simple support for mixed pseudo-type

cc @jpudil

src/FunctionSignatureAssertionError.php

@@ -91,4 +91,12 @@ final class FunctionSignatureAssertionError extends \AssertionError {
 			"Expected return type of type '$expectedReturnType', but given function declares '$actualReturnType'."
 		);
 	}
+
+	public static function parameterDoesNotAcceptAllRequiredValues(\ReflectionFunction $reflection, \ReflectionType $reflectionType): self
+	{
+		return new self(
+			$reflection,
+			"Parameter of given function does not accept all required values. Parameter restricts input values to {$reflectionType}, assertion requires mixed. (=must accept everything)"
+		);
+	}
 };