Teach PostTyper to handle untupled context closures (#17739)

Author: dwijnand

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -330,6 +330,9 @@ trait TreeInfo[T <: Untyped] { self: Trees.Instance[T] =>
     case _ => p(tree)
   }
 
+  /** The tree stripped of the possibly nested applications (term and type).
+   *  The original tree if it's not an application.
+   */
   def appliedCore(tree: Tree): Tree = tree match {
     case Apply(fn, _) => appliedCore(fn)
     case TypeApply(fn, _)       => appliedCore(fn)

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

@@ -261,9 +261,11 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
       def check(qual: Tree) =
         if !qual.tpe.isStable then
           report.error(em"Parameter untupling cannot be used for call-by-name parameters", tree.srcPos)
-      tree match
-        case Select(qual, _) => check(qual)    // simple select _n
-        case Apply(TypeApply(Select(qual, _), _), _) => check(qual) // generic select .apply[T](n)
+      appliedCore(closureBody(tree)) match
+        case Select(qual, _) => check(qual)
+        // simple select _n                    Select(qual, _n)
+        // generic select .apply[T](n)         Apply(TypeApply(Select(qual, _), _), _)
+        // context closure x ?=> f(using x)    Block(List(DefDef($anonfun, _, _, Apply(Select(Select(qual, _n), _), _)))
 
     def checkNotPackage(tree: Tree)(using Context): Tree =
       if !tree.symbol.is(Package) then tree

tests/pos/i16994.scala

@@ -0,0 +1,2 @@
+type ZZ = String ?=> Int
+def f(xs: ZZ*) = xs.zipWithIndex.foreach((f: ZZ, i) => f(using "s"))