Opaque types are not desugared on REPL code completion

REPL code completion computes completion candidates for a given
expression after typechecking it. Typechecking opaque types
desugars them from `<mods> type T = Int` to `<mods> type T = T.T`.
The completion relies on the position of the user's cursor
in the input expression to know what part of this expression to complete.
Since the cursor does not take the semantic info computed by typechecking,
the cursor points to `T.T` and not to `Int` on completion request.
Completion on synthetic `T.T` does not pick any candidates, which is
unexpected to the user. The expected behavior is to complete on `Int`,
the underlying type. This commit achieves the expected behavior by
making sure opaque types are treated as ordinary types on code completion.

Author: anatoliykmetyuk

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -806,7 +806,7 @@ object desugar {
       ctx.error(em"opaque type ${tdef.name} must be an alias type", tdef.sourcePos)
       tdef.withFlags(tdef.mods.flags &~ Opaque)
     }
-    else {
+    else if (!ctx.settings.YnoOpaqueTypes.value) {
       def completeForwarder(fwd: Tree) = tdef.rhs match {
         case LambdaTypeTree(tparams, tpt) =>
           val tparams1 =
@@ -828,6 +828,7 @@ object desugar {
           .withFlags(Synthetic | Opaque))
       Thicket(aliasType :: companions.toList)
     }
+    else tdef.withFlags(tdef.mods.flags &~ Opaque)
 
   /** The normalized name of `mdef`. This means
    *   1. Check that the name does not redefine a Scala core class.

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

@@ -144,6 +144,7 @@ class ScalaSettings extends Settings.SettingGroup {
   val YcheckAllPatmat: Setting[Boolean] = BooleanSetting("-Ycheck-all-patmat", "Check exhaustivity and redundancy of all pattern matching (used for testing the algorithm)")
   val YretainTrees: Setting[Boolean] = BooleanSetting("-Yretain-trees", "Retain trees for top-level classes, accessible from ClassSymbol#tree")
   val YshowTreeIds: Setting[Boolean] = BooleanSetting("-Yshow-tree-ids", "Uniquely tag all tree nodes in debugging output.")
+  val YnoOpaqueTypes: Setting[Boolean] = BooleanSetting("-Yno-opaque-types", "Opaque types are treated as ordinary transparent types.")
 
   val YprofileEnabled: Setting[Boolean] = BooleanSetting("-Yprofile-enabled", "Enable profiling.")
   val YprofileDestination: Setting[String] = StringSetting("-Yprofile-destination", "file", "where to send profiling output - specify a file, default is to the console.", "")

compiler/src/dotty/tools/repl/ReplCompiler.scala

@@ -277,6 +277,7 @@ class ReplCompiler extends Compiler {
     implicit val ctx: Context = state.context.fresh
       .setReporter(newStoreReporter)
       .setSetting(state.context.settings.YstopAfter, List("frontend"))
+      .setSetting(state.context.settings.YnoOpaqueTypes, true)
 
     wrapped(expr, src, state).flatMap { pkg =>
       val unit = CompilationUnit(src)