From bd846123ce47c937132dd47f13924490f78eec35 Mon Sep 17 00:00:00 2001
From: Matthew Brown <github@muglug.com>
Date: Sat, 11 Jan 2020 11:54:29 -0500
Subject: [PATCH] Fix #2560 - handle special case of array_filter on
 single-element array

---
 .../ArrayFilterReturnTypeProvider.php                | 12 ++++++++++++
 tests/FunctionCallTest.php                           |  7 +++++++
 2 files changed, 19 insertions(+)

diff --git a/src/Psalm/Internal/Provider/ReturnTypeProvider/ArrayFilterReturnTypeProvider.php b/src/Psalm/Internal/Provider/ReturnTypeProvider/ArrayFilterReturnTypeProvider.php
index 279a4b25e..7eb395814 100644
--- a/src/Psalm/Internal/Provider/ReturnTypeProvider/ArrayFilterReturnTypeProvider.php
+++ b/src/Psalm/Internal/Provider/ReturnTypeProvider/ArrayFilterReturnTypeProvider.php
@@ -77,6 +77,18 @@ class ArrayFilterReturnTypeProvider implements \Psalm\Plugin\Hook\FunctionReturn
                 $statements_source->getSuppressedIssues()
             );
 
+            if ($first_arg_array instanceof Type\Atomic\ObjectLike
+                && $first_arg_array->is_list
+                && $key_type->isSingleIntLiteral()
+                && $key_type->getSingleIntLiteral()->value === 0
+            ) {
+                return new Type\Union([
+                    new Type\Atomic\TList(
+                        $inner_type,
+                    ),
+                ]);
+            }
+
             if ($key_type->getLiteralStrings()) {
                 $key_type->addType(new Type\Atomic\TString);
             }
diff --git a/tests/FunctionCallTest.php b/tests/FunctionCallTest.php
index 1006f7778..1cefde30b 100644
--- a/tests/FunctionCallTest.php
+++ b/tests/FunctionCallTest.php
@@ -2422,6 +2422,13 @@ class FunctionCallTest extends TestCase
                     '$array' => 'list<int>',
                 ],
             ],
+            'specialCaseArrayFilterOnSingleEntry' => [
+                '<?php
+                    /** @psalm-return list<int> */
+                    function makeAList(int $ofThisInteger): array {
+                        return array_filter([$ofThisInteger]);
+                    }'
+            ],
         ];
     }