Re: c++/6601: [parser] Parse error when using :: qualified name in template

[Wolfgang Bangerth <bangerth@math.ethz.ch>]

The following reply was made to PR c++/6601; it has been noted by GNATS.

From: Wolfgang Bangerth <bangerth@math.ethz.ch>
To: Reiner Hauser <rhauser@fnal.gov>
Cc: gcc-gnats@gcc.gnu.org, <lerdsuwa@gcc.gnu.org>, <gcc-bugs@gcc.gnu.org>,
        <gcc-patches@gcc.gnu.org>, <pfeifer@dbai.tuwien.ac.at>
Subject: Re: c++/6601: [parser] Parse error when using :: qualified name in
 template
Date: Thu, 16 May 2002 10:05:23 +0200 (CEST)

 > Thanks, who would have thought about di-graphs ;) ? I hesitated a while
 > to submit a bug report, because I thought somebody else surely must have
 > caught this before. I finally did it since the Kai compiler compiled the
 > code without complaints, but it turns out that it has digraphs disabled
 > in the default mode and you have to give some --strict option to enable
 > them.
 > 
 > Maybe this can go into that 'bugs which aren't really bugs' web page you
 > got ? It might give people a clue when they run into similar problems.
 
 I append a patch to bugs.html below which lists this as a non-bug. It 
 might be worthwhile for both 3.1 and 3.2.
 Regards
   Wolfgang
 
 -------------------------------------------------------------------------
 Wolfgang Bangerth                  email:           bangerth@math.ethz.ch
                                    www: http://www.math.ethz.ch/~bangerth
 
 
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   <a href="http://www.validlab.com/goldberg/paper.ps">this paper</a>
   for more information.</p></dd>
   
 + <dt><em>Templates, scoping, and digraphs.</em></dt>
 + 
 + <dd><p>If you have a class in global namespace, say named
 + <code>X</code>, and want to give it as a template argument to some
 + other class, say <code>std::vector</code>, then this here fails with a
 + parser error: <code>std::vector&lt;::X&gt;</code>.
 + </p>
 + 
 + <p>
 + The reason is that the standard mandates that the sequence
 + <code>&lt;:</code> is treated as if it were the token
 + <code>[</code>, and the parser then reports a parse error before the
 + character <code>:</code> (by which it means the second
 + colon). There are several such combinations of characters, and 
 + they are called <em>di-graphs</em>.
 + </p>
 + 
 + <p>
 + The simplest way to avoid this is to write <code>std::vector&lt;
 + ::X&gt;</code>, i.e. place a space between the opening angle bracket
 + and the scope operator.
 + </p></dd>
 + 
 + 
   </dl>
   
   <h3><a name="missing">Missing features</a></h3>