Bug 15287 - [3.4/4.0 Regression] combination of operator[] and operator .* fails in templates

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gcc-3.4-syntax-err-index+objderef.cpp

Sample code to trigger the error

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2004-05-04 22:56

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Description:

Last confirmed: 2004-05-04 23:15

Opened: 2004-05-04 22:54

DESCRIPTION

G++ 3.4 wrongly recognizes some valid C++ syntax as an error:
This in particular happens when a class/structure is operated upon with
both `[]' and `.*' operators, like this:

(Class[C].*BoolMethod)();                                     //<1>

where Class is any C++ object; C is a size_t variable, and BoolMethod is a
pointer to method. An attachment is given as a concrete example of the
problem.

Some generic conditions are required to trigger the bug:

* This problem was originally discovered when `Class' is of the type
  `std::vector<something>'. But it turns out that any C++ object would
  trigger the error.

* The statement //<1> must be in a templatized function or class method.
  Look at the example: Uncomment line //[4] and comment //[5], then
  the error will not happen in `void Proc(_type2)'.


WORKAROUND

A temporary workaround is possible, by FIRST caching the `[]' operator
result, then applying the `.*' operator. But this may or may not work,
depending on what actually happens with the `[]' operator.

Consider my given example:
* If you compile with -DWORK_AROUND_BUG, the error messages disappear.
* If you compile with -DWORK_AROUND_BUG=-1 instead, the same error message
  would re-appear. The addition of parantheses does not help.


VERSION KNOWN TO BE AFFECTED: 3.4

My gcc gives the following version/compilation info:

[wirawan@snowflake tmp]$ /usr/local/gcc-3.4.0/bin/g++ -v
Reading specs from /usr/local/gcc-3.4.0/lib/gcc/i686-pc-linux-gnu/3.4.0/specs
Configured with: ../gcc-3.4.0/configure --prefix=/usr/local/gcc-3.4.0
--disable-nls --enable-languages=c,c++,f77 --enable-shared --with-cpu=i686
--with-tune=i686 --with-arch=i686 --enable-threads=posix
--enable-__cxa_atexit
Thread model: posix
gcc version 3.4.0


OTHER VERSIONS

This error does NOT happen with GNU C++ version 3.3.x and earlier.

------- Comment #2 From Andrew Pinski 2004-05-04 23:15 -------

Confirmed.
Reduced to:
-----------------------------
typedef __SIZE_TYPE__ size_t;
struct Structure {}; // empty, just a dummy placeholder
extern Structure& t;
struct Vector2
{
    typedef Structure& reference;
    reference operator[](int x) {return t;}
};
Vector2 Array;
template <int>//t
void Print(void (Structure::*BoolMethod)())
{
  size_t C = 0;
  (Array[C].*BoolMethod)();
}
-----------------------------

------- Comment #3 From Wolfgang Bangerth 2004-05-04 23:51 -------

Yes, and still a little smaller: 
---------------- 
struct S {}; 
 
struct Array { 
    S operator[](int); 
} array; 
 
void (S::*mem_fun_ptr)(); 
 
template <int> void foo() { 
  (array[0].*mem_fun_ptr)(); 
} 
-------------------- 
 
g/x> /home/bangerth/bin/gcc-3.3.4-pre/bin/c++ -c x.cc 
 
g/x> /home/bangerth/bin/gcc-3.4-pre/bin/c++ -c x.cc 
x.cc: In function `void foo()': 
x.cc:10: error: invalid operands of types `Array' and `int' to binary 
`operator+' 
 
Another workaround is to write 
  array.operator[](0) 
instead of 
  array[0] 
 
W.

GCC Bugzilla Bug 15287

[3.4/4.0 Regression] combination of operator[] and operator .* fails in templates

Last modified: 2004-10-30 21:11:40

Known to work:
Known to fail: