Allow private method invocation with self receiver (crystal-lang#6075)

Closes crystal-lang#5995

spec/compiler/semantic/visibility_modifiers_spec.cr

@@ -74,6 +74,22 @@ describe "Visibility modifiers" do
       "private method 'bar' called for Foo"
   end
 
+  it "allows invoking private method from the same class" do
+    assert_type(%(
+      class Foo
+        private def foo
+          1
+        end
+
+        def bar
+          self.foo
+        end
+      end
+
+      Foo.new.bar
+      )) { int32 }
+  end
+
   it "allows invoking protected method from the same class" do
     assert_type(%(
       class Foo

src/compiler/crystal/semantic/call_error.cr

@@ -568,8 +568,8 @@ class Crystal::Call
     case match.def.visibility
     when .private?
       if obj = @obj
-        if obj.is_a?(Var) && obj.name == "self" && match.def.name.ends_with?('=')
-          # Special case: private setter can be called with self
+        if obj.is_a?(Var) && obj.name == "self"
+          # Special case: private method can be called with self
           return
         end