表达式的类型

• Variables are typed on first assignment based on the source expression type and function parameters are optionally typed
  根据源表达式的类型，变量在第一次赋值的时候被类型化，而函数参数的类型化是可选的。
• It turns out that the compiler actually inserts casts into the code during compilation to account for the fact that type erasure reduces the precision of the types of certain expressions
  事实证明编译器确实在编译阶段将数据类型转换插入到代码中，这样做是为了解决类型擦除会降低某些表达式的类型的精度这一事实。