Fix problem when erasing erased context functions

Author: odersky

compiler/src/dotty/tools/dotc/transform/Erasure.scala

@@ -892,8 +892,9 @@ object Erasure {
             case closureDef(meth) =>
               val contextParams = meth.termParamss.head
               for param <- contextParams do
-                param.symbol.copySymDenotation(owner = sym).installAfter(erasurePhase)
-              vparams ++= contextParams
+                if !param.symbol.is(Flags.Erased) then
+                  param.symbol.copySymDenotation(owner = sym).installAfter(erasurePhase)
+                  vparams = vparams :+ param
               if crCount == 1 then meth.rhs.changeOwnerAfter(meth.symbol, sym, erasurePhase)
               else skipContextClosures(meth.rhs, crCount - 1)
 

library/src/scala/annotation/internal/ContextResultCount.scala

@@ -1,7 +1,7 @@
 package scala.annotation
 package internal
 
-/** An annotation that's aitomatically added for methods
+/** An annotation that's automatically added for methods
  *  that have one or more nested context closures as their right hand side.
  *  The parameter `n` is an Int Literal that tells how many nested closures
  *  there are.

tests/pos/CanThrow.scala

@@ -0,0 +1,11 @@
+package canThrowStrawman
+import language.experimental.erasedTerms
+
+class CanThrow[E <: Throwable]
+
+infix type throws1[R, E <: Throwable] = (erased CanThrow[E]) ?=> R
+
+class Fail extends Exception
+
+def foo(x: Boolean): Int throws1 Fail =
+  if x then 1 else throw Fail()