Proc from Symbol needs a receiver

So its arity should be -2 instead of -1.

[Bug #16640]
https://bugs.ruby-lang.org/issues/16640#change-84337

spec/ruby/core/symbol/to_proc_spec.rb

@@ -12,18 +12,20 @@ describe "Symbol#to_proc" do
     :to_s.to_proc.call(obj).should == "Received #to_s"
   end
 
-  it "produces a proc with arity -1" do
+  expected_arity = ruby_version_is("2.8") {-2} || -1
+  it "produces a proc with arity #{expected_arity}" do
     pr = :to_s.to_proc
-    pr.arity.should == -1
+    pr.arity.should == expected_arity
   end
 
   it "raises an ArgumentError when calling #call on the Proc without receiver" do
     -> { :object_id.to_proc.call }.should raise_error(ArgumentError, "no receiver given")
   end
 
-  it "produces a proc that always returns [[:rest]] for #parameters" do
+  expected_parameters = ruby_version_is("2.8") {[[:req], [:rest]]} || [[:rest]]
+  it "produces a proc that always returns #{expected_parameters} for #parameters" do
     pr = :to_s.to_proc
-    pr.parameters.should == [[:rest]]
+    pr.parameters.should == expected_parameters
   end
 
   it "passes along the block passed to Proc#call" do