template class with default parameter in template parameter declaration

[Marc Glisse]

On Tue, 8 Nov 2011, Ulrich Drepper wrote:

> Complicated title, here's a bit of code:
>
> #ifdef FIX
> # define PARM2 , class T5
> #else
> # define PARMS2
> #endif
>
>
> template<typename T1, typename T2 = int>
> struct cl1 {
> };
>
> template<template<class T3 PARM2> class T4 = cl1>
> struct cl2 {
> };
>
> cl2<> var;
>
> If compiled without FIX defined this will fail with gcc 4.3 and later.
> Haven't checked 4.2 but it works without the fix with gcc 4.1.  The
> strange thing is that it also worked with ancient compilers before the
> C++ frontend rewrite (gcc 3.2).  In short, when a expecting a template
> class in a template parameter list it now is not possible anymore to
> skip the default parameters.  Since this is an actual use of the class
> (in case the default is used) and the programmer declares to have no
> interest in the type of the second template parameter I wouldn't say
> it is needed but I haven't tracked a statement in the standard.
>
> Before changing too much code I want to make sure this new and very
> old behavior is what is required by the standard and not a bug which
> slipped in again.

It is announced in:
http://gcc.gnu.org/gcc-4.2/changes.html

It broke quite a bit of code (in particular with std::vector as cl1), but 
there were some exotic pieces of legal code that were broken by this 
extension, so it had to go.

-- 
Marc Glisse