From 9405c80db77e30e8c4f53c5d184392a92a9ae28c Mon Sep 17 00:00:00 2001
From: jvican <jorgevc@fastmail.es>
Date: Fri, 1 Jul 2016 10:38:52 +0200
Subject: [PATCH] Widen type of final vars #1285

When defining a final var, the type is not widened and, thus, it's as
specific as possible. However, when changing the value of the var, it
will give a compilation error because it cannot match the constant
types. This fix widens the type whenever it finds a final var.
---
 src/dotty/tools/dotc/typer/Typer.scala | 12 +++++++++++-
 tests/pos/WidenFinalVars.scala         |  5 +++++
 2 files changed, 16 insertions(+), 1 deletion(-)
 create mode 100644 tests/pos/WidenFinalVars.scala

diff --git a/src/dotty/tools/dotc/typer/Typer.scala b/src/dotty/tools/dotc/typer/Typer.scala
index ae8900c435e3..ccbefa7fcce8 100644
--- a/src/dotty/tools/dotc/typer/Typer.scala
+++ b/src/dotty/tools/dotc/typer/Typer.scala
@@ -1008,7 +1008,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
   def typedValDef(vdef: untpd.ValDef, sym: Symbol)(implicit ctx: Context) = track("typedValDef") {
     val ValDef(name, tpt, _) = vdef
     completeAnnotations(vdef, sym)
-    val tpt1 = checkSimpleKinded(typedType(tpt))
+    val tpt1 = widenIfFinalMutable(checkSimpleKinded(typedType(tpt)), sym)
     val rhs1 = vdef.rhs match {
       case rhs @ Ident(nme.WILDCARD) => rhs withType tpt1.tpe
       case rhs => typedExpr(rhs, tpt1.tpe)
@@ -1018,6 +1018,16 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
     vdef1
   }
 
+  /** Widen type if a valDef is both mutable and final, see #1285 */
+  private def widenIfFinalMutable(vdef: Tree, sym: Symbol)(implicit ctx: Context) = {
+    if (sym.is(allOf(Mutable, Final))) {
+      val widened = vdef.tpe.widen
+      vdef.withType(widened)
+      sym.info_=(widened)
+    }
+    vdef
+  }
+
   /** Add a @volitile to lazy vals when rewriting from Scala2 */
   private def patchIfLazy(vdef: ValDef)(implicit ctx: Context): Unit = {
     val sym = vdef.symbol
diff --git a/tests/pos/WidenFinalVars.scala b/tests/pos/WidenFinalVars.scala
new file mode 100644
index 000000000000..446b42cd427b
--- /dev/null
+++ b/tests/pos/WidenFinalVars.scala
@@ -0,0 +1,5 @@
+/* For more information, see #1285 */
+class Test {
+  final var x = false
+  x = true
+}
\ No newline at end of file