Fix #3045 - ignore mixed assignments to variables named $_ (#3211)

src/Psalm/Internal/Analyzer/Statements/Expression/AssignmentAnalyzer.php

@@ -32,6 +32,7 @@ use Psalm\Type;
 use function is_string;
 use function strpos;
 use function strtolower;
+use function substr;
 
 /**
  * @internal
@@ -342,6 +343,7 @@ class AssignmentAnalyzer
             if (!$assign_var instanceof PhpParser\Node\Expr\PropertyFetch
                 && !strpos($root_var_id ?? '', '->')
                 && !$comment_type
+                && substr($var_id ?? '', 0, 2) !== '$_'
             ) {
                 if (IssueBuffer::accepts(
                     new MixedAssignment(

tests/ReturnTypeTest.php

@@ -787,6 +787,17 @@ class ReturnTypeTest extends TestCase
                     '$res' => 'iterable<mixed, string>',
                 ],
             ],
+            'mixedAssignmentWithUnderscore' => [
+                '<?php
+                    $gen = (function (): Generator {
+                        yield 1 => \'a\';
+                        yield 2 => \'b\';
+                    })();
+
+                    foreach ($gen as $k => $_) {
+                        echo "$k\n";
+                    }'
+            ],
         ];
     }
 

tests/UnusedCodeTest.php

@@ -674,7 +674,6 @@ class UnusedCodeTest extends TestCase
 
                     $items = new IterableResult();
 
-                    /** @psalm-suppress MixedAssignment */
                     foreach ($items as $_item) {}'
             ],
             'usedThroughNewClassStringOfBase' => [