Fix type aliases in beta-reduction of polyfunctions

Fixes #17052

Author: nicolasstucki

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

@@ -117,7 +117,7 @@ object BetaReduce:
           case ref @ TypeRef(NoPrefix, _) =>
             ref.symbol
           case _ =>
-            val binding = TypeDef(newSymbol(ctx.owner, tparam.name, EmptyFlags, targ.tpe, coord = targ.span)).withSpan(targ.span)
+            val binding = TypeDef(newSymbol(ctx.owner, tparam.name, EmptyFlags, TypeAlias(targ.tpe), coord = targ.span)).withSpan(targ.span)
             bindings += binding
             binding.symbol
 

tests/pos/i17052.scala

@@ -0,0 +1,2 @@
+def test[F[_]](fAny: F[Any]) =
+  { [X] => (fx: F[X]) => { val fx2: F[X] = fx; () } }.apply[Any](fAny)