Catch unused foreach key when it’s already in scope

src/Psalm/Checker/Statements/Block/ForeachChecker.php

@@ -376,6 +376,11 @@ class ForeachChecker
                     $location,
                     $foreach_context->branch_point
                 );
+            } else {
+                $statements_checker->registerVariableAssignment(
+                    $key_var_id,
+                    $location
+                );
             }
 
             if ($stmt->byRef && $context->collect_references) {

tests/UnusedVariableTest.php

@@ -1314,6 +1314,19 @@ class UnusedVariableTest extends TestCase
                     echo $a;',
                 'error_message' => 'UnusedVariable',
             ],
+            'reusedKeyVar' => [
+                '<?php
+                    $key = "a";
+                    echo $key;
+
+                    $arr = ["foo" => "foo.foo"];
+
+                    foreach ($arr as $key => $v) {
+                        list($key) = explode(".", $v);
+                        echo $key;
+                    }',
+                'error_message' => 'UnusedVariable',
+            ],
         ];
     }
 }