Fix scala#6415: Enable REPL code completion for opaque types

The completion did not work because the desugared version of the
opaque type's definition shared the span with its private definition
inside the companion object.

Author: anatoliykmetyuk

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

@@ -820,7 +820,7 @@ object desugar {
       val moduleName = tdef.name.toTermName
       val localRef = Select(Ident(moduleName), tdef.name)
       localRef.pushAttachment(SuppressAccessCheck, ())
-      val aliasType = cpy.TypeDef(tdef)(rhs = completeForwarder(localRef))
+      val aliasType = cpy.TypeDef(tdef)(rhs = completeForwarder(localRef)).withSpan(Span(MaxOffset))
       val localType = tdef.withMods(Modifiers(Synthetic | Opaque).withPrivateWithin(tdef.name))
 
       val companions = moduleDef(ModuleDef(

compiler/test/dotty/tools/repl/TabcompleteTests.scala

@@ -128,4 +128,8 @@ class TabcompleteTests extends ReplTest {
   @Test def moduleCompletion = fromInitialState { implicit s =>
     assertEquals(List("Predef"), tabComplete("object Foo { type T = Pre"))
   }
+
+  @Test def i6415 = fromInitialState { implicit s =>
+    assertEquals(List("Predef"), tabComplete("opaque type T = Pre"))
+  }
 }