Erased Definitions - More Details

Rules

1. erased is a soft modifier. It can appear:

   • At the start of a parameter block of a method, function or class
   • In a method definition
   • In a val definition (but not lazy val or var)
   • In a class or trait definition

   erased val x = ...
   erased def f = ...
   
   def g(erased x: Int) = ...
   
   (erased x: Int, y: Int) => ...
   def h(x: (Int, erased Int) => Int) = ...
   
   class K(erased x: Int) { ... }
   erased class E {}
   

2. A reference to an erased val or def can only be used

   • Inside the expression of argument to an erased parameter
   • Inside the body of an erased val or def

3. Functions

   • (erased x1: T1, x2: T2, ..., xN: TN) => y : (erased T1, T2, ..., TN) => R
   • (given x1: T1, erased x2: T2, ..., xN: TN) => y: (given T1, erased T2, ..., TN) => R
   • (given erased T1) => R <:< erased T1 => R
   • (given T1, erased T2) => R <:< (T1, erased T2) => R
   • ...

   Note that there is no subtype relation between (erased T) => R and T => R (or (given erased T) => R and (given T) => R). The erased parameters must match exactly in their respective positions.

4. Eta expansion

   if def f(erased x: T): U then f: (erased T) => U.

5. Erasure semantics

   • All erased parameters are removed from the function
   • All argument to erased parameters are not passed to the function
   • All erased definitions are removed
   • (erased ET1, erased ET2, T1, ..., erased ETN, TM) => R are erased to (T1, ..., TM) => R.
   • (given erased ET1, erased ET2, T1, ..., erased ETN, TM) => R are erased to (given T1, ..., TM) => R.

6. Overloading

   Method with erased parameters will follow the normal overloading constraints after erasure.

7. Overriding

   • Member definitions overriding each other must both be erased or not be erased.
   • def foo(x: T): U cannot be overridden by def foo(erased x: T): U and vice-versa.

8. Type Restrictions

   • For dependent functions, erased parameters are limited to realizable types, that is, types that are inhabited by non-null values. This restriction stops us from using a bad bound introduced by an erased value, which leads to unsoundness (see #4060).
   • Polymorphic functions with erased parameters are currently not supported, and will be rejected by the compiler. This is purely an implementation restriction, and might be lifted in the future.