Modify stable path check for anytype

Author: noti0na1

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

@@ -116,14 +116,24 @@ class CheckRealizable(using Context) {
     case _: SingletonType | NoPrefix =>
       Realizable
     case tp =>
+      def checkAndType(x: TermRef, y: Type) =
+        (realizability(x) eq Realizable) && (y <:< x.widen)
+      val isStableAndType = tp match {
+        case AndType(tr: TermRef, tp1) =>
+          checkAndType(tr, tp1)
+        case AndType(tp1, tr: TermRef) =>
+          checkAndType(tr, tp1)
+        case _ => false
+      }
       def isConcrete(tp: Type): Boolean = tp.dealias match {
         case tp: TypeRef => tp.symbol.isClass
         case tp: TypeProxy => isConcrete(tp.underlying)
         case tp: AndType => isConcrete(tp.tp1) && isConcrete(tp.tp2)
         case tp: OrType  => isConcrete(tp.tp1) && isConcrete(tp.tp2)
         case _ => false
       }
-      if (!isConcrete(tp)) NotConcrete
+      if isStableAndType then Realizable
+      else if !isConcrete(tp) then NotConcrete
       else boundsRealizability(tp).andAlso(memberRealizability(tp))
   }
 

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

@@ -170,8 +170,9 @@ object Types {
       case tp: ExprType => tp.resultType.isStable
       case tp: AnnotatedType => tp.parent.isStable
       case tp: AndType =>
-        tp.tp1.isStable && (realizability(tp.tp2) eq Realizable) ||
-        tp.tp2.isStable && (realizability(tp.tp1) eq Realizable)
+        def checkAndStable(x: Type, y: Type) =
+          x.isStable && ((realizability(y) eq Realizable) || y <:< x.widen)
+        checkAndStable(tp.tp1, tp.tp2) || checkAndStable(tp.tp2, tp.tp1)
       case _ => false
     }
 

tests/explicit-nulls/neg/nn.scala

@@ -0,0 +1,10 @@
+// `.nn` extension method only strips away the outer Null.
+
+class Test {
+  val s1: String | Null = ???
+  val s2: String = s1.nn
+
+  val ss1: Array[String | Null] | Null = ???
+  val ss2: Array[String | Null] = ss1.nn
+  val ss3: Array[String] = ss1.nn // error
+}

tests/explicit-nulls/pos/flow-stable.scala

@@ -0,0 +1,10 @@
+class S {
+  def i[T](x: T): x.type = x
+
+  def f[T <: AnyRef](x: T | Null): x.type & T = {
+    if x != null then
+      i[x.type](x)
+    else
+      throw Exception()
+  }
+}