[Fix #6972] Fix a false positive for Style/MixinUsage

Fixes #6972

This PR fixes a false positive for `Style/MixinUsage` when using
inside block and `if` condition is after `include`.

Author: koic

CHANGELOG.md

@@ -8,6 +8,7 @@
 * [#6996](https://github.com/rubocop-hq/rubocop/pull/6996): Fix a false positive for `Style/RedundantFreeze` when freezing the result of `String#*`. ([@bquorning][])
 * [#6998](https://github.com/rubocop-hq/rubocop/pull/6998): Fix autocorrect of `Naming/RescuedExceptionsVariableName` to also rename all references to the variable. ([@Darhazer][])
 * [#6992](https://github.com/rubocop-hq/rubocop/pull/6992): Fix unknown default configuration for `Layout/IndentFirstParameter` cop. ([@drenmi][])
+* [#6972](https://github.com/rubocop-hq/rubocop/issues/6972): Fix a false positive for `Style/MixinUsage` when using inside block and `if` condition is after `include`. ([@koic][])
 
 ## 0.68.0 (2019-04-29)
 

lib/rubocop/ast/node/mixin/method_dispatch_node.rb

@@ -212,7 +212,7 @@ def binary_operation?
 
       def_node_matcher :macro_scope?, <<-PATTERN
         {^{({sclass class module block} ...) class_constructor?}
-         ^^{({sclass class module block} ... (begin ...)) class_constructor?}
+         ^^{({sclass class module block} ... ({begin if} ...)) class_constructor?}
          ^#macro_kwbegin_wrapper?
          #root_node?}
       PATTERN

spec/rubocop/cop/style/mixin_usage_spec.rb

@@ -62,6 +62,15 @@ class C
       RUBY
     end
 
+    it 'does not register an offense when using inside block ' \
+       'and `if` condition is after `include`' do
+      expect_no_offenses(<<-RUBY.strip_indent)
+        klass.class_eval do
+          include M if defined?(M)
+        end
+      RUBY
+    end
+
     it "doesn't register an offense when `include` call is a method argument" do
       expect_no_offenses(<<-RUBY.strip_indent)
         do_something(include(M))