Check level consistency of SingletonTypeTree as a type (#17209)

Fixes #8887

Author: nicolasstucki

compiler/src/dotty/tools/dotc/staging/CrossStageSafety.scala

@@ -60,7 +60,7 @@ class CrossStageSafety extends TreeMapWithStages {
         val healedType = healType(tree.srcPos)(tp1)
         if healedType == tree.tpe then tree
         else TypeTree(healedType).withSpan(tree.span)
-      case _: RefTree if tree.isType =>
+      case _: RefTree | _: SingletonTypeTree if tree.isType =>
         val healedType = healType(tree.srcPos)(tree.tpe)
         if healedType == tree.tpe then tree
         else TypeTree(healedType).withSpan(tree.span)

tests/neg-macros/i8887.scala

@@ -0,0 +1,5 @@
+import scala.quoted._
+
+def expr[X](x: Any)(using Quotes): Expr[Any] =
+  '{ foo[x.type] } // error
+def foo[X]: Any = ???

tests/neg-macros/quote-this-a.scala

@@ -4,9 +4,7 @@ class Foo {
 
   def f(using Quotes): Unit = '{
     def bar[T](x: T): T = x
-    bar[
-      this.type  // error
-      ] {
+    bar[this.type] {
       this  // error
     }
   }

tests/pos-macros/i8887.scala

@@ -0,0 +1,3 @@
+import scala.quoted._
+inline def foo(x: Any): Any = ${ expr[x.type] }
+def expr[X](using Quotes): Expr[Any] = ???