Allow calling a private method with `self.`

This makes it consistent with calling private attribute assignment
methods, which currently is allowed (e.g. `self.value =`).

Calling a private method in this way can be useful when trying to
assign the return value to a local variable with the same name.

[Feature #11297] [Feature #16123]

compile.c

@@ -6773,6 +6773,10 @@ compile_call(rb_iseq_t *iseq, LINK_ANCHOR *const ret, const NODE *const node, in
             iseq_block_param_id_p(iseq, node->nd_recv->nd_vid, &idx, &level)) {
             ADD_INSN2(recv, nd_line(node->nd_recv), getblockparamproxy, INT2FIX(idx + VM_ENV_DATA_SIZE - 1), INT2FIX(level));
         }
+        else if (private_recv_p(node)) {
+            ADD_INSN(recv, nd_line(node), putself);
+            flag |= VM_CALL_FCALL;
+        }
         else {
             CHECK(COMPILE(recv, "recv", node->nd_recv));
         }

spec/ruby/language/send_spec.rb

@@ -260,7 +260,7 @@ end
 describe "Invoking a private getter method" do
   it "does not permit self as a receiver" do
     receiver = LangSendSpecs::PrivateGetter.new
-    -> { receiver.call_self_foo }.should raise_error(NoMethodError)
+    -> { receiver.call_self_foo }.should_not raise_error(NoMethodError)
     -> { receiver.call_self_foo_or_equals(6) }.should raise_error(NoMethodError)
   end
 end

test/ruby/test_method.rb

@@ -673,7 +673,7 @@ class TestMethod < Test::Unit::TestCase
     assert_nothing_raised { mv3 }
 
     assert_nothing_raised { self.mv1 }
-    assert_raise(NoMethodError) { self.mv2 }
+    assert_nothing_raised { self.mv2 }
     assert_nothing_raised { self.mv3 }
 
     v = Visibility.new

test/ruby/test_module.rb

@@ -2479,7 +2479,8 @@ class TestModule < Test::Unit::TestCase
       assert_include(methods, :#{method}, ":#{method} should be private")
 
       assert_raise_with_message(NoMethodError, "private method `#{method}' called for main:Object") {
-        self.#{method}
+        recv = self
+        recv.#{method}
       }
     }
   end

test/ruby/test_refinement.rb

@@ -538,7 +538,7 @@ class TestRefinement < Test::Unit::TestCase
 
   def test_main_using_is_private
     assert_raise(NoMethodError) do
-      eval("self.using Module.new", Sandbox::BINDING)
+      eval("recv = self; recv.using Module.new", Sandbox::BINDING)
     end
   end