Add sigunature for Range#minmax

Author: ksss

core/range.rbs

@@ -825,6 +825,51 @@ class Range[out Elem] < Object
   #
   def min: ...
 
+  # <!--
+  #   rdoc-file=range.c
+  #   - minmax -> [object, object]
+  #   - minmax {|a, b| ... } -> [object, object]
+  # -->
+  # Returns a 2-element array containing the minimum and maximum value in `self`,
+  # either according to comparison method `#<=>` or a given block.
+  #
+  # With no block given, returns the minimum and maximum values, using `#<=>` for
+  # comparison:
+  #
+  #     (1..4).minmax     # => [1, 4]
+  #     (1...4).minmax    # => [1, 3]
+  #     ('a'..'d').minmax # => ["a", "d"]
+  #     (-4..-1).minmax   # => [-4, -1]
+  #
+  # With a block given, the block must return an integer:
+  #
+  # *   Negative if `a` is smaller than `b`.
+  # *   Zero if `a` and `b` are equal.
+  # *   Positive if `a` is larger than `b`.
+  #
+  # The block is called `self.size` times to compare elements; returns a 2-element
+  # Array containing the minimum and maximum values from `self`, per the block:
+  #
+  #     (1..4).minmax {|a, b| -(a <=> b) } # => [4, 1]
+  #
+  # Returns `[nil, nil]` if:
+  #
+  # *   The begin value of the range is larger than the end value:
+  #
+  #         (4..1).minmax                      # => [nil, nil]
+  #         (4..1).minmax {|a, b| -(a <=> b) } # => [nil, nil]
+  #
+  # *   The begin value of an exclusive range is equal to the end value:
+  #
+  #         (1...1).minmax                          # => [nil, nil]
+  #         (1...1).minmax  {|a, b| -(a <=> b) }    # => [nil, nil]
+  #
+  # Raises an exception if `self` is a beginless or an endless range.
+  #
+  # Related: Range#min, Range#max.
+  #
+  def minmax: ...
+
   # <!--
   #   rdoc-file=range.c
   #   - overlap?(range) -> true or false

test/stdlib/Range_test.rb

@@ -156,4 +156,11 @@ def test_max
     assert_send_type "(::Integer) -> ::Array[::Integer]", (1..4), :max, 2
     assert_send_type "(::Integer) { (::Integer, ::Integer) -> ::Integer } -> ::Array[::Integer]", (4..1), :max, 0 do |a, b| a <=> b end
   end
+
+  def test_minmax
+    assert_send_type "() -> [::Integer, ::Integer]", (1..4), :minmax
+    assert_send_type "() -> [nil, nil]", [], :minmax
+    assert_send_type "() { (::Integer, ::Integer) -> ::Integer } -> [::Integer, ::Integer]", (1..4), :minmax do |a, b| a.size <=> b.size end
+    assert_send_type "() { (::Integer, ::Integer) -> ::Integer } -> [nil, nil]", [], :minmax do |a, b| a <=> b end
+  end
 end