Make Lint/LiteralInCondition cop aware of ! and not

By this change, `Lint/LiteralInCondition` cop will register an offence for the following code.

```ruby
!'foo'

not 'foo'
```

CHANGELOG.md

@@ -39,6 +39,7 @@
 * [#4787](https://github.com/bbatsov/rubocop/pull/4787): Analyzing code that needs to support MRI 2.0 is no longer supported. ([@deivid-rodriguez][])
 * [#4787](https://github.com/bbatsov/rubocop/pull/4787): RuboCop no longer installs on MRI 2.0. ([@deivid-rodriguez][])
 * [#4266](https://github.com/bbatsov/rubocop/issues/4266): Download the inherited config files of a remote file from the same remote. ([@tdeo][])
+* [#4853](https://github.com/bbatsov/rubocop/pull/4853): Make `Lint/LiteralInCondition` cop aware of `!` and `not`. ([@pocke][])
 
 ## 0.50.0 (2017-09-14)
 

lib/rubocop/ast/node/send_node.rb

@@ -16,6 +16,10 @@ class SendNode < Node
       def node_parts
         to_a
       end
+
+      def negation_method?
+        keyword_bang? || keyword_not?
+      end
     end
   end
 end

lib/rubocop/cop/lint/literal_in_condition.rb

@@ -65,17 +65,23 @@ def on_case(case_node)
           end
         end
 
+        def on_send(node)
+          return unless node.negation_method?
+          check_for_literal(node)
+        end
+
         def message(node)
           format(MSG, node.source)
         end
 
         private
 
         def check_for_literal(node)
-          if node.condition.literal?
-            add_offense(node.condition)
+          cond = condition(node)
+          if cond.literal?
+            add_offense(cond)
           else
-            check_node(node.condition)
+            check_node(cond)
           end
         end
 
@@ -121,6 +127,14 @@ def check_case(case_node)
 
           handle_node(condition)
         end
+
+        def condition(node)
+          if node.send_type?
+            node.receiver
+          else
+            node.condition
+          end
+        end
       end
     end
   end

spec/rubocop/ast/send_node_spec.rb

@@ -858,4 +858,24 @@
       it { expect(send_node.def_modifier?).to be_truthy }
     end
   end
+
+  describe '#negation_method?' do
+    context 'with keyword `not`' do
+      let(:source) { 'not foo' }
+
+      it { expect(send_node).to be_negation_method }
+    end
+
+    context 'with bang method' do
+      let(:source) { '!foo' }
+
+      it { expect(send_node).to be_negation_method }
+    end
+
+    context 'with normal method' do
+      let(:source) { 'foo.bar' }
+
+      it { expect(send_node).not_to be_negation_method }
+    end
+  end
 end

spec/rubocop/cop/lint/literal_in_condition_spec.rb

@@ -131,6 +131,20 @@
       RUBY
       expect(cop.offenses).to be_empty
     end
+
+    it "registers an offense for `!#{lit}`" do
+      inspect_source(<<-RUBY.strip_indent)
+        !#{lit}
+      RUBY
+      expect(cop.offenses.size).to eq(1)
+    end
+
+    it "registers an offense for `not #{lit}`" do
+      inspect_source(<<-RUBY.strip_indent)
+        !#{lit}
+      RUBY
+      expect(cop.offenses.size).to eq(1)
+    end
   end
 
   it 'accepts array literal in case, if it has non-literal elements' do