Align isRealizable with isStable

A TermRef is stable if its underlying type is stable. Realizability should
behave the same way.
Part 1 of 9125f58.

Author: odersky

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

@@ -70,18 +70,20 @@ class CheckRealizable(implicit ctx: Context) {
   def realizability(tp: Type): Realizability = tp.dealias match {
     case tp: TermRef =>
       val sym = tp.symbol
-      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))
-        r andAlso {
-          sym.setFlag(Stable)
-          realizability(tp.prefix)
+      val r =
+        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))
+          r andAlso {
+            sym.setFlag(Stable)
+            realizability(tp.prefix)
+          }
         }
-      }
+      if (r == Realizable || tp.info.isStableRealizable) Realizable else r
     case _: SingletonType | NoPrefix =>
       Realizable
     case tp =>

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

@@ -18,6 +18,7 @@ import Denotations._
 import Periods._
 import util.Stats._
 import util.SimpleIdentitySet
+import CheckRealizable._
 import reporting.diagnostic.Message
 import ast.tpd._
 import ast.TreeTypeMap
@@ -157,6 +158,12 @@ object Types {
       case _ => false
     }
 
+    /** Does this type denote a realizable stable reference? Much more expensive to check
+     *  than isStable, that's why some of the checks are done later in PostTyper.
+     */
+    final def isStableRealizable(implicit ctx: Context): Boolean =
+      isStable && realizability(this) == Realizable
+
     /** Is this type a (possibly refined or applied or aliased) type reference
      *  to the given type symbol?
      *  @sym  The symbol to compare to. It must be a class symbol or abstract type.