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

proc.c

@@ -1091,7 +1091,8 @@ rb_vm_block_min_max_arity(const struct rb_block *block, int *max)
 	    return ifunc->argc.min;
 	}
       case block_type_symbol:
-	break;
+        *max = UNLIMITED_ARGUMENTS;
+        return 1;
     }
     *max = UNLIMITED_ARGUMENTS;
     return 0;

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

test/ruby/test_symbol.rb

@@ -157,6 +157,10 @@ class TestSymbol < Test::Unit::TestCase
     assert_predicate(:itself.to_proc, :lambda?)
   end
 
+  def test_to_proc_arity
+    assert_equal(-2, :itself.to_proc.arity)
+  end
+
   def test_to_proc_call_with_symbol_proc
     first = 1
     bug11594 = "[ruby-core:71088] [Bug #11594] corrupted the first local variable"
@@ -187,6 +191,10 @@ class TestSymbol < Test::Unit::TestCase
     assert_predicate(_test_to_proc_with_refinements_call(&:hoge), :lambda?)
   end
 
+  def test_to_proc_arity_with_refinements
+    assert_equal(-2, _test_to_proc_with_refinements_call(&:hoge).arity)
+  end
+
   def self._test_to_proc_arg_with_refinements_call(&block)
     block.call TestToPRocArgWithRefinements.new
   end
@@ -230,11 +238,11 @@ class TestSymbol < Test::Unit::TestCase
     begin;
       bug11845 = '[ruby-core:72381] [Bug #11845]'
       assert_nil(:class.to_proc.source_location, bug11845)
-      assert_equal([[:rest]], :class.to_proc.parameters, bug11845)
+      assert_equal([[:req], [:rest]], :class.to_proc.parameters, bug11845)
       c = Class.new {define_method(:klass, :class.to_proc)}
       m = c.instance_method(:klass)
       assert_nil(m.source_location, bug11845)
-      assert_equal([[:rest]], m.parameters, bug11845)
+      assert_equal([[:req], [:rest]], m.parameters, bug11845)
     end;
   end
 

vm_args.c

@@ -868,7 +868,7 @@ vm_caller_setup_arg_block(const rb_execution_context_t *ec, rb_control_frame_t *
                     rb_ary_push(callback_arg, block_code);
                     rb_ary_push(callback_arg, ref);
                     OBJ_FREEZE_RAW(callback_arg);
-                    func = rb_func_lambda_new(refine_sym_proc_call, callback_arg, 0, UNLIMITED_ARGUMENTS);
+                    func = rb_func_lambda_new(refine_sym_proc_call, callback_arg, 1, UNLIMITED_ARGUMENTS);
 		    rb_hash_aset(ref, block_code, func);
 		}
 		block_code = func;