[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]