.

Author: lihaoyi

content/unroll-default-arguments.md

@@ -468,9 +468,9 @@ use them when the given `p: Product` has a smaller `productArity` than the curre
 
 6. `@unroll` only supports `final` methods. `object` methods and constructors are naturally
    final, but `class` or `trait` methods that are `@unroll`ed need to be explicitly marked `final`.
-   Trying to implement the semantics of `@unroll` in the presence of downstream overrides,
-   where the downstream overrides can be compiled against by different versions of the upstream
-   code, has proved difficult. If we can come up with some implementation that works, we can
+   It has proved difficult to implement the semantics of `@unroll` in the presence of downstream 
+   overrides, `super`, etc. where the downstream overrides can be compiled against by different 
+   versions of the upstream code. If we can come up with some implementation that works, we can
    lift this restriction later, but for now I have not managed to do so and so this restriction
    stays.
 
@@ -696,6 +696,36 @@ allow anyone calling the original signature \to be forwarded to the narrower sig
 
 This is not currently implemented in `@unroll`, but would be a straightforward addition.
 
+### Incremental Forwarders or Direct Forwarders
+
+Given this:
+
+```scala
+def foo(s: String, n: Int = 1, @unroll b: Boolean = true, l: Long = 0) = s + n + b + l
+```
+
+There are two ways to do the forwarders. First option, which I used in above, is
+to have each forwarder directly call the primary method:
+
+```scala
+def foo(s: String, n: Int, b: Boolean) = foo(s, n, b, 0)
+def foo(s: String, n: Int) = foo(s, n, true, 0)
+```
+
+Second option is to have each forwarder incrementally call the next forwarder, which
+will eventually end up calling the primary method:
+
+```scala
+def foo(s: String, n: Int, b: Boolean) = foo(s, n, b, 0)
+def foo(s: String, n: Int) = foo(s, n, true)
+```
+
+The first option results in shorter stack traces, while the second option results in
+roughly half as much generated bytecode in the method bodies (though it's still `O(n^2)`).
+
+For now I chose the first option.
+
+
 ## Implementation & Testing
 
 This SIP has a full implementation for Scala {2.12, 2.13, 3} X {JVM, JS, Native}