Fix safeDealias

Providional type constructors are weird. They are still alias types but their
RHS is a possibly parameterized TypeBounds between Nothing and Any. safeDealias
should not "dealias" such a type to the upper bound `Any`.

Author: odersky

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

@@ -9,7 +9,7 @@ import SymDenotations.LazyType
 import Decorators._
 import util.Stats._
 import Names._
-import Flags.Module
+import Flags.{Module, Provisional}
 import dotty.tools.dotc.config.Config
 
 object TypeApplications {
@@ -284,7 +284,8 @@ class TypeApplications(val self: Type) extends AnyVal {
 
   /** Dealias type if it can be done without forcing the TypeRef's info */
   def safeDealias(using Context): Type = self match {
-    case self: TypeRef if self.denot.exists && self.symbol.isAliasType =>
+    case self: TypeRef
+    if self.denot.exists && self.symbol.isAliasType && !self.symbol.flagsUNSAFE.is(Provisional) =>
       self.superType.stripTypeVar.safeDealias
     case _ =>
       self

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -2035,10 +2035,10 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         // If it is an alias type, it would mean the type is cyclic
         // If it is an abstract type, it would mean the type is an irreducible
         // application of a higher-kinded type to a wildcard argument.
-        // Either way, the wildcard argument is illegal.
-        // The early test here is needed, so that we do not accidentally reduce
-        // an application of a Provisional type away so that the type constructor
-        // is no longer present on the roght hand side. See neg/i15507.scala.
+        // Either way, the wildcard argument is illegal. The early test of
+        // `checkNoWildcard` here is needed, so that we do not accidentally reduce
+        // an application of a Provisional type away, which would mean that the type constructor
+        // is no longer present on the right hand side. See neg/i15507.scala.
         checkedArgs = checkedArgs.mapconserve(checkNoWildcard)
       else if tycon == defn.throwsAlias
           && checkedArgs.length == 2