[Fix rubocop#120] Correct save-as-conditional checking

Fixes rubocop#120.

The Rails/SaveBang rule emitted incorrect errors about using the return
value of save-like calls in conditional nodes. It did not differentiate
between such calls in the body versus the condition, where only the
latter is necessarily a boolean expression.

lib/rubocop/cop/rails/save_bang.rb

@@ -212,7 +212,7 @@ def array_parent(node)
         end
 
         def check_used_in_conditional(node)
-          return false unless conditional?(node)
+          return false unless in_boolean_expr?(node)
 
           unless MODIFY_PERSIST_METHODS.include?(node.method_name)
             add_offense_for_node(node, CREATE_CONDITIONAL_MSG)
@@ -221,15 +221,20 @@ def check_used_in_conditional(node)
           true
         end
 
-        def conditional?(node) # rubocop:disable Metrics/CyclomaticComplexity
+        def in_boolean_expr?(node)
           node = node.block_node || node
+          parent = node.parent
+          return false unless parent
 
-          condition = node.parent
-          return false unless condition
+          predicate_node?(parent) || condition_of?(parent, node)
+        end
+
+        def predicate_node?(node)
+          node.or_type? || node.and_type? || single_negative?(node)
+        end
 
-          condition.if_type? || condition.case_type? ||
-            condition.or_type? || condition.and_type? ||
-            single_negative?(condition)
+        def condition_of?(parent, node)
+          (parent.if_type? || parent.case_type?) && node == parent.condition
         end
 
         def allowed_receiver?(node)

spec/rubocop/cop/rails/save_bang_spec.rb

@@ -223,6 +223,14 @@
       end
     end
 
+    it "when using #{method} in the body of a oneline if" do
+      inspect_source("object.#{method} if false")
+
+      expect(cop.messages)
+        .to eq(["Use `#{method}!` instead of `#{method}` " \
+                'if the return value is not checked.'])
+    end
+
     it "when using #{method} with a bunch of hashes & arrays" do
       inspect_source(<<~RUBY)
         return [{ success: object.#{method} }, true]