I think it is breaking change.

Maybe it is a breaking change but the behaviour before was certainly a bug.

So a method named method!= shouldn't work?

This PR breaks the following code:

class C
  def method!=(var)
    puts "assigning #{var}"
  end
end

C.new.method! = 4

Error:

Using compiled compiler at `.build/crystal'
Syntax error in test.cr:2: unexpected token: !=

  def method!=(var)
            ^

No, I don't think that code should work. It's easy enough to trim the ! off the property and use method = 4.

Merging. What do bang (foo.bar! = 1) or question (foo.bar? = 2) setters bring to the language? IMHO only weirdness, if not confusion.

@RX14 @ysbaddaden I agree, it's just confusing. I just wanted to point it out, as it wasn't mentionned anywhere and this PR is breaking it.

@bew Thanks for mentioning it, i'm sure it'll make it into the changelog.