Refinements to realizability

Unstable types can still be realizable (i.e. if their underlying
type is concrete with no bad bounds).

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

@@ -73,10 +73,16 @@ class CheckRealizable(implicit ctx: Context) {
       if (sym.is(Stable)) realizability(tp.prefix)
       else {
         val r =
-          if (!sym.isStable) NotStable
-          else if (!isLateInitialized(sym)) Realizable
-          else if (!sym.isEffectivelyFinal) new NotFinal(sym)
-          else realizability(tp.info).mapError(r => new ProblemInUnderlying(tp.info, r))
+          if (sym.isStable && !isLateInitialized(sym))
+            // it's realizable because we know that a value of type `tp` has been created at run-time
+            Realizable
+          else if (!sym.isEffectivelyFinal)
+            // it's potentially not realizable since it might be overridden with a member of nonrealizable type
+            new NotFinal(sym)
+          else
+            // otherwise we need to look at the info to determine realizability
+            // roughly: it's realizable if the info does not have bad bounds
+            realizability(tp.info).mapError(r => new ProblemInUnderlying(tp, r))
         r andAlso {
           sym.setFlag(Stable)
           realizability(tp.prefix)

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

@@ -149,7 +149,7 @@ object Types {
 
     /** Does this type denote a stable reference (i.e. singleton type)? */
     final def isStable(implicit ctx: Context): Boolean = stripTypeVar match {
-      case tp: TermRef => tp.termSymbol.isStable && tp.prefix.isStable || tp.info.isStable
+      case tp: TermRef => tp.symbol.isStable && tp.prefix.isStable || tp.info.isStable
       case _: SingletonType | NoPrefix => true
       case tp: RefinedOrRecType => tp.parent.isStable
       case tp: ExprType => tp.resultType.isStable
@@ -4506,6 +4506,8 @@ object Types {
       else if (lo `eq` hi) lo
       else Range(lower(lo), upper(hi))
 
+    protected def emptyRange = range(defn.NothingType, defn.AnyType)
+
     protected def isRange(tp: Type): Boolean = tp.isInstanceOf[Range]
 
     protected def lower(tp: Type): Type = tp match {
@@ -4630,7 +4632,7 @@ object Types {
       else tp.derivedTypeBounds(lo, hi)
 
     override protected def derivedSuperType(tp: SuperType, thistp: Type, supertp: Type): Type =
-      if (isRange(thistp) || isRange(supertp)) range(defn.NothingType, defn.AnyType)
+      if (isRange(thistp) || isRange(supertp)) emptyRange
       else tp.derivedSuperType(thistp, supertp)
 
     override protected def derivedAppliedType(tp: AppliedType, tycon: Type, args: List[Type]): Type =

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

@@ -105,7 +105,7 @@ trait TypeAssigner {
             case info: ClassInfo =>
               range(defn.NothingType, apply(classBound(info)))
             case _ =>
-              range(defn.NothingType, defn.AnyType) // should happen only in error cases
+              emptyRange // should happen only in error cases
           }
         case tp: ThisType if toAvoid(tp.cls) =>
           range(defn.NothingType, apply(classBound(tp.cls.classInfo)))

tests/neg/i50-volatile.scala

@@ -10,16 +10,23 @@ class Test {
   }
   lazy val o: A & B = ???
 
-  class Client extends o.Inner                          // old-error // old-error
+  class Client extends o.Inner                          // error
 
-  def xToString(x: o.X): String = x                     // old-error
+  def xToString(x: o.X): String = x                     // error
 
   def intToString(i: Int): String = xToString(i)
 }
-object Test2 {
 
-  import Test.o._                                       // error
+object Test2 {
+  trait A {
+    type X = String
+  }
+  trait B {
+    type X = Int
+  }
+  lazy val o: A & B = ???
 
-  def xToString(x: X): String = x
+  def xToString(x: o.X): String = x       // error
 
+  def intToString(i: Int): String = xToString(i)
 }

tests/neg/realizability.scala

@@ -0,0 +1,22 @@
+class C { type T }
+
+class Test {
+
+  type D <: C
+
+  lazy val a: C = ???
+  final lazy val b: C = ???
+  val c: D = ???
+  final lazy val d: D = ???
+
+  val x1: a.T = ???  // error: not a legal path, since a is lazy & non-final
+  val x2: b.T = ???  // OK, b is lazy but concrete
+  val x3: c.T = ???  // OK, c is abstract but strict
+  val x4: d.T = ???  // error: not a legal path since d is abstract and lazy
+
+  val y1: Singleton = a
+  val y2: Singleton = a
+  val y3: Singleton = a
+  val y4: Singleton = a
+
+}

tests/pos/i4031.scala

@@ -0,0 +1,8 @@
+object App {
+  trait A { type L >: Any}
+  def upcast(a: A, x: Any): a.L = x
+  lazy val p: A { type L <: Nothing } = p
+  val q = new A { type L = Any }
+  def coerce1(x: Any): Any = upcast(q, x)  // ok
+  def coerce3(x: Any): Any = upcast(p, x)  // ok, since dependent result type is not needed
+}