Avoid loading compilation units twice

For some class name foo.Foo we need to try to load the tasty from the its class.
If the class is not definded, we try load it from the obeject.

Previously, if both the class and the object extists we were loading the compilation unit twice.
Which would duplicate symbols and then fail with duplicated symbol definitions.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/FromTasty.scala

@@ -71,12 +71,13 @@ object FromTasty extends Driver {
       case unit: TASTYCompilationUnit =>
         assert(ctx.settings.YretainTrees.value)
         val className = unit.className.toTypeName
-        val compilationUnits = List(tree(className), tree(className.moduleClassName)).flatMap {
-          case Some((clsd, unpickled)) if !unpickled.isEmpty =>
-            List(CompilationUnit.mkCompilationUnit(clsd, unpickled, forceTrees = true))
-          case _ => Nil
+        def compilationUnit(className: TypeName): Option[CompilationUnit] = {
+          tree(className).flatMap { case (clsd, unpickled) =>
+            if (unpickled.isEmpty) None
+            else Some(CompilationUnit.mkCompilationUnit(clsd, unpickled, forceTrees = true))
+          }
         }
-        compilationUnits
+        compilationUnit(className).orElse(compilationUnit(className.moduleClassName)).toList
     }
 
     private def tree(className: TypeName)(implicit ctx: Context): Option[(ClassDenotation, tpd.Tree)] = {

tests/pos-from-tasty/simpleCaseObject.scala

@@ -0,0 +1,3 @@
+package foo
+
+case object Foo

tests/pos-from-tasty/simpleClassWithObject.scala

@@ -0,0 +1,4 @@
+package foo
+
+class Foo
+object Foo