SI-4043 Fix eta expansion of aliases with non trivial bounds

In the enclosed test case, which defines:

   trait PA[H1] {
     type Apply[A <: H1] = Any
   }

The type selection `PA[Int]#Apply` was being eta expanded
to `[A <: H1]Any`.

After this commit, it is expanded to `[A' <: Int]Any`. This avoids
a spurious kind conformance error in the test.

Author: retronym

src/reflect/scala/reflect/internal/Types.scala

@@ -2181,7 +2181,14 @@ trait Types
       // must initialise symbol, see test/files/pos/ticket0137.scala
       val tpars = initializedTypeParams
       if (tpars.isEmpty) this
-      else typeFunAnon(tpars, copyTypeRef(this, pre, sym, tpars map (_.tpeHK))) // todo: also beta-reduce?
+      else {
+        val map = newAsSeenFromMap(pre, sym.owner)
+        val tpars1 = map.mapOver(tpars)
+        if (tpars eq tpars1)
+          typeFunAnon(tpars, copyTypeRef(this, pre, sym, tpars map (_.tpeHK)))
+        else
+          typeFunAnon(tpars1, copyTypeRef(this, pre, sym, tpars map (_.tpeHK.substSym(tpars, tpars1))))
+      } // todo: also beta-reduce?
     }
 
     // only need to rebind type aliases, as typeRef already handles abstract types

test/files/pos/t4043.scala

@@ -0,0 +1,13 @@
+object TypeParam {
+  trait GC[K[_ <: H0], H0]
+ 
+  trait PA[H1] {
+    type Apply[A <: H1] = Any
+  }
+ 
+  // error:
+  // kinds of the type arguments ([A <: H1]Any,Int) do not conform to the expected kinds of the type parameters (type K,type H0) in trait GC.
+  // [A <: H1]Any's type parameters do not match type K's expected parameters:
+  // type A's bounds >: Nothing <: H1 are stricter than type _'s declared bounds >: Nothing <: H0
+  type a = GC[PA[Int]#Apply, Int]
+}