Add Range#minmax specs

This adds specs for Range#minmax, since the implementation provided by
Enumerable is overidden as of 2.7, to try to skip enumerating the range.

Author: sambostock

core/range/minmax_spec.rb

@@ -0,0 +1,55 @@
+require_relative '../../spec_helper'
+
+describe 'Range#minmax' do
+  before(:each) do
+    @x = mock('x')
+    @y = mock('y')
+
+    @x.should_receive(:<=>).with(@y).any_number_of_times.and_return(-1)
+    @x.should_receive(:<=>).with(@x).any_number_of_times.and_return(0)
+    @y.should_receive(:<=>).with(@x).any_number_of_times.and_return(1)
+    @y.should_receive(:<=>).with(@y).any_number_of_times.and_return(0)
+  end
+
+  describe 'by enumerating the range' do
+    before(:each) do
+      @x.should_receive(:succ).once.and_return(@y)
+    end
+
+    ruby_version_is ''...'2.7' do
+      it 'should return the minimum and maximum elements' do
+        (@x..@y).minmax.should == [@x, @y]
+      end
+    end
+
+    it 'should not return the end of the range as maximum if exclusive range' do
+      (@x...@y).minmax.should == [@x, @x]
+    end
+
+    it 'should respect custom comparison block' do
+      (@x..@y).minmax { |x, y| - (x <=> y) }.should == [@y, @x]
+    end
+  end
+
+  describe 'without enumerating the range' do
+    before(:each) do
+      # Prevent enumeration, but allow members to respond_to? :succ
+      @x.should_not_receive(:succ)
+      @y.should_not_receive(:succ)
+    end
+
+    ruby_version_is '2.7' do
+      it 'should return the minimum and maximum elements' do
+        (@x..@y).minmax.should == [@x, @y]
+      end
+    end
+
+    it 'should return nil pair if empty inclusive range' do
+      (@y..@x).minmax.should == [nil, nil]
+    end
+
+    it 'should return nil pair if empty exclusive range' do
+      (@x...@x).minmax.should == [nil, nil]
+    end
+  end
+end