Subtract block parameter captures in extern defs (#1023)

This fixes #448 by subtracting the block param captures, so the
following becomes OK:
```
extern def ifJS{ thn: => R }{ els: => R}: R =
  js { thn() }
  default { els() }
```

Note that the implementor of the extern is in charge of properly calling
the block parameters with the correct calling convention.

Author: marzipankaiser

effekt/shared/src/main/scala/effekt/Typer.scala

@@ -868,7 +868,14 @@ object Typer extends Phase[NameResolved, Typechecked] {
         sym.vparams foreach Context.bind
         sym.bparams foreach Context.bind
 
-        flowingInto(Context.lookupCapture(sym)) {
+        val functionCapture = Context.lookupCapture(sym)
+        val inferredCapture = Context.freshCaptVar(CaptUnificationVar.ExternDefRegion(d))
+
+
+        flowsIntoWithout(inferredCapture, functionCapture) {
+          sym.bparams.map(_.capture)
+        }
+        flowingInto(inferredCapture) {
 
           // Note: Externs are always annotated with a type
           val expectedReturnType = d.symbol.annotatedType.get.result
@@ -879,7 +886,6 @@ object Typer extends Phase[NameResolved, Typechecked] {
               checkStmt(body, Some(expectedReturnType))
             case u: source.ExternBody.Unsupported => u
           }
-
         }
 
         Result((), Pure)

effekt/shared/src/main/scala/effekt/symbols/symbols.scala

@@ -385,6 +385,7 @@ object CaptUnificationVar {
   case class RegionRegion(handler: source.Region) extends Role
   case class VarRegion(definition: source.VarDef) extends Role
   case class FunctionRegion(fun: source.FunDef) extends Role
+  case class ExternDefRegion(fun: source.ExternDef) extends Role
   case class BlockRegion(fun: source.DefDef) extends Role
   case class AnonymousFunctionRegion(fun: source.BlockLiteral) extends Role
   case class InferredBox(box: source.Box) extends Role

examples/neg/effekt_implemented_extern.check

@@ -1,3 +1,3 @@
-[error] examples/neg/effekt_implemented_extern.effekt:4:8: Used captures {io} are not in the allowed set {}
+[error] examples/neg/effekt_implemented_extern.effekt:4:8: Not allowed {io}
   js { test() }
-       ^^^^^^
+       ^^^^^^

examples/pos/ifbackend.check

@@ -0,0 +1 @@
+Hallo

examples/pos/ifbackend.effekt

@@ -0,0 +1,14 @@
+extern pure def ifJS[R]{ thn: => R }{ els: => R}: R =
+  js { thn() }
+  default { els() }
+
+extern pure def jsOnly(): String =
+  js """ "Hallo" """
+
+def main() = {
+  ifJS{
+    println(jsOnly())
+  }{
+    println("Hallo")
+  }
+}