WIP: Use initialization checker for soundness

-- Warning: tests/init/neg/i11572.scala:8:6 ------------------------------------
8 |  val t: Bounded = new Bounded {
  |      ^
  |      Access non-initialized value t. Calling trace:
  |       -> }	[ i11572.scala:11 ]
  |        -> override type T >: t.T <: t.T	[ i11572.scala:10 ]
1 warning found

Author: liufengyun

compiler/src/dotty/tools/dotc/transform/init/Summarization.scala

@@ -140,7 +140,7 @@ object Summarization {
         val Summary(pots, effs) = analyze(selector)
         val init = Summary(Potentials.empty, pots.promote(selector) ++ effs)
         cases.foldLeft(init) { (acc, cas) =>
-          acc union analyze(cas.body)
+          acc + analyze(cas.body)
         }
 
       // case CaseDef(pat, guard, body) =>
@@ -162,7 +162,7 @@ object Summarization {
 
       case Try(block, cases, finalizer) =>
         val Summary(pots, effs) =  cases.foldLeft(analyze(block)) { (acc, cas) =>
-          acc union analyze(cas.body)
+          acc + analyze(cas.body)
         }
         val Summary(_, eff2) = if (finalizer.isEmpty) Summary.empty else analyze(finalizer)
         Summary(pots, effs ++ eff2)
@@ -199,8 +199,22 @@ object Summarization {
           Summary(pots.promote(ddef) ++ effs)
         }
 
-      case _: TypeDef =>
-        Summary.empty
+      case tdef: TypeDef =>
+        if tdef.isClassDef then Summary.empty
+        else {
+          var summary = Summary.empty
+          val tp = tdef.symbol.info
+          val traverser = new TypeTraverser {
+            def traverse(tp: Type): Unit = tp match {
+              case tp: TermRef =>
+                summary = summary + analyze(tp, tdef.rhs)
+              case _ =>
+                traverseChildren(tp)
+            }
+          }
+          traverser.traverse(tp)
+          summary
+        }
 
       case _: Import | _: Export =>
         Summary.empty

compiler/src/dotty/tools/dotc/transform/init/Summary.scala

@@ -14,7 +14,7 @@ import config.Printers.init
 import Potentials._, Effects._, Util._
 
 case class Summary(pots: Potentials, effs: Effects) {
-  def union(summary2: Summary): Summary =
+  def +(summary2: Summary): Summary =
     Summary(pots ++ summary2.pots, this.effs ++ summary2.effs)
 
   def +(pot: Potential): Summary =

tests/init/neg/i11572.scala

@@ -0,0 +1,17 @@
+class A {
+  trait Cov[+X] {
+    def get: X
+  }
+  trait Bounded {
+    type T >: Cov[Int] <: Cov[String]
+  }
+  val t: Bounded = new Bounded {     // error
+    // Note: using this instead of t produces an error (as expected)
+    override type T >: t.T <: t.T
+  }
+
+  val covInt = new Cov[Int] {
+    override def get: Int = 3
+  }
+  val str: String = ((covInt: t.T): Cov[String]).get // ClassCastException: class Integer cannot be cast to class String
+}