Add regression test for phpstan/phpstan#12798

Test is_subclass_of in enum traits with BackedEnum and custom interface
checks. Verifies no false positive function.alreadyNarrowedType or
function.impossibleType errors are reported.

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

Author: phpstan-bot

tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleTest.php

@@ -1230,4 +1230,11 @@ public function testBug13687(): void
 		$this->analyse([__DIR__ . '/data/bug-13687.php'], []);
 	}
 
+	#[RequiresPhp('>= 8.1')]
+	public function testBug12798(): void
+	{
+		$this->treatPhpDocTypesAsCertain = true;
+		$this->analyse([__DIR__ . '/data/bug-12798.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Comparison/data/bug-12798.php

@@ -0,0 +1,49 @@
+<?php declare(strict_types = 1); // lint >= 8.1
+
+namespace Bug12798;
+
+interface Colorable
+{
+	public function color(): string;
+}
+
+trait HasColors
+{
+    /** @return array<string|int, string> */
+    public static function colors(): array {
+        return array_reduce(self::cases(), function (array $colors, self $case) {
+            $key = is_subclass_of($case, \BackedEnum::class) ? $case->value : $case->name;
+            $color = is_subclass_of($case, Colorable::class) ? $case->color() : 'gray';
+
+            $colors[$key] = $color;
+            return $colors;
+        }, []);
+    }
+}
+
+enum AlertLevelBacked: int implements Colorable
+{
+	use HasColors;
+
+	case Low      = 1;
+	case Medium   = 2;
+	case Critical = 3;
+
+	public function color(): string
+    {
+        return match ($this) {
+            self::Low      => 'green',
+            self::Medium   => 'yellow',
+            self::Critical => 'red',
+        };
+    }
+}
+
+enum AlertLevel
+{
+	use HasColors;
+
+	case Low;
+	case Medium;
+	case Critical;
+}