We need to centralize the definition of "immediate context" wrt template instantiation so we can eliminate spammy verbiage like the following in the description of is_assignable:

Only the validity of
the immediate context of
the assignment expression
is considered. [ Note: The
compilation of the
expression can result in
side effects such as the
instantiation of class
template specializations
and function template
specializations, the
generation of
implicitly-defined
functions, and so on. Such
side effects are not in the
“immediate context” and
can result in the program
being ill-formed. —end
note ]

In addition to all the current places where "immediate context" is described in the current standard, similar wording can be found in proposals for SFINAE-friendly iterator_traits, common_type, and invocation_traits.