Merge pull request #15343 from dotty-staging/revert-14026

Attempt to revert 14026

Author: odersky

compiler/src/dotty/tools/dotc/config/Config.scala

@@ -226,4 +226,10 @@ object Config {
    *  reduces the number of allocated denotations by ~50%.
    */
   inline val reuseSymDenotations = true
+
+  /** If true, check levels of type variables and create fresh ones as needed.
+   *  This is necessary for soundness (see 3ab18a9), but also causes several
+   *  regressions that should be fixed before turning this on.
+   */
+  inline val checkLevels = false
 }

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

@@ -92,9 +92,10 @@ trait ConstraintHandling {
 
   /** Is `level` <= `maxLevel` or legal in the current context? */
   def levelOK(level: Int, maxLevel: Int)(using Context): Boolean =
-    level <= maxLevel ||
-    ctx.isAfterTyper || !ctx.typerState.isCommittable || // Leaks in these cases shouldn't break soundness
-    level == Int.MaxValue // See `nestingLevel` above.
+    level <= maxLevel
+    || ctx.isAfterTyper || !ctx.typerState.isCommittable // Leaks in these cases shouldn't break soundness
+    || level == Int.MaxValue // See `nestingLevel` above.
+    || !Config.checkLevels
 
   /** If `param` is nested deeper than `maxLevel`, try to instantiate it to a
    *  fresh type variable of level `maxLevel` and return the new variable.

compiler/src/dotty/tools/dotc/core/tasty/TreePickler.scala

@@ -15,6 +15,7 @@ import Comments.{Comment, CommentsContext}
 import NameKinds._
 import StdNames.nme
 import transform.SymUtils._
+import config.Config
 import collection.mutable
 import dotty.tools.tasty.TastyFormat.ASTsSection
 
@@ -85,6 +86,11 @@ class TreePickler(pickler: TastyPickler) {
     case Some(label) =>
       if (label != NoAddr) writeRef(label) else pickleForwardSymRef(sym)
     case None =>
+      // See pos/t1957.scala for an example where this can happen.
+      // I believe it's a bug in typer: the type of an implicit argument refers
+      // to a closure parameter outside the closure itself. TODO: track this down, so that we
+      // can eliminate this case.
+      report.log(i"pickling reference to as yet undefined $sym in ${sym.owner}", sym.srcPos)
       pickleForwardSymRef(sym)
   }
 
@@ -197,7 +203,7 @@ class TreePickler(pickler: TastyPickler) {
       }
       else if (tpe.prefix == NoPrefix) {
         writeByte(if (tpe.isType) TYPEREFdirect else TERMREFdirect)
-        if !symRefs.contains(sym) && !sym.isPatternBound && !sym.hasAnnotation(defn.QuotedRuntimePatterns_patternTypeAnnot) then
+        if Config.checkLevels && !symRefs.contains(sym) && !sym.isPatternBound && !sym.hasAnnotation(defn.QuotedRuntimePatterns_patternTypeAnnot) then
           report.error(i"pickling reference to as yet undefined $tpe with symbol ${sym}", sym.srcPos)
         pickleSymRef(sym)
       }

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

@@ -1672,7 +1672,23 @@ class Namer { typer: Typer =>
             // This case applies if the closure result type contains uninstantiated
             // type variables. In this case, constrain the closure result from below
             // by the parameter-capture-avoiding type of the body.
-            typedAheadExpr(mdef.rhs, tpt.tpe).tpe
+            val rhsType = typedAheadExpr(mdef.rhs, tpt.tpe).tpe
+
+            // The following part is important since otherwise we might instantiate
+            // the closure result type with a plain functon type that refers
+            // to local parameters. An example where this happens in `dependent-closures.scala`
+            // If the code after `val rhsType` is commented out, this file fails pickling tests.
+            // AVOIDANCE TODO: Follow up why this happens, and whether there
+            // are better ways to achieve this. It would be good if we could get rid of this code.
+            // It seems at least partially redundant with the nesting level checking on TypeVar
+            // instantiation.
+            if !Config.checkLevels then
+              val hygienicType = TypeOps.avoid(rhsType, termParamss.flatten)
+              if (!hygienicType.isValueType || !(hygienicType <:< tpt.tpe))
+                report.error(i"return type ${tpt.tpe} of lambda cannot be made hygienic;\n" +
+                  i"it is not a supertype of the hygienic type $hygienicType", mdef.srcPos)
+            //println(i"lifting $rhsType over $termParamss -> $hygienicType = ${tpt.tpe}")
+            //println(TypeComparer.explained { implicit ctx => hygienicType <:< tpt.tpe })
           case _ =>
         }
         WildcardType

compiler/test/dotc/pos-test-pickling.blacklist

@@ -84,3 +84,6 @@ i4176-gadt.scala
 i13974a.scala
 
 java-inherited-type1
+
+# avoidance bug
+i15174.scala

tests/neg/i12640.scala

@@ -12,6 +12,6 @@ case class Empty[F[_]]() extends CpsStream[F,Nothing]
 
 def unfold[S,F[_]:CpsMonad,T](s0:S)(f:S => F[Option[(S,T)]]):F[CpsStream[F,T]] =
       summon[CpsMonad[F]].flatMap(f(s0)){
-         case Some(s1,a) => Cons(a, () => unfold(s1,f))  // error (used to crash)
+         case Some(s1,a) => Cons(a, () => unfold(s1,f))  // error (used to crash) // error
          case None => summon[CpsMonad[F]].pure(Empty[F]())
       }