Demo: Add conditional return type for $args array of get_posts - Can we?

functionMap.php

@@ -24,6 +24,7 @@
  * @link https://github.com/phpstan/phpstan-src/blob/1.10.x/resources/functionMap.php
  */
 return [
+    'get_posts' => ["(\$args is array{'fields': 'id=>parent'|'ids'} ? array<int, int> : array<int, WP_Post>)"],
     'addslashes_gpc' => ['T', '@phpstan-template' => 'T', 'gpc' => 'T'],
     'have_posts' => [null, '@phpstan-impure' => ''],
     'rawurlencode_deep' => ['T', '@phpstan-template' => 'T', 'value' => 'T'],

tests/TypeInferenceTest.php

@@ -20,6 +20,7 @@ public function dataFileAsserts(): iterable
         yield from $this->gatherAssertTypes(__DIR__ . '/data/get_post.php');
         yield from $this->gatherAssertTypes(__DIR__ . '/data/get_post_stati.php');
         yield from $this->gatherAssertTypes(__DIR__ . '/data/get_post_types.php');
+        yield from $this->gatherAssertTypes(__DIR__ . '/data/get_posts.php');
         yield from $this->gatherAssertTypes(__DIR__ . '/data/get_page_by_path.php');
         yield from $this->gatherAssertTypes(__DIR__ . '/data/get_permalink.php');
         yield from $this->gatherAssertTypes(__DIR__ . '/data/get_term_by.php');

tests/data/get_posts.php

@@ -0,0 +1,66 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PhpStubs\WordPress\Core\Tests;
+
+use function PHPStan\Testing\assertType;
+
+assertType('array<int, WP_Post>', get_posts());
+assertType('array<int, WP_Post>', get_posts(['key' => 'value']));
+assertType('array<int, WP_Post>', get_posts(['fields' => '']));
+assertType('array<int, int>', get_posts(['fields' => 'ids']));
+assertType('array<int, int>', get_posts(['fields' => 'id=>parent']));
+assertType('array<int, WP_Post>', get_posts(['fields' => 'Hello']));
+
+// Nonconstant array
+assertType('array<int, int|WP_Post>', get_posts((array)$_GET['array']));
+
+// Unions
+$union = $_GET['foo'] ? ['key' => 'value'] : ['some' => 'thing'];
+assertType('array<int, WP_Post>', get_posts($union));
+
+$union = $_GET['foo'] ? ['key' => 'value'] : ['fields' => 'ids'];
+assertType('array<int, int|WP_Post>', get_posts($union));
+
+$union = $_GET['foo'] ? ['key' => 'value'] : ['fields' => ''];
+assertType('array<int, WP_Post>', get_posts($union));
+
+$union = $_GET['foo'] ? ['key' => 'value'] : ['fields' => 'id=>parent'];
+assertType('array<int, int|WP_Post>', get_posts($union));
+
+$union = $_GET['foo'] ? ['fields' => ''] : ['fields' => 'ids'];
+assertType('array<int, int|WP_Post>', get_posts($union));
+
+$union = $_GET['foo'] ? ['fields' => ''] : ['fields' => 'id=>parent'];
+assertType('array<int, int|WP_Post>', get_posts($union));
+
+$union = $_GET['foo'] ? ['fields' => 'ids'] : ['fields' => 'id=>parent'];
+assertType('array<int, int>', get_posts($union));
+
+$union = $_GET['foo'] ? (array)$_GET['array'] : ['fields' => ''];
+assertType('array<int, int|WP_Post>', get_posts($union));
+
+$union = $_GET['foo'] ? (array)$_GET['array'] : ['fields' => 'ids'];
+assertType('array<int, int|WP_Post>', get_posts($union));
+
+$union = $_GET['foo'] ? (array)$_GET['array'] : ['fields' => 'id=>parent'];
+assertType('array<int, int|WP_Post>', get_posts($union));
+
+$union = $_GET['foo'] ? (string)$_GET['string'] : '';
+assertType('array<int, int|WP_Post>', get_posts(['fields' => $union]));
+
+$union = $_GET['foo'] ? (string)$_GET['string'] : 'ids';
+assertType('array<int, int|WP_Post>', get_posts(['fields' => $union]));
+
+$union = $_GET['foo'] ? (string)$_GET['string'] : 'id=>parent';
+assertType('array<int, int|WP_Post>', get_posts(['fields' => $union]));
+
+$union = $_GET['foo'] ? (string)$_GET['string'] : 'fields';
+assertType('array<int, WP_Post>', get_posts([$union => '']));
+
+$union = $_GET['foo'] ? (string)$_GET['string'] : 'fields';
+assertType('array<int, int|WP_Post>', get_posts([$union => 'ids']));
+
+$union = $_GET['foo'] ? (string)$_GET['string'] : 'fields';
+assertType('array<int, int|WP_Post>', get_posts([$union => 'id=>parent']));

wordpress-stubs.php

@@ -126599,6 +126599,7 @@ function is_post_publicly_viewable($post = \null)
      *   w?: int,
      *   year?: int,
      * } $args
+     * @phpstan-return ($args is array{'fields': 'id=>parent'|'ids'} ? array<int, int> : array<int, WP_Post>)
      */
     function get_posts($args = \null)
     {