c++/4672: Template parameter deduction fails for overloaded template functions

Thu Oct 25 16:26:00 GMT 2001

>Number:         4672
>Category:       c++
>Synopsis:       Template parameter deduction fails for overloaded template functions.
>Confidential:   no
>Severity:       serious
>Priority:       low
>Responsible:    unassigned
>State:          open
>Class:          rejects-legal
>Submitter-Id:   net
>Arrival-Date:   Thu Oct 25 16:26:12 PDT 2001
>Closed-Date:
>Last-Modified:
>Originator:     Sven Rosvall
>Release:        3.0
>Organization:
>Environment:
System: Linux harold.prqa.ie 2.2.14-6.1.1smp #1 SMP Thu Apr 13 19:55:55 EDT 2000 i686 unknown
Architecture: i686
host: i686-pc-linux-gnu
build: i686-pc-linux-gnu
target: i686-pc-linux-gnu
configured with: ./configure 
>Description:
This works fine with GCC 2.95.1 but not with 2.96 or 3.0.

We have also compared this code with the Comeau C/C++ 4.2.45.2
compiler for a third oppinion. It accepts the code without any
errors.

The problem is with template parameter deduction when there are two
overloaded template functions. One is taking a reference and the other
pointer. 

//Code example:
// checked_cast is intended to work as static_cast but for debug
// builds it will check that the cast is correct.

template<class T, class U> inline T checked_cast(U & from)
{
  try
  {
    dynamic_cast <T> (from);
  }
  catch (...)
  {
    // Error
  }
  return static_cast<T>(from);
}

template <class T, class U> inline T checked_cast(U * from)
{
  if (! dynamic_cast <T>(from))
  {
    // Error
  }
  return static_cast <T>(from);
}

class A
{
public:
  virtual ~A();
};

class C;

class B : public A
{
  void foo () const;
};

class C : public B
{
};

void B::foo () const
{
  C const & c1 = checked_cast <C const &> (*this);
  C const * c2 = checked_cast <C const *> (this);
}

Output from g++ (3.0)

/usr/local/bin/new-fronts/g++ -v input.cc
Reading specs from /usr/local/bin/new-fronts/../lib/gcc-lib/i686-pc-linux-gnu/3.0/specs
Configured with: ./configure 
Thread model: single
gcc version 3.0
 /usr/local/bin/new-fronts/../lib/gcc-lib/i686-pc-linux-gnu/3.0/cc1plus -v -iprefix /usr/local/bin/new-fronts/../lib/gcc-lib/i686-pc-linux-gnu/3.0/ -D__GNUC__=3 -D__GNUC_MINOR__=0 -D__GNUC_PATCHLEVEL__=0 -D__ELF__ -Dunix -Dlinux -D__ELF__ -D__unix__ -D__linux__ -D__unix -D__linux -Asystem=posix -D__NO_INLINE__ -D__STDC_HOSTED__=1 -D_GNU_SOURCE -Acpu=i386 -Amachine=i386 -Di386 -D__i386 -D__i386__ -D__tune_i686__ -D__tune_pentiumpro__ input.cc -D__GNUG__=3 -D__GXX_DEPRECATED -D__EXCEPTIONS -D__GXX_ABI_VERSION=100 -quiet -dumpbase input.cc -version -o /tmp/ccp1UP1Q.s
GNU CPP version 3.0 (cpplib) (i386 Linux/ELF)
GNU C++ version 3.0 (i686-pc-linux-gnu)
	compiled by GNU C version 3.0.
ignoring duplicate directory "/usr/local/include/g++-v3"
ignoring duplicate directory "/usr/local/include/g++-v3/i686-pc-linux-gnu"
ignoring duplicate directory "/usr/local/include/g++-v3/backward"
ignoring duplicate directory "/usr/local/lib/gcc-lib/i686-pc-linux-gnu/3.0/include"
ignoring duplicate directory "/usr/local/i686-pc-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/bin/lib/../include/g++-v3
 /usr/local/bin/lib/../include/g++-v3/i686-pc-linux-gnu
 /usr/local/bin/lib/../include/g++-v3/backward
 /usr/local/bin/lib/gcc-lib/i686-pc-linux-gnu/3.0/include
 /usr/local/bin/lib/../i686-pc-linux-gnu/include
 /usr/local/include
 /usr/include
End of search list.
input.cc: In member function `void B::foo() const':
input.cc:44: call of overloaded `U_checked_cast(const B* const)' is ambiguous
input.cc:2: candidates are: T U_checked_cast(U&) [with T = const C*, U = const 
   B* const]
input.cc:15:                 T U_checked_cast(U*) [with T = const C*, U = const 
   B]

Output from g++ (2.95.1)

g++ -v input.cc
Reading specs from /usr/local/lib/gcc-lib/i686-pc-linux-gnu/2.95.1/specs
gcc driver version 2.95.2 19991024 (release) executing gcc version 2.95.1
 /usr/local/lib/gcc-lib/i686-pc-linux-gnu/2.95.1/cpp -lang-c++ -v -D__GNUC__=2 -D__GNUG__=2 -D__GNUC_MINOR__=95 -D__cplusplus -D__ELF__ -Dunix -D__i386__ -Dlinux -D__ELF__ -D__unix__ -D__i386__ -D__linux__ -D__unix -D__linux -Asystem(posix) -D__EXCEPTIONS -Acpu(i386) -Amachine(i386) -Di386 -D__i386 -D__i386__ -Di686 -Dpentiumpro -D__i686 -D__i686__ -D__pentiumpro -D__pentiumpro__ input.cc /tmp/ccNhfq7X.ii
GNU CPP version 2.95.1 19990816 (release) (i386 Linux/ELF)
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/lib/gcc-lib/i686-pc-linux-gnu/2.95.1/../../../../include/g++-3
 /usr/local/include
 /usr/local/lib/gcc-lib/i686-pc-linux-gnu/2.95.1/../../../../i686-pc-linux-gnu/include
 /usr/local/lib/gcc-lib/i686-pc-linux-gnu/2.95.1/include
 /usr/include
End of search list.
The following default directories have been omitted from the search path:
End of omitted list.
 /usr/local/lib/gcc-lib/i686-pc-linux-gnu/2.95.1/cc1plus /tmp/ccNhfq7X.ii -quiet -dumpbase input.cc -version -o /tmp/ccSDYmSL.s
GNU C++ version 2.95.1 19990816 (release) (i686-pc-linux-gnu) compiled by GNU C version 2.95.1 19990816 (release).
 as -V -Qy -o /tmp/cc1jJrLA.o /tmp/ccSDYmSL.s
[snip]

>How-To-Repeat:

>Fix:
The workaround we use is to specify two parameters to checked_cast.
This works but does not look as nice as it is intended to tod.
>Release-Note:
>Audit-Trail:
>Unformatted: