Fix #1795: Avoid infinite recursion between member and asSeenFrom

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

@@ -133,6 +133,8 @@ object Config {
    */
   final val LogPendingFindMemberThreshold = 10
 
-  /** Maximal number of outstanding recursive calls to findMember  */
+  /** Maximal number of outstanding recursive calls to findMember before backing out
+   *  when findMemberLimit is set.
+   */
   final val PendingFindMemberLimit = LogPendingFindMemberThreshold * 4
 }

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

@@ -10,7 +10,8 @@ import NameOps._
 import Decorators._
 import StdNames._
 import Annotations._
-import util.SimpleMap
+import config.Config
+import util.{SimpleMap, Property}
 import collection.mutable
 import ast.tpd._
 
@@ -67,7 +68,10 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
           if (thiscls.derivesFrom(cls) && pre.baseTypeRef(thiscls).exists) {
             if (theMap != null && theMap.currentVariance <= 0 && !isLegalPrefix(pre)) {
               ctx.base.unsafeNonvariant = ctx.runId
-              AnnotatedType(pre, Annotation(defn.UnsafeNonvariantAnnot, Nil))
+              pre match {
+                case AnnotatedType(_, ann) if ann.symbol == defn.UnsafeNonvariantAnnot => pre
+                case _ => AnnotatedType(pre, Annotation(defn.UnsafeNonvariantAnnot, Nil))
+              }
             }
             else pre
           }
@@ -85,13 +89,15 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
           if (sym.isStatic) tp
           else {
             val pre1 = asSeenFrom(tp.prefix, pre, cls, theMap)
-            if (pre1.isUnsafeNonvariant)
-              pre1.member(tp.name).info match {
+            if (pre1.isUnsafeNonvariant) {
+              val safeCtx = ctx.withProperty(TypeOps.findMemberLimit, Some(()))
+              pre1.member(tp.name)(safeCtx).info match {
                 case TypeAlias(alias) =>
                   // try to follow aliases of this will avoid skolemization.
                   return alias
                 case _ =>
               }
+            }
             tp.derivedSelect(pre1)
           }
         case tp: ThisType =>
@@ -554,4 +560,10 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
 
 object TypeOps {
   @sharable var track = false // !!!DEBUG
+
+  /** When a property with this key is set in a context, it limit the number
+   *  of recursive member searches. If the limit is reached, findMember returns
+   *  NoDenotation.
+   */
+  val findMemberLimit = new Property.Key[Unit]
 }

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

@@ -573,8 +573,12 @@ object Types {
 
       { val recCount = ctx.findMemberCount + 1
         ctx.findMemberCount = recCount
-        if (recCount >= Config.LogPendingFindMemberThreshold)
+        if (recCount >= Config.LogPendingFindMemberThreshold) {
           ctx.pendingMemberSearches = name :: ctx.pendingMemberSearches
+          if (ctx.property(TypeOps.findMemberLimit).isDefined &&
+              ctx.findMemberCount > Config.PendingFindMemberLimit)
+            return NoDenotation
+        }
       }
 
       //assert(ctx.findMemberCount < 20)

tests/pos/i1795.scala

@@ -0,0 +1,13 @@
+sealed trait T1 {type M1}
+
+case object o1 extends T1
+
+sealed trait T2 {type M2}
+
+case object o2 extends T2
+
+class TestX {
+  type TestT1 <: T1 {type M1 = TestT2}
+  type TestT2 <: T2 {type M2 = TestT1}
+  //val x: TestT1 = o1
+}