Do not mutate to invalid AST on reqarg promotion

Closes #153

config/reek.yml

@@ -56,6 +56,7 @@ NestedIterators:
   - Mutant::CLI#parse
   - Mutant::Mutator::Util::Array::Element#dispatch
   - Mutant::Mutator::Node::Resbody#mutate_captures
+  - Mutant::Mutator::Node::Arguments#emit_argument_mutations
   - Mutant::Reporter::CLI::Report::Config#generic_stats
   - Mutant::RequireHighjack#infect
   - Mutant::RequireHighjack#desinfect

lib/mutant/mutator/node.rb

@@ -119,7 +119,7 @@ module Mutant
       # @api private
       #
       def mutate_child(index, mutator = Mutator, &block)
-        block ||= lambda { |_node| true }
+        block ||= ->(_node) { true }
         child = children.at(index)
         mutator.each(child, self) do |mutation|
           if block.call(mutation)
@@ -166,7 +166,7 @@ module Mutant
       # @api private
       #
       def emit_type(*children)
-        emit(new_self(*children))
+        emit(Parser::AST::Node.new(node.type, children))
       end
 
       # Emit singleton literals
@@ -200,16 +200,6 @@ module Mutant
         emit(N_NIL) unless asgn_left?
       end
 
-      # Return new self typed child
-      #
-      # @return [Parser::AST::Node]
-      #
-      # @api private
-      #
-      def new_self(*children)
-        Parser::AST::Node.new(node.type, children)
-      end
-
       # Emit values
       #
       # @param [Array<Object>] values

lib/mutant/mutator/node/and_asgn.rb

@@ -27,8 +27,7 @@ module Mutant
           emit_right_mutations
         end
 
-      end # OrAsgn
+      end # AndAsgn
     end # Node
   end # Mutator
 end # Mutant
-

lib/mutant/mutator/node/arguments.rb

@@ -17,10 +17,55 @@ module Mutant
         # @api private
         #
         def dispatch
-          emit_children_mutations
+          emit_argument_presence
+          emit_argument_mutations
           emit_mlhs_expansion
         end
 
+        # Emit argument presence mutation
+        #
+        # @return [undefined]
+        #
+        # @api private
+        #
+        def emit_argument_presence
+          emit_type
+          Mutator::Util::Array::Presence.each(children, self) do |children|
+            emit_type(*children)
+          end
+        end
+
+        # Emit argument mutations
+        #
+        # @return [undefined]
+        #
+        # @api private
+        #
+        def emit_argument_mutations
+          children.each_with_index do |child, index|
+            Mutator.each(child) do |mutant|
+              unless invalid_argument_replacement?(mutant, index)
+                emit_child_update(index, mutant)
+              end
+            end
+          end
+        end
+
+        # Test if child mutation is allowed
+        #
+        # @param [Parser::AST::Node]
+        #
+        # @return [Boolean]
+        #
+        # @api private
+        #
+        def invalid_argument_replacement?(mutant, index)
+          original = children.fetch(index)
+          original.type.equal?(:optarg) &&
+          mutant.type.equal?(:arg)      &&
+          children[0...index].any? { |node| node.type.equal?(:optarg) }
+        end
+
         # Emit mlhs expansions
         #
         # @return [undefined]
@@ -44,7 +89,7 @@ module Mutant
         #
         def mlhs_childs_with_index
           children.each_with_index.select do |child, _index|
-            child.type == :mlhs
+            child.type.equal?(:mlhs)
           end
         end
 

lib/mutant/mutator/node/next.rb

@@ -28,4 +28,3 @@ module Mutant
     end # Node
   end # Mutator
 end # Mutant
-

meta/def.rb

@@ -69,6 +69,25 @@ Mutant::Meta::Example.add do
   mutation 'def foo; end'
 end
 
+Mutant::Meta::Example.add do
+  source 'def foo(a = 0, b = 0); end'
+  mutation 'def foo(a = 0, b__mutant__ = 0); end'
+  mutation 'def foo(a__mutant__ = 0, b = 0); end'
+  mutation 'def foo(a = 0, b = 1); end'
+  mutation 'def foo(a = 0, b = -1); end'
+  mutation 'def foo(a = 0, b = self); end'
+  mutation 'def foo(a = 0, b = nil); end'
+  mutation 'def foo(a = -1, b = 0); end'
+  mutation 'def foo(a = self, b = 0); end'
+  mutation 'def foo(a = nil, b = 0); end'
+  mutation 'def foo(a = 1, b = 0); end'
+  mutation 'def foo(a = 0); end'
+  mutation 'def foo(b = 0); end'
+  mutation 'def foo(a, b = 0); end'
+  mutation 'def foo; end'
+  mutation 'def foo(a = 0, b = 0); raise; end'
+end
+
 Mutant::Meta::Example.add do
   source 'def foo(a = true); end'