Is stateful metaprogramming ill-formed (yet)?

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This is CWG active issue 2118:

Defining a friend function in a template, then referencing that function later provides a means of capturing and retrieving metaprogramming state. This technique is arcane and should be made ill-formed.

Notes from the May, 2015 meeting:

CWG agreed that such techniques should be ill-formed, although the mechanism for prohibiting them is as yet undetermined.

It’s still an active issue, nothing will change in C++17 at least for now. Though when such a prohibition mechanism is determined, this may be retroactively ruled as a DR.

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