Merge pull request #2267 from dotty-staging/fix-constant-lazy-vals

Fix #2266: Do not replace constant type lazy vals with constant.

4 files changed, 40 insertions, 7 deletions

diff --git a/compiler/src/dotty/tools/dotc/ast/TreeInfo.scala b/compiler/src/dotty/tools/dotc/ast/TreeInfo.scala
index f3bce4000..49187492e 100644
--- a/compiler/src/dotty/tools/dotc/ast/TreeInfo.scala
+++ b/compiler/src/dotty/tools/dotc/ast/TreeInfo.scala
@@ -3,7 +3,7 @@ package dotc
 package ast
 
 import core._
-import Flags._, Trees._, Types._, Contexts._
+import Flags._, Trees._, Types._, Contexts._, Constants._
 import Names._, StdNames._, NameOps._, Decorators._, Symbols._
 import util.HashSet
 import typer.ConstFold
@@ -426,8 +426,18 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
    */
   def constToLiteral(tree: Tree)(implicit ctx: Context): Tree = {
     val tree1 = ConstFold(tree)
+    def canInlineConstant(value: Constant): Boolean = {
+      val sym = tree1.symbol
+      isIdempotentExpr(tree1) && // see note in documentation
+      // lazy value must be initialized (would not be needed with isPureExpr)
+      !sym.is(Lazy) &&
+      // could hide initialization order issues (ex. val with constant type read before initialized)
+      (!ctx.owner.isLocalDummy || (!sym.is(Method) && !sym.is(Lazy) && value.isZero) ||
+        ctx.scala2Mode // ignore in Scala 2 because of inlined `final val` values
+      )
+    }
     tree1.tpe.widenTermRefExpr match {
-      case ConstantType(value) if isIdempotentExpr(tree1) => Literal(value)
+      case ConstantType(value) if canInlineConstant(value) => Literal(value)
       case _ => tree1
     }
   }
diff --git a/compiler/src/dotty/tools/dotc/core/Constants.scala b/compiler/src/dotty/tools/dotc/core/Constants.scala
index ed388b7ec..8ea285c8d 100644
--- a/compiler/src/dotty/tools/dotc/core/Constants.scala
+++ b/compiler/src/dotty/tools/dotc/core/Constants.scala
@@ -53,6 +53,20 @@ object Constants {
     def isNonUnitAnyVal       = BooleanTag <= tag && tag <= DoubleTag
     def isAnyVal              = UnitTag <= tag && tag <= DoubleTag
 
+    /** Is the zero or un-initialized value of the type */
+    def isZero(implicit ctx: Context): Boolean = tag match {
+      case BooleanTag => !value.asInstanceOf[Boolean]
+      case ByteTag    => value.asInstanceOf[Byte] == 0
+      case ShortTag   => value.asInstanceOf[Short] == 0
+      case CharTag    => value.asInstanceOf[Char] == 0
+      case IntTag     => value.asInstanceOf[Int] == 0
+      case LongTag    => value.asInstanceOf[Long] == 0L
+      case FloatTag   => value.asInstanceOf[Float] == 0.0
+      case DoubleTag  => value.asInstanceOf[Double] == 0.0
+      case NullTag    => true
+      case _          => false
+    }
+
     def tpe(implicit ctx: Context): Type = tag match {
       case UnitTag    => defn.UnitType
       case BooleanTag => defn.BooleanType
diff --git a/compiler/src/dotty/tools/dotc/transform/FirstTransform.scala b/compiler/src/dotty/tools/dotc/transform/FirstTransform.scala
index a3cf71ef2..767ee0901 100644
--- a/compiler/src/dotty/tools/dotc/transform/FirstTransform.scala
+++ b/compiler/src/dotty/tools/dotc/transform/FirstTransform.scala
@@ -9,7 +9,7 @@ import dotty.tools.dotc.transform.TreeTransforms._
 import ast.Trees._
 import Flags._
 import Types._
-import Constants.Constant
+import Constants._
 import Contexts.Context
 import Symbols._
 import SymDenotations._
@@ -34,6 +34,8 @@ import StdNames._
  *   - drops branches of ifs using the rules
  *          if (true) A else B  --> A
  *          if (false) A else B --> B
+ *          if (C: true) A else B  --> C; A
+ *          if (C: false) A else B  --> C; B
  */
 class FirstTransform extends MiniPhaseTransform with InfoTransformer with AnnotationTransformer { thisTransformer =>
   import ast.tpd._
@@ -190,11 +192,16 @@ class FirstTransform extends MiniPhaseTransform with InfoTransformer with Annota
   override def transformBlock(tree: Block)(implicit ctx: Context, info: TransformerInfo) =
     constToLiteral(tree)
 
-  override def transformIf(tree: If)(implicit ctx: Context, info: TransformerInfo) =
-    tree.cond match {
-      case Literal(Constant(c: Boolean)) => if (c) tree.thenp else tree.elsep
-      case _ => tree
+  override def transformIf(tree: If)(implicit ctx: Context, info: TransformerInfo) = {
+    tree.cond.tpe.widenTermRefExpr match {
+      case ConstantType(Constant(condVal: Boolean)) =>
+        val selected = if (condVal) tree.thenp else tree.elsep
+        if (isPureExpr(tree.cond)) selected
+        else Block(tree.cond :: Nil, selected)
+      case _ =>
+        tree
     }
+  }
 
   // invariants: all modules have companion objects
   // all types are TypeTrees
diff --git a/compiler/test/dotc/scala-collections.blacklist b/compiler/test/dotc/scala-collections.blacklist
index d6972a645..ae43e6eb4 100644
--- a/compiler/test/dotc/scala-collections.blacklist
+++ b/compiler/test/dotc/scala-collections.blacklist
@@ -81,3 +81,5 @@ scala/util/control/Exception.scala
 # 51 |  implicit def throwableSubtypeToCatcher[Ex <: Throwable: ClassTag, T](pf: PartialFunction[Ex, T]) =
 #    |                                                                                                 ^
 #    |                                                                     cyclic reference involving method mkCatcher
+
+scala/concurrent/duration/Duration.scala