Add regression tests for phpstan/phpstan#3770 and phpstan/phpstan#6822

Both issues report that closures cannot be marked as impure via PHPDoc.
These tests verify that @phpstan-impure on closures prevents incorrect
type narrowing in conditional branches.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: phpstan-bot

tests/PHPStan/Analyser/nsrt/bug-3770.php

@@ -0,0 +1,27 @@
+<?php declare(strict_types = 1);
+
+namespace Bug3770;
+
+use function PHPStan\Testing\assertType;
+
+// PHPDoc on closures should be respected for purity
+
+/** @phpstan-impure */
+$f = static function (string $input): bool {
+	return strlen($input) > rand(0, 10);
+};
+
+if ($f('hello')) {
+	// Should not narrow to true because closure is impure
+	assertType('bool', $f('hello'));
+}
+
+// Closure with @phpstan-pure should allow narrowing
+/** @phpstan-pure */
+$g = static function (string $input): bool {
+	return strlen($input) > 5;
+};
+
+if ($g('hello world')) {
+	assertType('true', $g('hello world'));
+}

tests/PHPStan/Analyser/nsrt/bug-6822.php

@@ -0,0 +1,28 @@
+<?php declare(strict_types = 1);
+
+namespace Bug6822;
+
+use function PHPStan\Testing\assertType;
+
+// Closures marked as @phpstan-impure should not have their return type narrowed
+
+/** @phpstan-impure */
+$closure = function (): bool {
+	return (bool) rand(0, 1);
+};
+
+assertType('bool', $closure());
+
+if ($closure()) {
+	assertType('bool', $closure());
+}
+
+// Same with an explicit impure closure assigned to a variable
+/** @phpstan-impure */
+$impureFn = function (): int {
+	return rand(0, 100);
+};
+
+if ($impureFn() > 50) {
+	assertType('int<0, 100>', $impureFn());
+}