[ruby-core:99850] [Ruby master Bug#14437] Integer == doesn't work with coerce since 2.4 (and != since 1.9). Should it?

Issue #14437 has been updated by jeremyevans0 (Jeremy Evans).

Status changed from Open to Closed

taw (Tomasz Wegrzanowski) wrote in #note-2:
> Looking at the code fix_equal(x,y) when y was of non-core class used to call num_equal,(x,y) which then did reverse call for y==x,
> so it was possible to have custom classes, which have meaningful interaction with 42 == obj
> 
> It still does this, except now it casts the result to true/false.
> 
> ~~~
> static VALUE
> num_equal(VALUE x, VALUE y)
> {
>     VALUE result;
>     if (x == y) return Qtrue;
>     result = num_funcall1(y, id_eq, x);
>     if (RTEST(result)) return Qtrue;
>     return Qfalse;
> }
> ~~~
> 
> I can see it comes from this change https://github.com/ruby/ruby/commit/ffa371d9aa1af1f22c41063add9af3e4922f2f12
> 
> 
> Why was this changed? It's messing up with my use case here, and there's a bunch of other ruby code (like rspec)
> which make == return something else than true/false.

The `Integer#==` method is documented to only return true or false (see https://docs.ruby-lang.org/en/master/Integer.html#method-i-3D-3D), so returning other values would be considered a bug.  Code that wants `==` to return a custom value should now use `obj == integer` instead of `integer == obj`.

----------------------------------------
Bug #14437: Integer == doesn't work with coerce since 2.4 (and != since 1.9). Should it?
https://bugs.ruby-lang.org/issues/14437#change-87377

* Author: taw (Tomasz Wegrzanowski)
* Status: Closed
* Priority: Normal
* ruby -v: 2.5.0
* Backport: 2.3: UNKNOWN, 2.4: UNKNOWN, 2.5: UNKNOWN
----------------------------------------
Here's extracted test sample:

~~~ruby
class Item
  def initialize(value)
    @value = value
  end

  def coerce(other)
    [Item.new(other), self]
  end

  def ==(other)
    Item.new("#{inspect} == #{other.inspect}")
  end

  def !=(other)
    Item.new("#{inspect} != #{other.inspect}")
  end

  def inspect
    "(#{@value})"
  end
end

a = Item.new("a")
p [RUBY_VERSION, 42 == a, 42 != a, a == 42, a == a, a != 42, a != a]
~~~

I'd expect it to print: `["2.x.x", ((a) == 42), ((a) != 42), ((a) == 42), ((a) == (a)), ((a) != 42), ((a) != (a))]`

What happens instead is:

~~~
["1.9.3", ((a) == 42), false, ((a) == 42), ((a) == (a)), ((a) != 42), ((a) != (a))]
["2.2.0", ((a) == 42), false, ((a) == 42), ((a) == (a)), ((a) != 42), ((a) != (a))]
["2.3.3", ((a) == 42), false, ((a) == 42), ((a) == (a)), ((a) != 42), ((a) != (a))]
["2.4.1", true, false, ((a) == 42), ((a) == (a)), ((a) != 42), ((a) != (a))]
["2.5.0", true, false, ((a) == 42), ((a) == (a)), ((a) != 42), ((a) != (a))]
~~~

So != never used coerce, and now == doesn't use coerce either.
Using Bignum value instead of 42 in this example breaks it even pre-2.4.

So, the question is:
* is using coerce like this not supported, and it was just an accident that it used to work? (and I should redefine Integer#== and Integer#!=)
* or is it meant to work, and it's a ruby bug?

For context, it's a problem for z3 gem, which builds big mathematical expressions like Z3.Int("a")+Z3.Int("b") == 4
and then uses Microsoft Z3 solver to solve them. Not being able to use == / != because of this issue would really reduce its usability.

Using coerce this way works just fine with +, -, *, >=, etc., it's just == and != which don't work.



-- 
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