Re: c++/3585: wrong function overload resolution?

[Martin Sebor <sebor at roguewave dot com>]

The following reply was made to PR c++/3585; it has been noted by GNATS.

From: Martin Sebor <sebor@roguewave.com>
To: gcc-gnats@gcc.gnu.org
Cc:  
Subject: Re: c++/3585: wrong function overload resolution?
Date: Fri, 06 Jul 2001 10:53:13 -0600

 gawrilow@math.tu-berlin.de wrote:
 > 
 > >Number:         3585
 > >Category:       c++
 > >Synopsis:       wrong function overload resolution?
 ...
 > >Description:
 > Let's consider the following fragment:
 > 
 > void f(int& x) { }  // #1
 > 
 > void f(int x) { }   // #2
 > 
 > int temp() { return 0; }
 > 
 > int main() {
 >   int var=0;
 >   f(var);       // should resolve to #1
 >   f(temp());    // resolves to #2
 > }
 > 
 > The gcc rejects f(var) as ambiguous reasoning that #1 and #2
 > were equally good. IMHO, resolving to #2 includes an
 > lvalue-to-rvalue conversion, where #1 incurs an identity
 > conversion, thus being better. Am I missing something?
 
 I think so: Reference Binding is considered the Identity conversion
 (see 13.3.3.1.4, p1).
 
 > 
 > The things become yet prettier as we move to templates.
 > Let's change the definitions of f's to the following:
 > 
 > template <class T> void f(T& x) { } // #1
 > template <class T> void f(T x) { } // #2
 > 
 > gcc 3.0 still considers this ambiguous, while 2.95 has
 > chosen #1 for f(var) without any complains.
 
 The call to f (var) is also ambiguous here. Argument deduction
 finds [T = int] in both cases and you're left with the same as
 above.
 
 > 
 > Thus the final questions: who understands the overload
 > resolution rules better: gcc 3.0, gcc 2.95,
 > me (obviously not!), or somebody else?
 
 I'd say 3.0 is correct here and 2.95 is in error. Most other
 compilers agree.
 
 > 
 > How could I otherwise distinct between rvalue and lvalue
 > arguments to an overloaded function, without using
 > f(const T&) ?
 
 Here's how you can tell an lvalue from an rvalue:
 
 bool is_lvalue (...) { return false; }
 
 template <class T> 
 bool is_lvalue (T&) { return true; }
 
 Regards
 Martin