Workaround #742 and add a test for it.

Making a correct fix could take some time,
and I want to find other issues before I start working on this one.

Author: DarkDimius

src/dotty/tools/dotc/core/Contexts.scala

@@ -177,25 +177,26 @@ object Contexts {
     /** The new implicit references that are introduced by this scope */
     private var implicitsCache: ContextualImplicits = null
     def implicits: ContextualImplicits = {
-      if (implicitsCache == null )
-        implicitsCache = {
-          val implicitRefs: List[TermRef] =
-            if (isClassDefContext)
-              try owner.thisType.implicitMembers
-              catch {
-                case ex: CyclicReference => Nil
-              }
-            else if (isImportContext) importInfo.importedImplicits
-            else if (isNonEmptyScopeContext) scope.implicitDecls
-            else Nil
-          val outerImplicits =
-            if (isImportContext && importInfo.hiddenRoot.exists)
-              outer.implicits exclude importInfo.hiddenRoot
-            else
-              outer.implicits
-          if (implicitRefs.isEmpty) outerImplicits
-          else new ContextualImplicits(implicitRefs, outerImplicits)(this)
+      if (implicitsCache == null ) {
+        val outerImplicits =
+          if (isImportContext && importInfo.hiddenRoot.exists)
+            outer.implicits exclude importInfo.hiddenRoot
+          else
+            outer.implicits
+        try
+          implicitsCache = {
+            val implicitRefs: List[TermRef] =
+              if (isClassDefContext) owner.thisType.implicitMembers
+              else if (isImportContext) importInfo.importedImplicits
+              else if (isNonEmptyScopeContext) scope.implicitDecls
+              else Nil
+            if (implicitRefs.isEmpty) outerImplicits
+            else new ContextualImplicits(implicitRefs, outerImplicits)(this)
+          }
+        catch {
+          case ex: CyclicReference => implicitsCache = outerImplicits
         }
+      }
       implicitsCache
     }
 

tests/pending/run/StackMap.scala

@@ -0,0 +1,7 @@
+object Test {
+  var implicitsCache = null
+  
+  def main(args: Array[String]): Unit = {
+     implicitsCache = try{null} catch { case ex: Exception => null }
+  }
+}