Add test to cover assigning a numbered parameter

spec/ruby/language/numbered_parameters_spec.rb

@@ -80,6 +80,20 @@
       -> { eval("['a'].map { |x| _1 }") }.should raise_error(SyntaxError, /ordinary parameter is defined/)
     end
 
+    describe "assigning to a numbered parameter" do
+      ruby_version_is '2.7'...'3.0' do
+        it "warns" do
+          -> { eval("proc { _1 = 0 }") }.should complain(/warning: `_1' is reserved for numbered parameter; consider another name/)
+        end
+      end
+
+      ruby_version_is '3.0' do
+        it "raises SyntaxError" do
+          -> { eval("proc { _1 = 0 }") }.should raise_error(SyntaxError, /_1 is reserved for numbered parameter/)
+        end
+      end
+    end
+
     it "affects block arity" do
       -> { _1 }.arity.should == 1
       -> { _2 }.arity.should == 2