Fix eta-expanding guards in adaptType

Author: dwijnand

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -4281,7 +4281,16 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
 
     def adaptType(tp: Type): Tree = {
       val tree1 =
-        if ((pt eq AnyTypeConstructorProto) || tp.typeParamSymbols.isEmpty) tree
+        if pt eq AnyTypeConstructorProto then tree
+        else if tp.typeParamSymbols.isEmpty || tp.typeParamSymbols.isEmpty then
+          // call typeParamSymbols twice, to get the stable results
+          // (see also note inside typeParamSymbols)
+          // given `type LifecycleF = [_] =>> Any` in pos/i19942.scala
+          // with an Ident of LifecycleF, calling typeParams will return:
+          // 1. [type _] a list of the symbol _ in the type def tree, on the first call
+          // 2. [+_] a list of a lambda param, afterwards
+          // we only want to eta-expand if there are real type param symbols, so we check twice
+          tree
         else {
           if (ctx.isJava)
             // Cook raw type

tests/pos/i19942.scala

@@ -0,0 +1,23 @@
+type LifecycleF = [_] =>> Any
+trait Lifecycle[+F[_], +A]
+
+trait LifecycleTag[R]
+object LifecycleTag:
+  implicit def resourceTag[R <: Lifecycle[F0, A0], F0[_], A0]: LifecycleTag[R] = ???
+
+trait MakeDSL[T]:
+  final def fromResource[R <: Lifecycle[LifecycleF, T]](implicit tag: LifecycleTag[R]): Any = ???
+
+object distage:
+  // Cannot be defined in the same scope as rest of code
+  final type Identity[+A] = A
+import distage.*
+
+trait Resource
+trait DependentResource() extends Lifecycle[Identity, Resource]
+
+@main def Test =  {
+  val dsl: MakeDSL[Resource] = ???
+  val fails = dsl.fromResource[DependentResource]
+  val works = dsl.fromResource[DependentResource](using LifecycleTag.resourceTag[DependentResource, Identity, Resource])
+}