AppliedTypes returned by baseType shouldn't have type lambdas as constructors

In tests/pos/argDenot-alpakka.min.scala, we want

    `(([O] =>> Foo[O, S]) @UV)[Int]`.baseType(`Foo`)

to return `Foo[Int]` rather than an applied type lambda.

This could be achieved by removing the initial if branch from the AppliedType
case in baseTypeOf, since the recursive fallback will always work, but it makes
sense to keep a special case for performance, so we just explicitly add as a
condition to the fast-path that the type constructor of the AppliedType can't be
a lambda.

Author: smarter

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

@@ -2225,19 +2225,13 @@ object SymDenotations {
           case tp @ AppliedType(tycon, args) =>
             def computeApplied = {
               btrCache(tp) = NoPrefix
-              extension (tp: Type) def typeSymbol2: Symbol = tp match
-                case tp: TypeRef     => tp.symbol
-                case tp: TypeVar     => tp.underlying.typeSymbol2
-                case tp: AppliedType => tp.underlying.typeSymbol2
-                case _               => NoSymbol
               val baseTp =
-                if (tycon.typeSymbol2 eq symbol) tp
-                else (tycon.typeParams: @unchecked) match {
+                if (tycon.typeSymbol eq symbol) && !tycon.isLambdaSub then tp
+                else (tycon.typeParams: @unchecked) match
                   case LambdaParam(_, _) :: _ =>
                     recur(tp.superType)
                   case tparams: List[Symbol @unchecked] =>
                     recur(tycon).substApprox(tparams, args)
-                }
               record(tp, baseTp)
               baseTp
             }