Fix phpstan/phpstan#3585: Reassignment of $this is not an error

Makefile

@@ -126,6 +126,7 @@ lint:
 		--exclude tests/PHPStan/Rules/Properties/data/property-override-attr-missing.php \
 		--exclude tests/PHPStan/Rules/Properties/data/override-attr-on-property.php \
 		--exclude tests/PHPStan/Rules/Properties/data/property-override-attr.php \
+		--exclude tests/PHPStan/Rules/Operators/data/bug-3585.php \
 		src tests
 
 install-paratest:

src/Rules/Operators/InvalidAssignVarRule.php

@@ -72,7 +72,7 @@ public function processNode(Node $node, Scope $scope): array
 	private function containsNonAssignableExpression(Expr $expr): bool
 	{
 		if ($expr instanceof Expr\Variable) {
-			return false;
+			return $expr->name === 'this';
 		}
 
 		if ($expr instanceof Expr\PropertyFetch) {

tests/PHPStan/Rules/Operators/InvalidAssignVarRuleTest.php

@@ -59,4 +59,34 @@ public function testRule(): void
 		]);
 	}
 
+	public function testBug3585(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-3585.php'], [
+			[
+				'Expression on left side of assignment is not assignable.',
+				9,
+			],
+			[
+				'Expression on left side of assignment is not assignable.',
+				10,
+			],
+			[
+				'Expression on left side of assignment is not assignable.',
+				11,
+			],
+			[
+				'Expression on left side of assignment is not assignable.',
+				12,
+			],
+			[
+				'Expression on left side of assignment is not assignable.',
+				17,
+			],
+			[
+				'Expression on left side of assignment is not assignable.',
+				23,
+			],
+		]);
+	}
+
 }

tests/PHPStan/Rules/Operators/data/bug-3585.php

@@ -0,0 +1,24 @@
+<?php declare(strict_types = 1);
+
+namespace Bug3585;
+
+class Foo
+{
+	public function doFoo(): void
+	{
+		$this = 1;
+		$this = new self();
+		$this .= 'foo';
+		[$this] = [1];
+	}
+
+	public static function doBar(): void
+	{
+		$this = 1; // allowed in static context? Actually no, PHP still forbids it
+	}
+}
+
+function baz(): void
+{
+	$this = 1; // PHP forbids this too
+}