Issue #9251 has been updated by shyouhei (Shyouhei Urabe).

Category set to doc

So this is a doc issue.  The book is wrong here.  We have never changed operator preceedence for decades.
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Bug #9251: ! operator has lower precedence than = in an assignment expression
https://bugs.ruby-lang.org/issues/9251#change-43692

Author: rits (First Last)
Status: Feedback
Priority: Low
Assignee: 
Category: doc
Target version: 
ruby -v: ruby 2.0.0p353 (2013-11-22 revision 43784) [i686-linux]
Backport: 1.9.3: UNKNOWN, 2.0.0: UNKNOWN


irb(main):001:0> !a = 1
(irb):1: warning: found = in conditional, should be ==
=> false
irb(main):002:0> a
=> 1

! is supposed to have higher precedence than =, so !a = 1 should be (!a) = 1, and thus an error, not !(a = 1) 



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