Fix #4884: asSeenFrom on incorrect prefix in type applications

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

@@ -65,10 +65,14 @@ object Checking {
    *     cannot handle those.
    */
   def checkAppliedType(tree: AppliedTypeTree, boundsCheck: Boolean)(implicit ctx: Context) = {
+    def kind(tp: Type): Type = tp match {
+      case nTp: NamedType => nTp.info
+      case _ => tp
+    }
     val AppliedTypeTree(tycon, args) = tree
     // If `args` is a list of named arguments, return corresponding type parameters,
     // otherwise return type parameters unchanged
-    val tparams = tycon.tpe.typeParams
+    val tparams = kind(tycon.tpe).typeParams
     def argNamed(tparam: ParamInfo) = args.find {
       case NamedArg(name, _) => name == tparam.paramName
       case _ => false

tests/pos/i4884.scala

@@ -0,0 +1,41 @@
+object Test {
+  trait A
+  trait TestConstructor1 { type F[_ <: A] }
+  trait TestConstructor2[D] {
+    type F[_ <: D]
+    class G[X <: D]
+    trait TestConstructor3[E] {
+      type G[_ <: D & E]
+      class H[X <: D & E]
+    }
+  }
+
+  val v1: TestConstructor1 => Unit = { f =>
+    type P[a <: A] = f.F[a] // OK
+  }
+
+  val v2: TestConstructor2[A] => Unit = { f =>
+    type P[a <: A] = f.F[a] // Error! Type argument a does not conform to upper bound D
+  }
+
+  def f2(f: TestConstructor2[A]): Unit = {
+    type P[a <: A] = f.F[a] // Error! Type argument a does not conform to upper bound D
+  }
+
+  // val v3: (f: TestConstructor2[A]) => (g: f.TestConstructor3[A]) => Unit = { f => g =>
+  //   type P[a <: A] = f.F[a] // Error! Type argument a does not conform to upper bound D
+  //   // type Q[a <: A] = g.G[a]
+  //   // type R[a <: A] = (f.F & g.G)[a]
+  //   // type R[a <: A] = ([X] => f.F[X] & g.G[X])[a]
+  // }
+  def f3(f: TestConstructor2[A], g: f.TestConstructor3[A]): Unit = {
+    type P[a <: A] = f.F[a] // Error! Type argument a does not conform to upper bound D
+    type Q[a <: A] = g.G[a]
+    // type R[a <: A] = (f.F & g.G)[a] // compiler error
+    type R[a <: A] = ([X <: A] => f.F[X] & g.G[X])[a]
+    type S[a <: A] = f.G[a] & g.H[a]
+  }
+  //val v4: (f: TestConstructor2[A]) => (g: f.TestConstructor3[A]) => Unit = {f => ???} // crash
+  //val v5: (f: TestConstructor2[A]) => (g: f.TestConstructor3[A]) => Unit = {(f: TestConstructor2[A]) => ???} // crash
+  //val v6: (f: TestConstructor2[A]) => (g: f.TestConstructor3[A]) => Unit = {(f: TestConstructor2[A]) => (g: f.TestConstructor3[A]) => ???} // crash
+}