Relax subtype matching to work with non-finite union guard types

Replace the `count(getFiniteTypes()) > 1` restriction with a
`!$guardType instanceof MixedType` check. This allows conditional
expressions with non-finite union guards (e.g. int|string) to match
when the specified type is a strict subtype, while still preventing
incorrect matches against mixed-based types that would cause *NEVER*
type collapses.

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

Author: phpstan-bot

src/Analyser/MutatingScope.php

@@ -3226,12 +3226,13 @@ public function filterBySpecifiedTypes(SpecifiedTypes $specifiedTypes): self
 						if ($specifiedHolder->equals($conditionalTypeHolder)) {
 							continue;
 						}
+						$guardType = $conditionalTypeHolder->getType();
 						if (
 							$conditionalExpression->getTypeHolder()->getCertainty()->yes()
 							&& $specifiedHolder->getCertainty()->yes()
 							&& $conditionalTypeHolder->getCertainty()->yes()
-							&& count($conditionalTypeHolder->getType()->getFiniteTypes()) > 1
-							&& $conditionalTypeHolder->getType()->isSuperTypeOf($specifiedHolder->getType())->yes()
+							&& !$guardType instanceof MixedType
+							&& $guardType->isSuperTypeOf($specifiedHolder->getType())->yes()
 						) {
 							$subtypeMatch = true;
 							continue;

tests/PHPStan/Rules/Variables/DefinedVariableRuleTest.php

@@ -1485,6 +1485,15 @@ public function testBug12597(): void
 		$this->analyse([__DIR__ . '/data/bug-12597.php'], []);
 	}
 
+	public function testBug12597NonFinite(): void
+	{
+		$this->cliArgumentsVariablesRegistered = true;
+		$this->polluteScopeWithLoopInitialAssignments = false;
+		$this->checkMaybeUndefinedVariables = true;
+		$this->polluteScopeWithAlwaysIterableForeach = true;
+		$this->analyse([__DIR__ . '/data/bug-12597-non-finite.php'], []);
+	}
+
 	public function testBug14117(): void
 	{
 		$this->cliArgumentsVariablesRegistered = true;

tests/PHPStan/Rules/Variables/data/bug-12597-non-finite.php

@@ -0,0 +1,19 @@
+<?php declare(strict_types = 1);
+
+namespace Bug12597NonFinite;
+
+class HelloWorld
+{
+	public function test(mixed $type): void
+	{
+		if (is_int($type) || is_string($type)) {
+			$message = 'Hello!';
+		}
+
+		if (is_int($type)) {
+			$this->message($message);
+		}
+	}
+
+	public function message(string $message): void {}
+}