Fix #518 - improve dead code detection to cover variable method calls

src/Psalm/Checker/Statements/Expression/Call/MethodCallChecker.php

@@ -48,6 +48,12 @@ class MethodCallChecker extends \Psalm\Checker\Statements\Expression\CallChecker
             return false;
         }
 
+        if (!is_string($stmt->name)) {
+            if (ExpressionChecker::analyze($statements_checker, $stmt->name, $context) === false) {
+                return false;
+            }
+        }
+
         $method_id = null;
 
         if ($stmt->var instanceof PhpParser\Node\Expr\Variable) {

tests/UnusedCodeTest.php

@@ -623,6 +623,23 @@ class UnusedCodeTest extends TestCase
 
                     new A([1, 2, 3]);',
             ],
+            'usedMethodCallVariable' => [
+                '<?php
+                    function reindex(array $arr, string $methodName): array {
+                        $ret = [];
+
+                        foreach ($arr as $element) {
+                            $ret[$element->$methodName()] = true;
+                        }
+
+                        return $ret;
+                    }',
+                'error_levels' => [
+                    'MixedAssignment',
+                    'MixedMethodCall',
+                    'MixedArrayOffset',
+                ],
+            ],
         ];
     }