为什么在编译时不检查lambda返回类型？

在第一个例子，

MyInterface::getLength

"I am NOT anInteger"

T

R

MyInterface

Serializable & Comparable<? extendsSerializable & Comparable<?>>

// it compiles since String is a SerializableFunction<MyInterface, Serializable> function = MyInterface::getLength;Builder.of(MyInterface.class).with(function, "I am NOT an Integer");

MyInterface::getLength``Function<MyInterface, Integer>

// it doesn't compile since String isn't an IntegerFunction<MyInterface, Integer> function = MyInterface::getLength;Builder.of(MyInterface.class).with(function, "I am NOT an Integer");