Partially fix #1500: Implicit search breaks at a certain depth

The issue fixed here was introduced by 71027f15. The added `csyms.isEmpty` condition on `case nil =>` is always true, which is clearely a bug.

t1500c still fails with covariant (or contravariant) type parameters on `::`, but this seams to be a more complicated issue involving the typer.

1 files changed, 5 insertions, 4 deletions

diff --git a/src/dotty/tools/dotc/typer/Implicits.scala b/src/dotty/tools/dotc/typer/Implicits.scala
index 0a3307140..2a1c18f7d 100644
--- a/src/dotty/tools/dotc/typer/Implicits.scala
+++ b/src/dotty/tools/dotc/typer/Implicits.scala
@@ -801,14 +801,15 @@ class SearchHistory(val searchDepth: Int, val seen: Map[ClassSymbol, Int]) {
       def updateMap(csyms: List[ClassSymbol], seen: Map[ClassSymbol, Int]): SearchHistory = csyms match {
         case csym :: csyms1 =>
           seen get csym match {
+            // proto complexity is >= than the last time it was seen → diverge
             case Some(prevSize) if size >= prevSize => this
             case _ => updateMap(csyms1, seen.updated(csym, size))
           }
-        case nil =>
-          if (csyms.isEmpty) this
-          else new SearchHistory(searchDepth + 1, seen)
+        case _ =>
+          new SearchHistory(searchDepth + 1, seen)
       }
-      updateMap(proto.classSymbols, seen)
+      if (proto.classSymbols.isEmpty) this
+      else updateMap(proto.classSymbols, seen)
     }
   }
 }