Fix #3433: Fix determining typeParams for structural type members

Author: odersky

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

@@ -172,6 +172,7 @@ class TypeApplications(val self: Type) extends AnyVal {
       case self: TypeRef =>
         val tsym = self.symbol
         if (tsym.isClass) tsym.typeParams
+        else if (!tsym.exists) self.info.typeParams
         else if (!tsym.isCompleting) tsym.info.typeParams
         else Nil
       case self: AppliedType =>

tests/pos/i3433.scala

@@ -0,0 +1,4 @@
+object Test {
+  type Const[t] = { type L[x] = t }
+  type Bar[a] = Const[Int]#L[a]
+}