Fix false positives for is_a() and instanceof checks on $this in traits

- When analyzing trait methods, $this is bound to each using class separately
- is_a($this, SomeClass::class) was reported as "always true" in SomeClass context
  and "always false" in SomeClass2 context, but both are false positives
- Added trait-aware check in ImpossibleCheckTypeHelper to treat $this as uncertain
  when inside a trait, since the same code is shared across multiple classes
- New regression test in tests/PHPStan/Rules/Comparison/data/bug-13023.php

Closes phpstan/phpstan#13023

Author: github-actions[bot]

src/Rules/Comparison/ImpossibleCheckTypeHelper.php

@@ -310,6 +310,11 @@ public function findSpecifiedType(
 				continue;
 			}
 
+			if ($scope->isInTrait() && $sureType[0] instanceof Expr\Variable && $sureType[0]->name === 'this') {
+				$results[] = TrinaryLogic::createMaybe();
+				continue;
+			}
+
 			if ($this->treatPhpDocTypesAsCertain) {
 				$argumentType = $scope->getType($sureType[0]);
 			} else {
@@ -336,6 +341,11 @@ public function findSpecifiedType(
 				continue;
 			}
 
+			if ($scope->isInTrait() && $sureNotType[0] instanceof Expr\Variable && $sureNotType[0]->name === 'this') {
+				$results[] = TrinaryLogic::createMaybe();
+				continue;
+			}
+
 			if ($this->treatPhpDocTypesAsCertain) {
 				$argumentType = $scope->getType($sureNotType[0]);
 			} else {

tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleTest.php

@@ -1207,4 +1207,10 @@ public function testBug13799(): void
 		]);
 	}
 
+	public function testBug13023(): void
+	{
+		$this->treatPhpDocTypesAsCertain = true;
+		$this->analyse([__DIR__ . '/data/bug-13023.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Comparison/data/bug-13023.php

@@ -0,0 +1,25 @@
+<?php declare(strict_types = 1);
+
+namespace Bug13023;
+
+class SomeClass
+{
+	use MyTrait;
+}
+
+class SomeClass2
+{
+	use MyTrait;
+}
+
+trait MyTrait
+{
+	public function getRandom(): int
+	{
+		$value = random_int(1, 100);
+		if (is_a($this, SomeClass::class)) {
+			return $value * $value;
+		}
+		return $value;
+	}
+}