Fix issue with unset var in loop

src/Psalm/Checker/StatementsChecker.php

@@ -400,6 +400,12 @@ class StatementsChecker extends SourceChecker implements StatementsSource
                     }
                 }
 
+                foreach ($asserted_vars as $var_id) {
+                    if (!isset($loop_context->vars_in_scope[$var_id])) {
+                        $loop_context->vars_in_scope[$var_id] = $pre_loop_context->vars_in_scope[$var_id];
+                    }
+                }
+
                 // if there are no changes to the types, no need to re-examine
                 if (!$has_changes) {
                     break;

tests/LoopScopeTest.php

@@ -550,6 +550,24 @@ class LoopScopeTest extends PHPUnit_Framework_TestCase
         $file_checker->visitAndAnalyzeMethods();
     }
 
+    /**
+     * @return void
+     */
+    public function testUnsetInLoop()
+    {
+        $stmts = self::$parser->parse('<?php
+        $a = null;
+
+        foreach ([1, 2, 3] as $i) {
+            $a = $i;
+            unset($i);
+        }
+        ');
+
+        $file_checker = new FileChecker('somefile.php', $this->project_checker, $stmts);
+        $file_checker->visitAndAnalyzeMethods();
+    }
+
     /**
      * @return void
      */