Add regression test for #6567

Closes phpstan/phpstan#6567

Amp-Thread-ID: https://ampcode.com/threads/T-01a1819f-fbb7-7460-b1bd-aab545ccfc0a

Author: ondrejmirtes

tests/PHPStan/Rules/Functions/ReturnTypeRuleTest.php

@@ -542,6 +542,29 @@ public function testBug11507(): void
 		$this->analyse([__DIR__ . '/data/bug-11507.php'], []);
 	}
 
+	public function testBug6567(): void
+	{
+		$this->checkNullables = true;
+		$this->checkExplicitMixed = false;
+		$this->analyse([__DIR__ . '/data/bug-6567.php'], [
+			[
+				'Function Bug6567\add() should return T of int but returns int.',
+				11,
+				'Type int is not always the same as T. It breaks the contract for some argument types, typically subtypes.',
+			],
+			[
+				'Function Bug6567\testBool() should return T of bool but returns false.',
+				27,
+				'Type false is not always the same as T. It breaks the contract for some argument types, typically subtypes.',
+			],
+			[
+				'Function Bug6567\testFloat() should return T of float but returns float.',
+				36,
+				'Type float is not always the same as T. It breaks the contract for some argument types, typically subtypes.',
+			],
+		]);
+	}
+
 	public function testBug15245(): void
 	{
 		$this->checkNullables = true;

tests/PHPStan/Rules/Functions/data/bug-6567.php

@@ -0,0 +1,37 @@
+<?php declare(strict_types = 1);
+
+namespace Bug6567;
+
+/**
+ * @template T of int
+ * @param T $p
+ * @return T
+ */
+function add($p) {
+	return $p + 1000;
+}
+
+/**
+ * @param int<0,100> $p
+ */
+function test($p): void {
+	\PHPStan\dumpType(add($p));
+}
+
+/**
+ * @template T of bool
+ * @param T $p
+ * @return T
+ */
+function testBool($p) {
+	return false;
+}
+
+/**
+ * @template T of float
+ * @param T $p
+ * @return T
+ */
+function testFloat($p) {
+	return $p + 1;
+}