Align isRealizable with isStable

A TermRef is stable if its underlying type is stable. Realizability should
behave the same way. This obviates the change in z1720.scala.

Author: odersky

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

@@ -72,16 +72,18 @@ 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)) realizability(tp.prefix)
-          else if (!sym.isEffectivelyFinal) new NotFinal(sym)
-          else realizability(tp.info).mapError(r => new ProblemInUnderlying(tp.info, r))
-        if (r == Realizable) sym.setFlag(Stable)
-        r
-      }
+      val r =
+        if (sym.is(Stable)) realizability(tp.prefix)
+        else {
+          val r =
+            if (!sym.isStable) NotStable
+            else if (!isLateInitialized(sym)) realizability(tp.prefix)
+            else if (!sym.isEffectivelyFinal) new NotFinal(sym)
+            else realizability(tp.info).mapError(r => new ProblemInUnderlying(tp.info, r))
+          if (r == Realizable) sym.setFlag(Stable)
+          r
+        }
+      if (r == Realizable || tp.info.isStableRealizable) Realizable else r
     case _: SingletonType | NoPrefix =>
       Realizable
     case tp =>

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

@@ -159,7 +159,7 @@ object Types {
     /** 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 isRealizable(implicit ctx: Context): Boolean =
+    final def isStableRealizable(implicit ctx: Context): Boolean =
       isStable && realizability(this) == Realizable
 
     /** Is this type a (possibly refined or applied or aliased) type reference

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

@@ -350,7 +350,7 @@ trait TypeAssigner {
    */
   def safeSubstParam(tp: Type, pref: ParamRef, argType: Type, initVariance: Int = 1)(implicit ctx: Context) = {
     val tp1 = tp.substParam(pref, argType)
-    if ((tp1 eq tp) || argType.isRealizable) tp1
+    if ((tp1 eq tp) || argType.isStableRealizable) tp1
     else {
       val widenedArgType = argType.widen
       if (realizability(widenedArgType) == Realizable)

tests/neg/z1720.scala

@@ -3,7 +3,7 @@ package test
 class Thing {
   def info: Info[this.type] = InfoRepository.getInfo(this)
   def info2: Info[this.type] = {
-    val self: this.type = this
+    def self: this.type = this
     InfoRepository.getInfo(self)
   }
 }