I have sent the following letter (with minor alterations) to the head of ACM SIGPLAN (Association for Computer Machinery, special interest group on programming languages), and the steering committees of CAV (International conference on Computer-Aided Verification) and VMCAI (International conference on Verification, Model-Checking and Abstract Interpretation):

Recent developments in US politics seem to indicate that US visa policies in the forthcoming years are likely to be haphazard, with abrupt changes, people with valid visas being denied entry, researchers from certain citizenships or religions being denied entry, perhaps the Visa waiver program being scrapped.

In these circumstances, I wonder about the wisdom of holding scientific conferences in the United States.

Do you have an official position on such matters? If not,shouldn't this be discussed?

Michael Hicks, head of SIGPLAN, informed me that ACM supported the Computer Research Association's statement on President Trump's executive order. The ACM has issued its own statement. The CAV steering committee has informed me that CAV 2018 (as well as 2017) is to take place in Europe, thus 2019 and 2020 are to take place in North America but that logistical and political factors will be taken into account when choosing the location.

In the meantime, several colleagues have expressed concerns about possible gratuitous harassment of students etc. that are going to conferences in the United States.

I remind prospective students or postgraduate researchers that face or may face difficulties coming to the United States that our excellent research facilities in France can welcome you.

PS ACM SIGPLAN's executive committee has issued an official statement  ending with:

While we hope that the EO is rescinded (and will work toward this outcome), we are developing contingency plans for organizing future SIGPLAN events so as to maximize inclusiveness of the worldwide scientific community. We welcome your input.