Add support and tests for $this type assertions

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());
+	}
+};