c++/2645: Boost-related const & typdef in template puzzle

[pcarlini@unitus.it]

>Number:         2645
>Category:       c++
>Synopsis:       Boost-related const & typdef in template puzzle
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Apr 25 13:36:01 PDT 2001
>Closed-Date:
>Last-Modified:
>Originator:     Paolo Carlini
>Release:        3.0 20010423 (prerelease) & Codesourcery
>Organization:
>Environment:
i686, Linux2.2.19, glibc2.2.2, binutils2.11
>Description:
This puzzle surfaced while analyzing Boost's testsuite
failure of call_traits_test.cpp.
That code assumes that two typedefs (for a const int&) are
equivalent to an explicit const int & in order to specify
a template type, but this is not the case for Gcc2.95.2
and Gcc3.0.
The following code, which does compile (and should not!),
illustrates the case:

template <typename T>
struct call_traits
{
public:
   typedef T type_less_spec;
};

template <typename T>
struct call_traits<T&>
{
   typedef T type_more_spec;
};


int main()
{
   int num;

   // Two typedefs lead to the instant. of the less spec. ("wrong") template
   typedef int& r_type;
   typedef const r_type cr_type;
   call_traits<cr_type>::type_less_spec var1 = num;
   //                    ^^^^^^^^^^^^^^

   // The explicit type leads to the instantiation of the "correct" one
   call_traits<const int&>::type_more_spec var2 = num;
   //                       ^^^^^^^^^^^^^^

   // As happen with a single typedef!
   typedef const int& std_cr_type;
   call_traits<std_cr_type>::type_more_spec var3 = num;

   
   // As happen, indeed, without the cv-qualifier
   call_traits<r_type>::type_more_spec var4;
}


>How-To-Repeat:
Before submitting the bug report, I cross checked with
two other (commonly believed) very ISO-compliant compilers,
which, indeed, rejected the testcase.
>Fix:
????
>Release-Note:
>Audit-Trail:
>Unformatted: