Fix numeric coercion when #coerce isn't public

Author: andrykonchin

CHANGELOG.md

@@ -17,6 +17,7 @@ Compatibility:
 * Fix explicitly inherited `Struct` subclasses and don't provide `#members` method (#3802, @andrykonchin).
 * Support Digest plugins (#1390, @nirvdrum).
 * Joni has been updated from 2.2.1 to 2.2.6 (@andrykonchin).
+* Fix numeric coercing when `#coerce` method is not public (#3848, @andrykonchin).
 
 Performance:
 

spec/ruby/core/integer/divide_spec.rb

@@ -106,4 +106,21 @@
       -> { @bignum / :symbol }.should raise_error(TypeError)
     end
   end
+
+  it "coerces the RHS and calls #coerce" do
+    obj = mock("integer plus")
+    obj.should_receive(:coerce).with(6).and_return([6, 3])
+    (6 / obj).should == 2
+  end
+
+  it "coerces the RHS and calls #coerce even if it's private" do
+    obj = Object.new
+    class << obj
+      private def coerce(n)
+        [n, 3]
+      end
+    end
+
+    (6 / obj).should == 2
+  end
 end

spec/ruby/core/integer/minus_spec.rb

@@ -40,4 +40,21 @@
       -> { @bignum - :symbol }.should raise_error(TypeError)
     end
   end
+
+  it "coerces the RHS and calls #coerce" do
+    obj = mock("integer plus")
+    obj.should_receive(:coerce).with(5).and_return([5, 10])
+    (5 - obj).should == -5
+  end
+
+  it "coerces the RHS and calls #coerce even if it's private" do
+    obj = Object.new
+    class << obj
+      private def coerce(n)
+        [n, 10]
+      end
+    end
+
+    (5 - obj).should == -5
+  end
 end

spec/ruby/core/integer/plus_spec.rb

@@ -55,4 +55,21 @@ def +(other)
     RUBY
     ruby_exe(code).should == "-1"
   end
+
+  it "coerces the RHS and calls #coerce" do
+    obj = mock("integer plus")
+    obj.should_receive(:coerce).with(6).and_return([6, 3])
+    (6 + obj).should == 9
+  end
+
+  it "coerces the RHS and calls #coerce even if it's private" do
+    obj = Object.new
+    class << obj
+      private def coerce(n)
+        [n, 3]
+      end
+    end
+
+    (6 + obj).should == 9
+  end
 end

src/main/ruby/truffleruby/core/numeric.rb

@@ -179,9 +179,9 @@ def coerce(other)
 
   def math_coerce(other, error = :coerce_error)
     other = Truffle::Interop.unbox_if_needed(other)
-    return math_coerce_error(other, error) unless other.respond_to? :coerce
+    return math_coerce_error(other, error) unless other.respond_to?(:coerce, true)
 
-    values = other.coerce(self)
+    values = other.__send__(:coerce, self)
 
     unless Primitive.is_a?(values, Array) && values.length == 2
       if error == :no_error