Do not mutate calls to block given

lib/mutant/mutator/call.rb

@@ -67,11 +67,13 @@ module Mutant
       # @api private
       #
       def emit_implicit_self_receiver
+        # FIXME: Edge case that is currently not very well undestood
+        return if name == :block_given?
         return unless self?
         mutant = dup_node
         mutant.privately = true
         # TODO: Fix rubinius to allow this as an attr_accessor
-        mutant.instance_variable_set(:@vcall_style,true)
+        mutant.instance_variable_set(:@vcall_style, true)
         emit(mutant)
       end
 

spec/unit/mutant/mutator/call/mutation_spec.rb

@@ -2,6 +2,20 @@ require 'spec_helper'
 
 describe Mutant::Mutator, 'call' do
   context 'send without arguments' do
+    # This could not be reproduced in a test case but happens in the mutant source code?
+    context 'block given' do
+      context 'implicit self' do
+        let(:source) { 'block_given?' }
+
+        it_should_behave_like 'a noop mutator'
+      end
+      
+      context 'explicit self' do
+        let(:source) { 'self.block_given?' }
+
+        it_should_behave_like 'a noop mutator'
+      end
+    end
     context 'with self as receiver' do
 
       context 'implicit' do