Fix conditional return type on $this not using subtracted type

- When a method has a conditional return type like `@return ($this is self::A ? int : null)`,
  and `$this` has a subtracted type (e.g. `$this(X~X::A)` from narrowing), the conditional
  was not properly resolved because the subtracted type was not propagated
- The root cause was in `StaticType::transformStaticType` which transforms `StaticType`
  instances in return types but did not propagate the calling type's subtracted type
- Added subtracted type propagation in `StaticType::transformStaticType` to match the
  behavior of `CalledOnTypeUnresolvedMethodPrototypeReflection::transformStaticType`
- New regression test in tests/PHPStan/Analyser/nsrt/bug-12244.php

Closes phpstan/phpstan#12244

Author: github-actions[bot]

src/Type/StaticType.php

@@ -369,6 +369,13 @@ private function transformStaticType(Type $type, ClassMemberAccessAnswerer $scop
 					$type = new self($type->getClassReflection(), $type->getSubtractedType());
 				}
 
+				if ($this->getSubtractedType() !== null && $type->getSubtractedType() === null) {
+					$type = $type->changeSubtractedType($this->getSubtractedType());
+					if (!$type instanceof StaticType) {
+						return $traverse($type);
+					}
+				}
+
 				if (!$isFinal || $type instanceof ThisType) {
 					return $traverse($type);
 				}

tests/PHPStan/Analyser/nsrt/bug-12244.php

@@ -0,0 +1,32 @@
+<?php // lint >= 8.1
+
+declare(strict_types = 1);
+
+namespace Bug12244;
+
+use function PHPStan\Testing\assertType;
+
+enum X: string {
+	case A = 'a';
+	case B = 'b';
+	case C = 'c';
+
+	/** @return ($this is self::A ? int : null) */
+	public function get(): ?int {
+		return ($this === self::A) ? 123 : null;
+	}
+
+	public function doSomething(): void {
+		if ($this !== self::A) {
+			assertType('$this(Bug12244\X~Bug12244\X::A)', $this);
+			assertType('null', $this->get());
+		}
+	}
+
+	public static function doSomethingFor(X $x): void {
+		if ($x !== self::A) {
+			assertType('Bug12244\X~Bug12244\X::A', $x);
+			assertType('null', $x->get());
+		}
+	}
+}