[Bug c++/41614] Compilation error with member templates

["redi at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #3 from redi at gcc dot gnu dot org  2009-10-07 09:48 -------
(In reply to comment #2)
> Thanks for the fast reply! I had tried compilation with VC++ 2008 and there it
> worked, so I thought it would be valid.

You should be wary of using VC++ to determine if code is valid, it's not often
right!

> Maybe a more expressive error message could help. In this shortened example the
> error message is ... well... okay, but in my reallife project I get this as a
> first error, which is totally misleading (and lead me into thinking this is a
> compiler bug):
> 
> C:\Programme\C++\Projects\FreeFileSync\algorithm.cpp|565|error: no match for
> 'operator<' in 'dbLeft->DetectChanges::detectFileChange [with
> FreeFileSync::SelectedSide side = side, bool respectFiltering = true] <
> (FreeFileSync::SelectedSide)0u'|
> C:\Programme\C++\wxWidgets\include\wx\string.h|1572|note: candidates are: bool
> operator<(const wxString&, const wxString&)|
> C:\Programme\C++\wxWidgets\include\wx\string.h|1574|note:                 bool
> operator<(const wxString&, const wxChar*)|
> C:\Programme\C++\wxWidgets\include\wx\string.h|1576|note:                 bool
> operator<(const wxChar*, const wxString&)|
> C:\Programme\C++\wxWidgets\include\wx\longlong.h|1040|note:                
> bool operator<(long int, const wxLongLong&)|
> C:\Programme\C++\wxWidgets\include\wx\longlong.h|1053|note:                
> bool operator<(long unsigned int, const wxULongLong&)|
> 

Although it may be confusing, the message is technically correct, due to the
complicated parsing rules of C++.

no match for
'operator<' in 'dbLeft->DetectChanges::detectFileChange [...] <
(FreeFileSync::SelectedSide)0u'

says that the expression is being interpreted as 'x < y' where x is the member
'detectFileChange' and y is an integer constant. There is no appropriate
operator< available so it lists the visible overloads of operator<

The rules of C++ specifically say that the compiler must interpret it like
that, see 14.3 [temp.names] paragraph 4. The way to resolve it is to say you
are calling a template member function, by inserting the keyword 'template'
before the template-id, as I showed.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41614