C++ FE stripping const qualifiers from function prototypes (bug or feature?)

[Diego Novillo <dnovillo at google dot com>]

Apologies if this is a basic misunderstanding of how C++ works, but to
me this looks like a bug.  I am tracking down an ODR violation message
that LTO is emitting when linking two TUs that include the same header
file, but whose IL was generated with different front ends:

$ cat f.h
typedef const char * (*Callback)(void *, const char *, const int);

$ cat f1.cc
#include "f.h"
extern "C" { extern Callback p; };

$ cat f2.c
#include "f.h"
Callback p = 0;

$ g++ -flto -c f1.cc
$ gcc -flto -c f2.c
$ gcc -flto -o f f1.o f2.o
f2.c:2: error: type of 'p' does not match original declaration
f1.cc:2: error: previously declared here

The reason why we get into this error is that in the C++ FE, the
typedef for Callback removes 'const' from the third argument.  This is
what I see when debugging cc1plus just before it writes the IL to
disk:

(gdb) ptu 0x7ffff79f2cc0
$NODE =$10 = 0x7ffff79f2cc0
const char * (*JSErrorCallback) (void *, const char *, int)

Notice how the third argument is 'int' instead of 'const int'.  Is
this the way C++ is supposed to behave or is this a bug in the FE?


Thanks.  Diego.