[cfe-dev] Explaining unqualified lookup in templates

Richard Smith richard at metafoo.co.uk
Wed Apr 4 10:35:30 PDT 2012

On Wed, Apr 4, 2012 at 5:35 AM, Bruce Merry <bmerry at gmail.com> wrote:

> Hi
> With reference to http://clang.llvm.org/compatibility.html#dep_lookup:
> I've just run into exactly the problem with overloaded operator<< that
> this page describes. I've modified my code as suggested, but out of
> interest I'm curious as to why the unqualified lookup on dependent
> names is done immediately rather than deferred until template
> instantiation (when the argument-dependent lookup is done). It seems
> to contradict the C++11 spec 14.6.2 (it looks like C++98 has the same
> language as well), which says:
> "If an operand of an operator is a type-dependent expression, the
> operator also denotes a dependent name. Such names are unbound and are
> looked up at the point of the template instantiation ( in
> both the context of the template definition and the context of the
> point of instantiation."
> In the example at the URL above, std::cout<<value has a second
> argument which is type-dependent, so I would expect operator<< to be
> looked up in the context of the point of instantiation, at which point
> the appropriate overload is defined.

Notionally, yes, name lookup occurs at the point of instantiation. But that
doesn't mean that name lookup finds names which have been declared since
the template was defined. The relevant section is

"For a function call that depends on a template parameter, the candidate
functions are found using the usual
lookup rules (3.4.1, 3.4.2, 3.4.3) except that:
— For the part of the lookup using unqualified name lookup (3.4.1) or
qualified name lookup (3.4.3), only
function declarations from the template definition context are found.
— For the part of the lookup using associated namespaces (3.4.2), only
function declarations found in
either the template definition context or the template instantiation context
are found."
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