c++/7562: strange behaviour with typedef and consts

[Nicolas Noble]

>Number:         7562
>Category:       c++
>Synopsis:       const with typedefs doesn't work as expected.
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    unassigned
>State:          open
>Class:          rejects-legal
>Submitter-Id:   net
>Arrival-Date:   Fri Aug 09 13:46:00 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     
>Release:        3.1
>Organization:
>Environment:
System: Linux the-babel-tower 2.4.18 #2 Sun Apr 28 23:46:35 CEST 2002 i686 unknown
Architecture: i686

	
host: i386-slackware-linux-gnu
build: i386-slackware-linux-gnu
target: i386-slackware-linux-gnu
configured with: ../gcc-3.1/configure --prefix=/usr --enable-shared --with-gnu-ld --enable-threads --verbose --target=i386-slackware-linux --host=i386-slackware-linux
>Description:
Here is a problem I saw first with the zlib header. It's very easy to understand, I think you'll be able to understand
with the How-To-Repeat section. In brief, it's when having a "const" with a previously typedef'ed type, the compiler
forgets the "const", which only produces a warning in C compilation mode, but produces an error in C++ compilation mode.
>How-To-Repeat:
Here is the simple source code:

$ cat typedef.cc 
typedef void * voidp;

void func1(const voidp p) { }

void func2(const void * p) {
    func1(p);
}

and what happen:

$ gcc-3.1 -c typedef.cc 
typedef.cc: In function `void func2(const void*)':
typedef.cc:6: invalid conversion from `const void*' to `void*'

which is the worse case I think. With a .c extension it only does that:
$ gcc-3.1 -c typedef.c  
typedef.c: In function `func2':
typedef.c:6: warning: passing arg 1 of `func1' discards qualifiers from pointer target type


>Fix:
Actually the only "fix" I was able to think of was to forget typedef'ing the voidp, but this is a pain with the zlib.h
since it has "voidp" everywhere. Another solution, but very ugly, is to #define voidp void * for example.

>Release-Note:
>Audit-Trail:
>Unformatted: