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diff --git a/spec/ruby/core/proc/parameters_spec.rb b/spec/ruby/core/proc/parameters_spec.rb
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+++ b/spec/ruby/core/proc/parameters_spec.rb
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+require_relative '../../spec_helper'
+
+describe "Proc#parameters" do
+  it "returns an empty Array for a proc expecting no parameters" do
+    proc {}.parameters.should == []
+  end
+
+  it "returns an Array of Arrays for a proc expecting parameters" do
+    p = proc {|x| }
+    p.parameters.should.instance_of?(Array)
+    p.parameters.first.should.instance_of?(Array)
+  end
+
+  it "sets the first element of each sub-Array to :opt for optional arguments" do
+    proc {|x| }.parameters.first.first.should == :opt
+    proc {|y,*x| }.parameters.first.first.should == :opt
+  end
+
+  it "regards named parameters in procs as optional" do
+    proc {|x| }.parameters.first.first.should == :opt
+  end
+
+  it "sets the first element of each sub-Array to :req for required argument if lambda keyword used" do
+    proc {|x| }.parameters(lambda: true).first.first.should == :req
+    proc {|y,*x| }.parameters(lambda: true).first.first.should == :req
+  end
+
+  it "regards named parameters in procs as required if lambda keyword used" do
+    proc {|x| }.parameters(lambda: true).first.first.should == :req
+  end
+
+  it "regards named parameters in lambda as optional if lambda: false keyword used" do
+    -> x { }.parameters(lambda: false).first.first.should == :opt
+  end
+
+  it "regards named parameters in procs and lambdas as required if lambda keyword is truthy" do
+    proc {|x| }.parameters(lambda: 123).first.first.should == :req
+    -> x { }.parameters(lambda: 123).first.first.should == :req
+  end
+
+  it "ignores the lambda keyword if it is nil" do
+    proc {|x|}.parameters(lambda: nil).first.first.should == :opt
+    -> x { }.parameters(lambda: nil).first.first.should == :req
+  end
+
+  it "regards optional keyword parameters in procs as optional" do
+    proc {|x: :y| }.parameters.first.first.should == :key
+  end
+
+  it "regards parameters with default values as optional" do
+    -> x=1 { }.parameters.first.first.should == :opt
+    proc {|x=1| }.parameters.first.first.should == :opt
+  end
+
+  it "sets the first element of each sub-Array to :req for required arguments" do
+    -> x, y=[] { }.parameters.first.first.should == :req
+    -> y, *x { }.parameters.first.first.should == :req
+  end
+
+  it "regards named parameters in lambdas as required" do
+    -> x { }.parameters.first.first.should == :req
+  end
+
+  it "regards keyword parameters in lambdas as required" do
+    -> x: { }.parameters.first.first.should == :keyreq
+  end
+
+  it "sets the first element of each sub-Array to :rest for parameters prefixed with asterisks" do
+    -> *x { }.parameters.first.first.should == :rest
+    -> x, *y { }.parameters.last.first.should == :rest
+    proc {|*x| }.parameters.first.first.should == :rest
+    proc {|x,*y| }.parameters.last.first.should == :rest
+  end
+
+  it "sets the first element of each sub-Array to :keyrest for parameters prefixed with double asterisks" do
+    -> **x { }.parameters.first.first.should == :keyrest
+    -> x, **y { }.parameters.last.first.should == :keyrest
+    proc {|**x| }.parameters.first.first.should == :keyrest
+    proc {|x,**y| }.parameters.last.first.should == :keyrest
+  end
+
+  it "sets the first element of each sub-Array to :block for parameters prefixed with ampersands" do
+    -> &x { }.parameters.first.first.should == :block
+    -> x, &y { }.parameters.last.first.should == :block
+    proc {|&x| }.parameters.first.first.should == :block
+    proc {|x,&y| }.parameters.last.first.should == :block
+  end
+
+  it "sets the second element of each sub-Array to the name of the argument" do
+    -> x { }.parameters.first.last.should == :x
+    -> x=Math::PI { }.parameters.first.last.should == :x
+    -> an_argument, glark, &foo { }.parameters[1].last.should == :glark
+    -> *rest { }.parameters.first.last.should == :rest
+    -> &block { }.parameters.first.last.should == :block
+    proc {|x| }.parameters.first.last.should == :x
+    proc {|x=Math::PI| }.parameters.first.last.should == :x
+    proc {|an_argument, glark, &foo| }.parameters[1].last.should == :glark
+    proc {|*rest| }.parameters.first.last.should == :rest
+    proc {|&block| }.parameters.first.last.should == :block
+  end
+
+  it "ignores unnamed rest arguments" do
+    -> x {}.parameters.should == [[:req, :x]]
+  end
+
+  it "ignores implicit rest arguments" do
+    proc { |x, | }.parameters.should == [[:opt, :x]]
+    -> x { }.parameters.should == [[:req, :x]]
+  end
+
+  it "adds rest arg with name * for \"star\" argument" do
+    -> * {}.parameters.should == [[:rest, :*]]
+  end
+
+  it "adds keyrest arg with ** as a name for \"double star\" argument" do
+    -> ** {}.parameters.should == [[:keyrest, :**]]
+  end
+
+  it "adds block arg with name & for anonymous block argument" do
+    -> & {}.parameters.should == [[:block, :&]]
+  end
+
+  it "does not add locals as block options with a block and splat" do
+    -> *args, &blk do
+      local_is_not_parameter = {}
+    end.parameters.should == [[:rest, :args], [:block, :blk]]
+    proc do |*args, &blk|
+      local_is_not_parameter = {}
+    end.parameters.should == [[:rest, :args], [:block, :blk]]
+  end
+
+  it "returns all parameters defined with the name _ as _" do
+    proc = proc {|_, _, _ = 1, *_, _:, _: 2, **_, &_| }
+    proc.parameters.should == [
+      [:opt, :_],
+      [:opt, :_],
+      [:opt, :_],
+      [:rest, :_],
+      [:keyreq, :_],
+      [:key, :_],
+      [:keyrest, :_],
+      [:block, :_]
+    ]
+
+    lambda = -> _, _, _ = 1, *_, _:, _: 2, **_, &_ {}
+    lambda.parameters.should == [
+      [:req, :_],
+      [:req, :_],
+      [:opt, :_],
+      [:rest, :_],
+      [:keyreq, :_],
+      [:key, :_],
+      [:keyrest, :_],
+      [:block, :_]
+    ]
+  end
+
+  it "returns :nokey for **nil parameter" do
+    proc { |**nil| }.parameters.should == [[:nokey]]
+  end
+
+  ruby_version_is "3.4"..."4.0" do
+    it "handles the usage of `it` as a parameter" do
+      eval("proc { it }").parameters.should == [[:opt, nil]]
+      eval("lambda { it }").parameters.should == [[:req]]
+    end
+  end
+
+  ruby_version_is "4.0" do
+    it "handles the usage of `it` as a parameter" do
+      eval("proc { it }").parameters.should == [[:opt]]
+      eval("lambda { it }").parameters.should == [[:req]]
+    end
+  end
+
+  ruby_version_is "4.1" do
+    it "returns :noblock for &nil parameter" do
+      eval <<~RUBY
+        proc { |&nil| }.parameters.should == [[:noblock]]
+      RUBY
+    end
+  end
+end