c++/5413: g++ fails to understand class template's typedef

[Wirawan Purwanto]

The following reply was made to PR c++/5413; it has been noted by GNATS.

From: "Wirawan Purwanto" <wirawan@camelot.physics.wm.edu>
To: <gcc-gnats@gcc.gnu.org>,
	<gcc-prs@gcc.gnu.org>,
	<wirawan0@hotmail.com>,
	<gcc-bugs@gcc.gnu.org>,
	<nobody@gcc.gnu.org>
Cc:  
Subject: Re: c++/5413: g++ fails to understand class template's typedef
Date: Thu, 17 Jan 2002 13:16:29 -0500

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 xref:
 http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&p
 r=5413
 
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 /*
 
 Help GCC developer!
 
 I found an annoying bug in g++ recently. A simplified test program is
 attached. This file alone is enough to reproduce the error.
 
 I have tested this program under the following compilers:
 
 1. g++ 2.95.2, mingw32 distribution. Typing "g++ -v" returns:
 
    Reading specs from =
 e:\APPS\GCC2.95.2\BIN\..\lib\gcc-lib\mingw32\2.95.2\specs
    gcc driver version 2.95.2 19991024 (release) executing gcc version =
 2.95.2-mingw
    snapshot 20010329
 
 2. g++ 2.95.3-6, distributed in mingw32 version 1.1. Typing "g++ -v" =
 returns:
 
    Reading specs from =
 c:/usr/gcc/bin/../lib/gcc-lib/mingw32/2.95.3-6/specs
    gcc version 2.95.3-6 (mingw special)
 
 3. g++ 2.95.3, in Mandrake Linux version . Typing "g++ -v" returns:
 
    Reading specs from /usr/lib/gcc-lib/i586-mandrake-linux/2.95.3/specs
    gcc version 2.95.3 19991030 (prerelease)
 
 This error is related to the template feature in the compiler.
 The error can be briefly desrcribed as follows:
 
 Suppose we declare a class template called PARENT, and in this class we
 make a typedef PARENT_type. Any type should work (I have tried arrays
 and structs and ints).
 
 Then we declare another class template, called CHILD, whose base class
 is _another_ class template (given in the template parameter of CHILD).
 Now we want to use the typedefs declared in the base class. For example,
 here I typed:
 
   typedef typename parent_template<payload_data>::PARENT_type =
 CHILD_type1;
 
 in the declaration of class CHILD. This is OK as long as we use
 CHILD_type1 within the declaration of the class. But once we declare it
 OUTSIDE the class declaration (i.e. in the definition of method
 
   CHILD<parent_template>::this_is_refused(CHILD_type1 xx)           <--+
                                                                        |
 below, the compiler complains (line number can vary, but it refers to  |
 the line containing { } after the statement above): -------------------+
 
   gcc-template-bug1.cpp:100: `parent_template<payload_data>' is not a
   class, struct, or union type
   gcc-template-bug1.cpp:100: syntax error before `{'
 
 The fact that parent_template<payload_data> is used as a base class
 implies that parent_template<payload_data> must be a class. Why can't we
 assume that here? BTW I have checked this testcase against Borland C++ =
 5.5
 and Compaq C++ (in Alpha machine), they don't complain about this. Also
 ecgs 2.96 for linux-alpha does not complain--but AFAIK this version is
 not supported.
 
 Please help me! Is there any solution or patch to this problem? I would
 appreciate if you cc: also your reply to my address below.
 
 Wirawan
 <wirawan0@hotmail.com>
 
  */
 
 template <class Payload =3D int>
 class PARENT
 {
 public:
 //  this is a custom typedef defined in the parent
     typedef int PARENT_type;
 //  this is an "extra" payload that can be customized by the user
     Payload Data;
 };
 
 // this is the custom payload, defined by the user
 struct payload_data { int x; };
 
 // now we use the PARENT class template to define the CHILD. But here
 // the CHILD will have a _specific_ payload (don't ask why---I did =
 employ
 // this organization in my own program).
 template <template <class e> class parent_template>
 class CHILD: public parent_template<payload_data>
 {
 public:
     //    typedef typeof(typename =
 parent_template<payload_data>::PARENT_type) CHILD_type1;
 //    typedef parent_template<payload_data> prnt;
 //    typedef typename prnt::PARENT_type CHILD_type1;
     typedef typename parent_template<payload_data>::PARENT_type =
 CHILD_type1;
     typedef int CHILD_type2[4];
     void this_is_fine(CHILD_type2 xx);
     void this_is_refused(CHILD_type1 xx); // this is not regarded as =
 error
 };
 
 template <template <class e> class parent_template>
 void CHILD<parent_template>::this_is_fine(CHILD_type2 xx)
 { }
 
 template <template <class e> class parent_template>
 void CHILD<parent_template>::this_is_refused(CHILD_type1 xx)
 { }                          // ^ but above line is regarded as =
 erroneous!
 
 CHILD<PARENT> sistem;
 
 int main()
 {
     CHILD<PARENT>::PARENT_type uu;                  // however, this is =
 OK
     PARENT<double>::PARENT_type xyz;
     return 0;
 }
 
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