Make dependently-typed members of u.tasty stable

Without this change, `showExtractors` is unstable and so is
`showExtractors.tasty`, so `showExtractors.showTypeOrBounds`'s type gets
approximated from `showExtractors.tasty.TypeOrBounds` to its bounds (`Any` or
`Nothing` depending on variance), so the additions to the testcase fail. The
same applies to other members.

I also tried replacing `tasty.Tree` with `T#Tree` (and so on), but apparently
those two types aren't considered equal, at least by Scalac (which is arguably a
bug).

Relates to code added in #4577.

Author: Blaisorblade

compiler/src/dotty/tools/dotc/tasty/TastyImpl.scala

@@ -29,9 +29,9 @@ object TastyImpl extends scala.tasty.Tasty {
 
   // ===== Show =====================================================
 
-  def defaultShow: Show[this.type] = showExtractors
+  val showExtractors: Show[this.type] = new ShowExtractors(this)
 
-  def showExtractors: Show[this.type] = new ShowExtractors(this)
+  val defaultShow: Show[this.type] = showExtractors
 
   // TODO
   // def showSourceCode: Show[this.type] = ???

library/src/scala/tasty/Tasty.scala

@@ -19,9 +19,9 @@ abstract class Tasty { tasty =>
 
   // ===== Show =====================================================
 
-  implicit def defaultShow: Show[tasty.type]
+  implicit val defaultShow: Show[tasty.type]
 
-  def showExtractors: Show[tasty.type]
+  val showExtractors: Show[tasty.type]
 
   // TODO
   // def showSourceCode: Show[tasty.type]

tests/run/tasty-macro-assert/quoted_1.scala

@@ -22,8 +22,16 @@ object Asserts {
 
     def isOps(tpe: TypeOrBounds): Boolean = tpe match {
       case Type.SymRef(DefDef("Ops", _, _, _, _),
-        Type.SymRef(ValDef("Asserts", _, _),
+        pre @ Type.SymRef(ValDef("Asserts", _, _),
           Type.ThisType(Type.SymRef(PackageDef("<empty>", _), NoPrefix())))) =>
+        val tpeShow = tpe.show
+        val tpeShow1 = showExtractors.showTypeOrBounds(tpe)(u.context)
+        val preShow = pre.show
+        val preShow1 = showExtractors.showTypeOrBounds(pre)(u.context)
+        assert(tpeShow == tpeShow1)
+        assert(preShow == preShow1)
+        println(tpeShow)
+        println(preShow)
         true
       case _ => false
     }