Repeat:
template <typename T>
struct base
{
  base();
  base(unsigned);
};

template <typename T>
struct derived : base<T>
{
  derived() { }
};

#if 1
template <typename V>
struct Y
{
  Y(unsigned = 0);
};

template <>
Y<char>::Y(unsigned) { }
#else
# include <locale>
#endif

template struct derived<double>;
>Fix:
No idea. The bug is not expressed if any of the following
changes is made:

1. change the argument type of base::base from unsigned
   to int.

2. remove the default argument to Y::Y.

3. move the specialization to the top of the file.

4. another class is derived from base<double> before the
   explicit specialization.
>Release-Note:
>Audit-Trail:
>Unformatted: