Trimming a lowercase string gives a lowercase string

stubs/core.stub

@@ -313,3 +313,18 @@ function is_callable(mixed $value, bool $syntax_only = false, ?string &$callable
  * @return ($num is float ? float : $num is int ? non-negative-int : float|non-negative-int)
  */
 function abs($num) {}
+
+/**
+ * @return ($string is lowercase-string ? lowercase-string : string)
+ */
+function trim(string $string, string $characters = " \n\r\t\v\x00"): string {}
+
+/**
+ * @return ($string is lowercase-string ? lowercase-string : string)
+ */
+function ltrim(string $string, string $characters = " \n\r\t\v\x00"): string {}
+
+/**
+ * @return ($string is lowercase-string ? lowercase-string : string)
+ */
+function rtrim(string $string, string $characters = " \n\r\t\v\x00"): string {}

tests/PHPStan/Analyser/nsrt/lowercase-string-trim.php

@@ -0,0 +1,29 @@
+<?php
+
+namespace LowercaseStringTrim;
+
+use function PHPStan\Testing\assertType;
+
+class Foo
+{
+
+	/**
+	 * @param lowercase-string $lowercase
+	 */
+	public function doTrim(string $lowercase, string $string): void
+	{
+		assertType('lowercase-string', trim($lowercase));
+		assertType('lowercase-string', ltrim($lowercase));
+		assertType('lowercase-string', rtrim($lowercase));
+		assertType('lowercase-string', trim($lowercase, $string));
+		assertType('lowercase-string', ltrim($lowercase, $string));
+		assertType('lowercase-string', rtrim($lowercase, $string));
+		assertType('string', trim($string));
+		assertType('string', ltrim($string));
+		assertType('string', rtrim($string));
+		assertType('string', trim($string, $lowercase));
+		assertType('string', ltrim($string, $lowercase));
+		assertType('string', rtrim($string, $lowercase));
+	}
+
+}