Add support and tests for $this type assertions

Author: ekisu

tests/PHPStan/Analyser/NodeScopeResolverTest.php

@@ -1151,6 +1151,7 @@ public function dataFileAsserts(): iterable
 yield from $this->gatherAssertTypes(__DIR__ . '/data/assert-empty.php');
 yield from $this->gatherAssertTypes(__DIR__ . '/data/assert-method.php');
 yield from $this->gatherAssertTypes(__DIR__ . '/data/assert-property.php');
+yield from $this->gatherAssertTypes(__DIR__ . '/data/assert-this.php');
 yield from $this->gatherAssertTypes(__DIR__ . '/data/assert-methods.php');
 yield from $this->gatherAssertTypes(__DIR__ . '/data/assert-intersected.php');
 yield from $this->gatherAssertTypes(__DIR__ . '/data/assert-invariant.php');

tests/PHPStan/Analyser/data/assert-this.php

@@ -0,0 +1,84 @@
+<?php
+
+namespace AssertThis;
+
+use RuntimeException;
+
+use function PHPStan\Testing\assertType;
+
+/**
+ * @template TOk
+ * @template TErr
+ */
+interface Result {
+/**
+ * @phpstan-assert-if-true Ok<TOk> $this
+ * @phpstan-assert-if-false Err<TErr> $this
+ */
+public function isOk(): bool;
+
+/**
+ * @return TOk|never
+ */
+public function unwrap();
+}
+
+/**
+ * @template TOk
+ * @template-implements Result<TOk, never>
+ */
+class Ok implements Result {
+public function __construct(private $value) {
+}
+
+/**
+ * @return true
+ */
+public function isOk(): bool {
+return true;
+}
+
+/**
+ * @return TOk
+ */
+public function unwrap() {
+return $this->value;
+}
+}
+
+/**
+ * @template TErr
+ * @template-implements Result<never, TErr>
+ */
+class Err implements Result {
+public function __construct(private $value) {
+}
+
+/**
+ * @return false
+ */
+public function isOk(): bool {
+return false;
+}
+
+/**
+ * @return never
+ */
+public function unwrap() {
+throw new RuntimeException('Tried to unwrap() an Err value');
+}
+}
+
+function () {
+/** @var Result<int, string> $result */
+$result = new Ok(123);
+assertType('AssertThis\\Result<int, string>', $result);
+
+if ($result->isOk()) {
+assertType('AssertThis\\Ok<int>', $result);
+assertType('int', $result->unwrap());
+} else { 
+assertType('AssertThis\\Err<string>', $result);
+assertType('never', $result->unwrap());
+}
+};