Methods in fluent interfaces are considered impure

src/Reflection/Annotations/AnnotationMethodReflection.php

@@ -10,6 +10,7 @@
 use PHPStan\TrinaryLogic;
 use PHPStan\Type\Generic\TemplateTypeMap;
 use PHPStan\Type\MixedType;
+use PHPStan\Type\ThisType;
 use PHPStan\Type\Type;
 
 class AnnotationMethodReflection implements ExtendedMethodReflection
@@ -122,6 +123,10 @@ public function hasSideEffects(): TrinaryLogic
 			return TrinaryLogic::createYes();
 		}
 
+		if ((new ThisType($this->declaringClass))->isSuperTypeOf($this->returnType)->yes()) {
+			return TrinaryLogic::createYes();
+		}
+
 		return TrinaryLogic::createMaybe();
 	}
 

src/Reflection/Php/PhpMethodReflection.php

@@ -30,6 +30,7 @@
 use PHPStan\Type\MixedType;
 use PHPStan\Type\ObjectWithoutClassType;
 use PHPStan\Type\StringType;
+use PHPStan\Type\ThisType;
 use PHPStan\Type\Type;
 use PHPStan\Type\TypehintHelper;
 use PHPStan\Type\VoidType;
@@ -430,6 +431,10 @@ public function hasSideEffects(): TrinaryLogic
 			return TrinaryLogic::createFromBoolean(!$this->isPure);
 		}
 
+		if ((new ThisType($this->declaringClass))->isSuperTypeOf($this->getReturnType())->yes()) {
+			return TrinaryLogic::createYes();
+		}
+
 		return TrinaryLogic::createMaybe();
 	}
 

tests/PHPStan/Analyser/data/impure-method.php

@@ -4,6 +4,9 @@
 
 use function PHPStan\Testing\assertType;
 
+/**
+ * @method $this phpDocReturnThis()
+ */
 class Foo
 {
 
@@ -15,6 +18,14 @@ public function voidMethod(): void
 		$this->fooProp = rand(0, 1);
 	}
 
+	/**
+	 * @return $this
+	 */
+	public function returnsThis()
+	{
+		$this->fooProp = rand(0, 1);
+	}
+
 	public function ordinaryMethod(): int
 	{
 		return 1;
@@ -51,6 +62,24 @@ public function doFoo(): void
 		assertType('int', $this->fooProp);
 	}
 
+	public function doFluent(): void
+	{
+		$this->fooProp = 1;
+		assertType('1', $this->fooProp);
+
+		$this->returnsThis();
+		assertType('int', $this->fooProp);
+	}
+
+	public function doFluent2(): void
+	{
+		$this->fooProp = 1;
+		assertType('1', $this->fooProp);
+
+		$this->phpDocReturnThis();
+		assertType('int', $this->fooProp);
+	}
+
 	public function doBar(): void
 	{
 		$this->fooProp = 1;