Fix possibly-private error

src/Psalm/Checker/Statements/Expression/CallChecker.php

@@ -791,6 +791,15 @@ class CallChecker
 
                 $method_id = $fq_class_name . '::' . strtolower($stmt->name);
 
+                if ($var_id === '$this' &&
+                    $context->self &&
+                    $fq_class_name !== $context->self &&
+                    MethodChecker::methodExists($context->self . '::' . strtolower($stmt->name), $file_checker)
+                ) {
+                    $method_id = $context->self . '::' . strtolower($stmt->name);
+                    $fq_class_name = $context->self;
+                }
+
                 if ($intersection_types && !MethodChecker::methodExists($method_id, $file_checker)) {
                     foreach ($intersection_types as $intersection_type) {
                         $method_id = $intersection_type->value . '::' . strtolower($stmt->name);

tests/ClassScopeTest.php

@@ -428,4 +428,30 @@ class ClassScopeTest extends PHPUnit_Framework_TestCase
         $file_checker = new FileChecker('somefile.php', $this->project_checker, $stmts);
         $file_checker->visitAndAnalyzeMethods();
     }
+
+    /**
+     * @return void
+     */
+    public function testDefinedPrivateMethod()
+    {
+        $stmts = self::$parser->parse('<?php
+        class A {
+            public function foo() : void {
+                if ($this instanceof B) {
+                    $this->boop();
+                }
+            }
+
+            private function boop() : void {}
+        }
+
+        class B extends A {
+            private function boop() : void {}
+        }
+        ');
+
+        $file_checker = new FileChecker('somefile.php', $this->project_checker, $stmts);
+        $context = new Context();
+        $file_checker->visitAndAnalyzeMethods($context);
+    }
 }