Fix implicitScope for the case when implicit scopes cannot be cached

This fixes #347.

When Config.cacheImplicitScopes == false or an implicit scope is
requested for a type for which it cannot be cached, this change
causes the same computation to be applied as if the scope were
looked up in the implicit scope cache and not found.

Author: olhotak

src/dotty/tools/dotc/typer/Implicits.scala

@@ -337,25 +337,29 @@ trait ImplicitRunInfo { self: RunInfo =>
    /** The implicit scope of type `tp`
      *  @param isLifted    Type `tp` is the result of a `liftToClasses` application
      */
-    def iscope(tp: Type, isLifted: Boolean = false): OfTypeImplicits =
+    def iscope(tp: Type, isLifted: Boolean = false): OfTypeImplicits = {
+      def computeIScope(cacheResult: Boolean) = {
+        val savedEphemeral = ctx.typerState.ephemeral
+        ctx.typerState.ephemeral = false
+        try {
+          val liftedTp = if (isLifted) tp else liftToClasses(tp)
+          val result =
+            if (liftedTp ne tp) iscope(liftedTp, isLifted = true)
+            else ofTypeImplicits(collectCompanions(tp))
+          if (ctx.typerState.ephemeral) record("ephemeral cache miss: implicitScope")
+          else if(cacheResult) implicitScopeCache(tp) = result
+          result
+        }
+        finally ctx.typerState.ephemeral |= savedEphemeral
+      }
+
       if (tp.hash == NotCached || !Config.cacheImplicitScopes)
-        ofTypeImplicits(collectCompanions(tp))
+        computeIScope(cacheResult = false)
       else implicitScopeCache get tp match {
         case Some(is) => is
-        case None =>
-          val savedEphemeral = ctx.typerState.ephemeral
-          ctx.typerState.ephemeral = false
-          try {
-            val liftedTp = if (isLifted) tp else liftToClasses(tp)
-            val result =
-              if (liftedTp ne tp) iscope(liftedTp, isLifted = true)
-              else ofTypeImplicits(collectCompanions(tp))
-            if (ctx.typerState.ephemeral) record("ephemeral cache miss: implicitScope")
-            else implicitScopeCache(tp) = result
-            result
-          }
-          finally ctx.typerState.ephemeral |= savedEphemeral
-        }
+        case None => computeIScope(cacheResult = true)
+      }
+    }
 
     iscope(tp)
   }