Fix scala#4031: Check arguments of dependent methods for realizability

The first attempt required changing z1720.scala; but that became unnecessary
after aligning isRealizable with isStable.
A TermRef is stable if its underlying type is stable. Realizability should
behave the same way.

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

@@ -4513,6 +4513,8 @@ object Types {
       else if (lo `eq` hi) lo
       else Range(lower(lo), upper(hi))
 
+    protected def emptyRange: Type = range(defn.NothingType, defn.AnyType)
+
     protected def isRange(tp: Type): Boolean = tp.isInstanceOf[Range]
 
     protected def lower(tp: Type): Type = tp match {
@@ -4586,7 +4588,7 @@ object Types {
           forwarded.orElse(
             range(super.derivedSelect(tp, preLo).loBound, super.derivedSelect(tp, preHi).hiBound))
         case _ =>
-          super.derivedSelect(tp, pre) match {
+          if (pre == defn.AnyType) pre else super.derivedSelect(tp, pre) match {
             case TypeBounds(lo, hi) => range(lo, hi)
             case tp => tp
           }
@@ -4637,7 +4639,7 @@ 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/Applications.scala

@@ -478,8 +478,9 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
             addArg(typedArg(arg, formal), formal)
             if (methodType.isParamDependent && typeOfArg(arg).exists)
               // `typeOfArg(arg)` could be missing because the evaluation of `arg` produced type errors
-              safeSubstParam(_, methodType.paramRefs(n), typeOfArg(arg))
-            else identity
+              safeSubstParam(_, methodType.paramRefs(n), typeOfArg(arg), initVariance = -1)
+            else
+              identity
           }
 
           def missingArg(n: Int): Unit = {

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

@@ -13,6 +13,7 @@ import NameOps._
 import collection.mutable
 import reporting.diagnostic.messages._
 import Checking.checkNoPrivateLeaks
+import CheckRealizable._
 
 trait TypeAssigner {
   import tpd._
@@ -105,7 +106,7 @@ 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)))
@@ -340,13 +341,31 @@ trait TypeAssigner {
     }
   }
 
-  /** Substitute argument type `argType` for parameter `pref` in type `tp`,
-   *  skolemizing the argument type if it is not stable and `pref` occurs in `tp`.
+  /** Substitute argument type `argType` for parameter `pref` in type `tp`, but
+   *  take special measures if the argument is not realizable:
+   *  1. If the widened argument type is known to have good bounds,
+   *     substitute the skolemized argument type.
+   *  2. If the widened argument type is not known to have good bounds, eliminate all references
+   *     to the parameter in `tp`.
+   *  (2) is necessary since even with a skolemized type we might break subtyping if
+   *  bounds are bad. This could lead to errors not being detected. A test case is the second
+   *  failure in neg/i4031.scala
    */
-  def safeSubstParam(tp: Type, pref: ParamRef, argType: Type)(implicit ctx: Context): Type = {
+  def safeSubstParam(tp: Type, pref: ParamRef, argType: Type, initVariance: Int = 1)(implicit ctx: Context): Type = {
     val tp1 = tp.substParam(pref, argType)
-    if ((tp1 eq tp) || argType.isStable) tp1
-    else tp.substParam(pref, SkolemType(argType.widen))
+    if ((tp1 eq tp) || argType.isStableRealizable) tp1
+    else {
+      val widenedArgType = argType.widen
+      if (realizability(widenedArgType) == Realizable)
+        tp.substParam(pref, SkolemType(widenedArgType))
+      else {
+        val avoidParam = new ApproximatingTypeMap {
+          variance = initVariance
+          def apply(t: Type) = if (t `eq` pref) emptyRange else mapOver(t)
+        }
+        avoidParam(tp)
+      }
+    }
   }
 
   /** Substitute types of all arguments `args` for corresponding `params` in `tp`.

tests/neg/i4031-posttyper.scala

@@ -0,0 +1,7 @@
+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 coerce2(x: Any): Int = upcast(p, x): p.L // error: not a legal path
+}

tests/neg/i4031.scala

@@ -0,0 +1,17 @@
+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 coerce(x: Any): Int = upcast(p, x)  // error: not a legal path
+
+  def compare(x: A, y: x.L) = assert(x == y)
+  def compare2(x: A)(y: x.type) = assert(x == y)
+
+
+  def main(args: Array[String]): Unit = {
+    println(coerce("Uh oh!"))
+    compare(p, p) // error: not a legal path
+    compare2(p)(p) // error: not a legal path
+  }
+}

tests/neg/i50-volatile.scala

@@ -12,14 +12,21 @@ class Test {
 
   class Client extends o.Inner                          // old-error // old-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
+}