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Bug introduced in 10.4.1 or earlier and persisting through 11.1.0 or later

Using x++ or x=x+1 inside Defer generates strange results.

Evaluating the following commands one by one (in three different input cells), with Shift+Enter, one gets: 

x=1;

Defer[x++]; 

x
(* 15 but I have also seen 17! *)

Even stranger, if one creates a notebook with only those same commands, restarts the kernel, and does Evaluation ► Evaluate Notebook then the command x returns 1 instead of 15.

Any explanations?

Improve this question
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9
  • $\begingroup$ I get 1 everytime, as I would expect... $\endgroup$
    – ktm
    Oct 8, 2016 at 22:17
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    $\begingroup$ First check: turn off the Suggestions Bar and try again. $\endgroup$
    – Mr.Wizard
    Oct 9, 2016 at 2:18
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    $\begingroup$ @Mr.Wizard - Turning off the Suggestions Bar fixes the problem. Thanks. $\endgroup$
    – Soldalma
    Oct 9, 2016 at 11:03
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    $\begingroup$ @Mr.Wizard What do we do with questions like this? Close it as a duplicate of what you linked? Close it as a duplicate of another question about a Suggestion Bar bug? Post an answer? I'm really disturbed by the amount of problems the Suggestion Bar causes. I think it says more about how difficult it is to manipulate code in Mathematica than about WRI quality control. I would never have dared to attempt to implement something like this myself—the risk of evaluation leaks is much too great. $\endgroup$
    – Szabolcs
    Oct 9, 2016 at 12:50
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    $\begingroup$ @Szabolcs - I reported the bug. $\endgroup$
    – Soldalma
    Oct 9, 2016 at 18:27

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