Fix forward arg mutations

[Fix #1316]

* Do not emit naked forward arg argument promotion
* Do not mutate away forard args, this would syntactically
  invalidate the generated source.

Changelog.md

@@ -1,3 +1,9 @@
+# v0.11.6 2022-04-10
+
+* [#1317](https://github.com/mbj/mutant/pull/1317)
+
+  Fix forward arg mutations.
+
 # v0.11.5 2022-04-03
 
 * [#1314](https://github.com/mbj/mutant/pull/1314)

Gemfile.lock

@@ -1,7 +1,7 @@
 PATH
   remote: .
   specs:
-    mutant (0.11.5)
+    mutant (0.11.6)
       diff-lcs (~> 1.3)
       parser (~> 3.1.0)
       regexp_parser (~> 2.0, >= 2.0.3)

lib/mutant/mutator/node/arguments.rb

@@ -19,17 +19,27 @@ def dispatch
         end
 
         def emit_argument_presence
-          emit_type unless removed_block_arg?(EMPTY_ARRAY)
+          emit_type unless removed_block_arg?(EMPTY_ARRAY) || forward_arg?
 
-          Util::Array::Presence.call(children).each do |children|
-            unless removed_block_arg?(children) || (children.one? && n_mlhs?(children.first))
-              emit_type(*children)
+          children.each_with_index do |removed, index|
+            new_arguments = children.dup
+            new_arguments.delete_at(index)
+            unless n_forward_arg?(removed) || removed_block_arg?(new_arguments) || only_mlhs?(new_arguments)
+              emit_type(*new_arguments)
             end
           end
         end
 
-        def removed_block_arg?(children)
-          anonymous_block_arg? && children.none?(&ANONYMOUS_BLOCKARG_PRED)
+        def only_mlhs?(new_arguments)
+          new_arguments.one? && n_mlhs?(new_arguments.first)
+        end
+
+        def forward_arg?
+          children.last && n_forward_arg?(children.last)
+        end
+
+        def removed_block_arg?(new_arguments)
+          anonymous_block_arg? && new_arguments.none?(&ANONYMOUS_BLOCKARG_PRED)
         end
 
         def anonymous_block_arg?

lib/mutant/mutator/node/send.rb

@@ -241,7 +241,9 @@ def emit_argument_propagation
 
           argument = Mutant::Util.one(arguments)
 
-          emit_propagation(argument) unless n_kwargs?(argument)
+          return if n_kwargs?(argument) || n_forwarded_args?(argument)
+
+          emit_propagation(argument)
         end
 
         def mutate_receiver

lib/mutant/version.rb

@@ -2,5 +2,5 @@
 
 module Mutant
   # Current mutant version
-  VERSION = '0.11.5'
+  VERSION = '0.11.6'
 end # Mutant

meta/def.rb

@@ -187,3 +187,29 @@
   mutation 'def self.foo(a, b); raise; end'
   mutation 'def self.foo(a, b); super; end'
 end
+
+Mutant::Meta::Example.add :def do
+  source 'def foo(...); end'
+
+  mutation 'def foo(...); raise; end'
+  mutation 'def foo(...); super; end'
+end
+
+Mutant::Meta::Example.add :def do
+  source 'def foo(a, ...); end'
+
+  mutation 'def foo(_a, ...); end'
+  mutation 'def foo(a, ...); raise; end'
+  mutation 'def foo(a, ...); super; end'
+  mutation 'def foo(...); end'
+end
+
+Mutant::Meta::Example.add :def, :send do
+  source 'def foo(...); bar(...) end'
+
+  mutation 'def foo(...); bar; end'
+  mutation 'def foo(...); raise; end'
+  mutation 'def foo(...); super; end'
+  mutation 'def foo(...); nil; end'
+  mutation 'def foo(...); end'
+end