Fix #3917: Properly desugar Ident

compiler/sjs/backend/sjs/JSCodeGen.scala

@@ -850,21 +850,6 @@ class JSCodeGen()(implicit ctx: Context) {
     }
   } // end of genStatOrExpr()
 
-  // !!! DUPLICATE code with DottyBackendInterface
-  private def desugarIdent(i: Ident): Option[Select] = {
-    i.tpe match {
-      case TermRef(prefix: TermRef, name) =>
-        Some(tpd.ref(prefix).select(i.symbol))
-      case TermRef(prefix: ThisType, name) =>
-        Some(tpd.This(prefix.cls).select(i.symbol))
-      /*case TermRef(NoPrefix, name) =>
-        if (i.symbol is Method) Some(This(i.symbol.topLevelClass).select(i.symbol)) // workaround #342 todo: remove after fixed
-        else None*/
-      case _ =>
-        None
-    }
-  }
-
   private def qualifierOf(fun: Tree): Tree = fun match {
     case fun: Ident =>
       fun.tpe match {

compiler/src/dotty/tools/backend/jvm/DottyBackendInterface.scala

@@ -444,15 +444,7 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
   def desugarIdent(i: Ident): Option[tpd.Select] = {
     var found = desugared.get(i.tpe)
     if (found == null) {
-      i.tpe match {
-        case TermRef(prefix: TermRef, _) =>
-          found = tpd.ref(prefix).select(i.symbol)
-        case TermRef(prefix: ThisType, _) =>
-          found = tpd.This(prefix.cls).select(i.symbol)
-        case TermRef(NoPrefix, _) =>
-          if (i.symbol is Flags.Method) found = This(i.symbol.topLevelClass).select(i.symbol) // workaround #342 todo: remove after fixed
-        case _ =>
-      }
+      found = tpd.desugarIdent(i).orNull
       if (found != null) desugared.put(i.tpe, found)
     }
     if (found == null) None else Some(found)

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

@@ -826,14 +826,13 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
      */
     def becomes(rhs: Tree)(implicit ctx: Context): Tree =
       if (tree.symbol is Method) {
-        val setr = tree match {
-          case Ident(_) =>
-            val setter = tree.symbol.setter
-            assert(setter.exists, tree.symbol.showLocated)
-            ref(tree.symbol.setter)
-          case Select(qual, _) => qual.select(tree.symbol.setter)
+        val setter = tree.symbol.setter
+        assert(setter.exists, tree.symbol.showLocated)
+        val qual = tree match {
+          case id: Ident => desugarIdentPrefix(id)
+          case Select(qual, _) => qual
         }
-        setr.appliedTo(rhs)
+        qual.select(setter).appliedTo(rhs)
       }
       else Assign(tree, rhs)
 
@@ -1012,5 +1011,21 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
     val encoding = ctx.settings.encoding.value
     if (file != null && file.exists) new SourceFile(file, Codec(encoding)) else NoSource
   }
+
+  /** Desugar identifier into a select node. Return None if not possible */
+  def desugarIdent(tree: Ident)(implicit ctx: Context): Option[Select] = {
+    val qual = desugarIdentPrefix(tree)
+    if (qual.isEmpty) None
+    else Some(qual.select(tree.symbol))
+  }
+
+  private def desugarIdentPrefix(tree: Ident)(implicit ctx: Context): Tree = tree.tpe match {
+    case TermRef(prefix: TermRef, _) =>
+      ref(prefix)
+    case TermRef(prefix: ThisType, _) =>
+      This(prefix.cls)
+    case _ =>
+      EmptyTree
+  }
 }
 

compiler/src/dotty/tools/dotc/transform/localopt/ConstantFold.scala

@@ -6,7 +6,6 @@ import core.Symbols._
 import core.Types._
 import typer.ConstFold
 import ast.Trees._
-import Simplify.desugarIdent
 
 /** Various constant folding.
  *

compiler/src/dotty/tools/dotc/transform/localopt/InlineCaseIntrinsics.scala

@@ -10,7 +10,6 @@ import core.Flags._
 import core.TypeApplications.noBounds
 import ast.Trees._
 import transform.SymUtils._
-import Simplify.desugarIdent
 import dotty.tools.dotc.ast.tpd
 
 /** Inline case class specific methods using desugarings assumptions.

compiler/src/dotty/tools/dotc/transform/localopt/Simplify.scala

@@ -149,16 +149,6 @@ class Simplify extends MiniPhase with IdentityDenotTransformer {
 
 object Simplify {
   import tpd._
-  // TODO: This function is duplicated in jvm/DottyBackendInterface.scala, let's factor these out!
-  def desugarIdent(i: Ident)(implicit ctx: Context): Option[Select] = {
-    i.tpe match {
-      case TermRef(prefix: TermRef, _) =>
-        Some(ref(prefix).select(i.symbol))
-      case TermRef(prefix: ThisType, _) =>
-        Some(This(prefix.cls).select(i.symbol))
-      case _ => None
-    }
-  }
 
   /** Is this tree mutable, or java.lang.System.{in, out, err}? These three
    *  System members are the only static final fields that are mutable.

tests/pos/i3917.scala

@@ -0,0 +1,27 @@
+class A {
+  var a = false
+}
+
+object B {
+  var b = false
+}
+
+class C {
+  var c = false
+}
+
+object C extends A {
+  def test = {
+    a = true
+    C.a = true
+    this.a = true
+    C.this.a = true
+
+    import B._
+    b = true
+
+    val c0 = new C
+    import c0._
+    c = true
+  }
+}