Change DecrementInteger and IncrementInteger mutators behaviour

src/Mutator/Number/DecrementInteger.php

@@ -74,7 +74,15 @@ public static function getDefinition(): ?Definition
      */
     public function mutate(Node $node): iterable
     {
-        yield new Node\Scalar\LNumber($node->value - 1);
+        $parentNode = ParentConnector::getParent($node);
+
+        $value = $node->value - 1;
+
+        if ($parentNode instanceof Node\Expr\UnaryMinus) {
+            $value = $node->value + 1;
+        }
+
+        yield new Node\Scalar\LNumber($value);
     }
 
     public function canMutate(Node $node): bool

src/Mutator/Number/IncrementInteger.php

@@ -38,6 +38,7 @@
 use Infection\Mutator\Definition;
 use Infection\Mutator\GetMutatorName;
 use Infection\Mutator\MutatorCategory;
+use Infection\PhpParser\Visitor\ParentConnector;
 use PhpParser\Node;
 
 /**
@@ -63,7 +64,15 @@ public static function getDefinition(): ?Definition
      */
     public function mutate(Node $node): iterable
     {
-        yield new Node\Scalar\LNumber($node->value + 1);
+        $parentNode = ParentConnector::getParent($node);
+
+        $value = $node->value + 1;
+
+        if ($parentNode instanceof Node\Expr\UnaryMinus) {
+            $value = $node->value - 1;
+        }
+
+        yield new Node\Scalar\LNumber($value);
     }
 
     public function canMutate(Node $node): bool

tests/phpunit/Mutator/Number/DecrementIntegerTest.php

@@ -300,7 +300,7 @@ public function mutationsProvider(): iterable
             <<<'PHP'
 <?php
 
-if ($foo === -9) {
+if ($foo === -11) {
     echo 'bar';
 }
 PHP

tests/phpunit/Mutator/Number/IncrementIntegerTest.php

@@ -110,7 +110,7 @@ public function mutationsProvider(): iterable
 PHP
         ];
 
-        yield 'It decrements a negative integer' => [
+        yield 'It increments a negative integer' => [
             <<<'PHP'
 <?php
 
@@ -122,7 +122,7 @@ public function mutationsProvider(): iterable
             <<<'PHP'
 <?php
 
-if ($foo === -11) {
+if ($foo === -9) {
     echo 'bar';
 }
 PHP