Paragraph 14.8.2/8 of the C++11 Standard specifies the conditions under which a substitution failure shall or shall not result in a "hard" compilation error (thereby causing compilation to fail) or in a "soft" error which would just cause the compiler to discard a template from a set of candidates for overload resolution (without making compilation fail and enabling the well-known SFINAE idiom):
If a substitution results in an invalid type or expression, type deduction fails. An invalid type or expression is one that would be ill-formed if written using the substituted arguments. [ Note: Access checking is done as part of the substitution process. —end note ] Only invalid types and expressions in the immediate context of the function type and its template parameter types can result in a deduction failure. [...]
The words "immediate context" appear only 8 times in the whole C++11 Standard, and each time they are followed by (or occur as part of) an instance of the following (non-normative) text:
[Note: The evaluation of the substituted types and expressions can result in side effects such as the instantiation of class template specializations and/or function template specializations, the generation of implicitly-defined functions, etc. Such side effects are not in the “immediate context” and can result in the program being ill-formed.—end note ]
The note gives a (not very generous) hint on what is meant by immediate context, but at least for me this is often not enough to decide whether a substitution is or is not supposed to cause a "hard" compilation error.
Could you provide an explanation, a decision procedure, and/or some concrete examples to help figuring out in what cases a substitution error does and does not occur in the "immediate context" of the function type and its template parameter types?