Re: c++/6057: expression mangling doesn't work for operator new

[Richard Smith <richard at ex-parrot dot com>]

The following reply was made to PR c++/6057; it has been noted by GNATS.

From: Richard Smith <richard@ex-parrot.com>
To: <nathan@gcc.gnu.org>,  <gcc-bugs@gcc.gnu.org>,  <gcc-prs@gcc.gnu.org>, 
     <mmitchel@gcc.gnu.org>,  <richard@ex-parrot.com>, 
     <gcc-gnats@gcc.gnu.org>
Cc:  
Subject: Re: c++/6057: expression mangling doesn't work for operator new
Date: Fri, 13 Sep 2002 17:45:49 +0100 (BST)

 On 13 Sep 2002 nathan@gcc.gnu.org wrote:
 
 > is this fixed now?
 
 No, the bug is still there in cvs on the mainline.   Try, for example,
 compiling the following code
 
   template <unsigned int> struct helper {};
   template <class T> static void check( helper< sizeof( new T ) > * );
 
   int main()
   {
     check<int>(0);
   }
 
 The problem was with the new expression, not the sizeof expression.
 There was an unrelated bug with mangling of sizeof expression, which has
 now been fixed (can't find PR number at the moment); but that only applied
 where the argument of the sizeof expression was a type that was dependent
 on a template parameter, not an expression.
 
 The segfault is actually occuring on the line
 
   code = TREE_CODE (expr);	// Line 1790 in revision 1.57
 
 at the start of write_expression in mangle.c because expr is
 a NULL pointer.  This is happening because when write_expression is called
 to write the NEW_EXPR it goes down the 'default' case statement at the end
 of the function, and iterates over each of the tree operands.  The three
 operands to the NEW_EXPR tree node are the placement expresion (which is
 of type TREE_LIST, or, more frequently a NULL pointer), the type which is
 being created (which is a type not an expression), and the initialiser
 list (again, of type TREE_LIST, or a NULL pointer).
 
 The second operand of the NEW_EXPR needs to have write_type, not
 write_expression called on it; the first and third operands of NEW_EXPR
 need to be mangled in some new way, which is what the
 write_expression_list function in my patch does (it also copes with a
 NULL pointer argument, which write_expression doesn't).
 
 The following test case showes the other failure modes of the existing
 code
 
   struct foo {} f;
   typedef unsigned int size_t;
   void *operator new( size_t, foo );
 
   template <unsigned int> struct helper {};
   template <class T> static void check( helper< sizeof( new(f) T(0) ) > * );
 
   int main()
   {
     check<int>(0);
   }
 
 where the segfault occurs because write_expression can't cope being passed
 a TREE_LIST.
 
 As I mentioned in the original PR, a more general solution would be
 preferable:  I know that the same problems apply to function calls,
 
   template <class T> T make_type();
   template <unsigned int> struct helper {};
   template <class T> static void check( helper< sizeof( make_type<T>() ) > * );
 
   int main()
   {
     check<int>(0);
   }
 
 where the segfault occurs because CALL_EXPR's operands can't be handled.
 
 
 http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=6057
 
 --
 Richard Smith