This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2024-11-11
[Voted into WP at July, 2009 meeting.]
The overload resolution rules for ranking a template against a non-template function differ for conversion functions in a surprising way. 12.2.4 [over.match.best] lists four checks, the last three concern this report. For the non-conversion operator case, checks 2 and 3 are applicable, whereas for the conversion operator case checks 3 and 4 are applicable. Checks 2 and 4 concern the ranking of argument and return value conversion sequences respectively. Check 3 concerns only the templatedness of the functions being ranked, and will prefer a non-template to a template. Notice that this check happens after argument conversion sequence ranking, but before return value conversion sequence ranking. This has the effect of always selecting a non-template conversion operator, as the following example shows:
struct C
{
inline operator int () { return 1; }
template <class T> inline operator T () { return 0; }
};
inline long f (long x) { return x; }
int
main (int argc, char *argv[])
{
return f (C ());
}
The non-templated C::operator int function will be selected, rather than the apparently better C::operator long<long> instantiation. This is a surprise, and resulted in a bug report where the user expected the template to be selected. In addition some C++ compilers have implemented the overload ranking as if checks 3 and 4 were transposed.
Is this ordering accidental, or is there a rationale?
Notes from the April, 2005 meeting:
The CWG agreed that the template/non-template distinction should be the final tie-breaker.
Proposed resolution (March, 2007):
In the second bulleted list of 12.2.4 [over.match.best] paragraph 1, move the second and third bullets to the end of the list, to read as follows:
for some argument j, ICSj(F1) is a better conversion sequence than ICSj(F2), or, if not that,
the context is an initialization by user-defined conversion (see 9.4 [dcl.init], 12.2.2.6 [over.match.conv], and 12.2.2.7 [over.match.ref]) and the standard conversion sequence from the return type of F1 to the destination type (i.e., the type of the entity being initialized) is a better conversion sequence than the standard conversion sequence from the return type of F2 to the destination type, [Example: ... —end example] or, if not that,
- F1 is a non-template function and F2 is a function template specialization, or, if not that,
F1 and F2 are function template specializations, and the function template for F1 is more specialized than the template for F2 according to the partial ordering rules described in 13.7.7.3 [temp.func.order].