Allow contextual functions with erased parameters to be integrated (#17313)

## Fix #17147

- Allow contextual functions with erased parameters to be integrated

<!-- TODO description of the change -->


<!-- Ideally should have a called "Fix #XYZ: A SHORT FIX DESCRIPTION"
-->

Author: nicolasstucki

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -1509,7 +1509,6 @@ class Definitions {
 
   /** Is an context function class.
    *   - ContextFunctionN for N >= 0
-   *   - ErasedContextFunctionN for N > 0
    */
   def isContextFunctionClass(cls: Symbol): Boolean = scalaClassName(cls).isContextFunction
 

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

@@ -20,7 +20,7 @@ object ContextFunctionResults:
    */
   def annotateContextResults(mdef: DefDef)(using Context): Unit =
     def contextResultCount(rhs: Tree, tp: Type): Int = tp match
-      case defn.ContextFunctionType(_, resTpe, erasedParams) if !erasedParams.contains(true) /* Only enable for non-erased functions */ =>
+      case defn.ContextFunctionType(_, resTpe, _) =>
         rhs match
           case closureDef(meth) => 1 + contextResultCount(meth.rhs, resTpe)
           case _ => 0
@@ -116,8 +116,14 @@ object ContextFunctionResults:
       atPhase(erasurePhase)(integrateSelect(tree, n))
     else tree match
       case Select(qual, name) =>
-        if name == nme.apply && defn.isContextFunctionClass(tree.symbol.maybeOwner) then
-          integrateSelect(qual, n + 1)
+        if name == nme.apply then
+          qual.tpe match
+            case defn.ContextFunctionType(_, _, _) =>
+              integrateSelect(qual, n + 1)
+            case _ if defn.isContextFunctionClass(tree.symbol.maybeOwner) => // for TermRefs
+              integrateSelect(qual, n + 1)
+            case _ =>
+              n > 0 && contextResultCount(tree.symbol) >= n
         else
           n > 0 && contextResultCount(tree.symbol) >= n
       case Ident(name) =>

tests/pos-custom-args/erased/tailrec.scala

@@ -0,0 +1,20 @@
+import scala.annotation.tailrec
+
+erased class Foo1
+class Foo2
+
+@tailrec
+final def test1(n: Int, acc: Int): (Foo1, Foo2) ?=> Int =
+  if n <= 0 then acc
+  else test1(n - 1, acc * n)
+
+@tailrec
+final def test2(n: Int, acc: Int): Foo1 ?=> Int =
+  if n <= 0 then acc
+  else test2(n - 1, acc * n)
+
+@main def Test() =
+  given Foo1 = Foo1()
+  given Foo2 = Foo2()
+  test1(10, 0)
+  test2(10, 0)