Merge pull request #773 from dotty-staging/final-vals2

Memoize: duplicate scala2 behaviour: don't create fields for final vals.

7 files changed, 87 insertions, 23 deletions

diff --git a/src/dotty/tools/dotc/Compiler.scala b/src/dotty/tools/dotc/Compiler.scala
index e4b328a82..c537d8885 100644
--- a/src/dotty/tools/dotc/Compiler.scala
+++ b/src/dotty/tools/dotc/Compiler.scala
@@ -56,7 +56,6 @@ class Compiler {
       List(new VCInlineMethods,
            new SeqLiterals,
            new InterceptedMethods,
-           new Literalize,
            new Getters,
            new ClassTags,
            new ElimByName,
diff --git a/src/dotty/tools/dotc/ast/TreeInfo.scala b/src/dotty/tools/dotc/ast/TreeInfo.scala
index 82c8c9d60..1266ebbd9 100644
--- a/src/dotty/tools/dotc/ast/TreeInfo.scala
+++ b/src/dotty/tools/dotc/ast/TreeInfo.scala
@@ -326,18 +326,26 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
       // see a detailed explanation of this trick in `GenSymbols.reifyFreeTerm`
       free.symbol.hasStableFlag && isIdempotentExpr(free)
 */
-    case Apply(fn, Nil) =>
+    case Apply(fn, args) =>
+      def isKnownPureOp(sym: Symbol) =
+        sym.owner.isPrimitiveValueClass || sym.owner == defn.StringClass
       // Note: After uncurry, field accesses are represented as Apply(getter, Nil),
       // so an Apply can also be pure.
-      if (fn.symbol is Stable) exprPurity(fn) else Impure
+      if (args.isEmpty && fn.symbol.is(Stable)) exprPurity(fn)
+      else if (tree.tpe.isInstanceOf[ConstantType] && isKnownPureOp(tree.symbol))
+        // A constant expression with pure arguments is pure.
+        minOf(exprPurity(fn), args.map(exprPurity))
+      else Impure
     case Typed(expr, _) =>
       exprPurity(expr)
     case Block(stats, expr) =>
-      (exprPurity(expr) /: stats.map(statPurity))(_ min _)
+      minOf(exprPurity(expr), stats.map(statPurity))
     case _ =>
       Impure
   }
 
+  private def minOf(l0: PurityLevel, ls: List[PurityLevel]) = (l0 /: ls)(_ min _)
+
   def isPureExpr(tree: tpd.Tree)(implicit ctx: Context) = exprPurity(tree) == Pure
   def isIdempotentExpr(tree: tpd.Tree)(implicit ctx: Context) = exprPurity(tree) >= Idempotent
 
diff --git a/src/dotty/tools/dotc/core/Types.scala b/src/dotty/tools/dotc/core/Types.scala
index 5fd3b81d0..d04da30c0 100644
--- a/src/dotty/tools/dotc/core/Types.scala
+++ b/src/dotty/tools/dotc/core/Types.scala
@@ -715,6 +715,14 @@ object Types {
       case _ => this
     }
 
+    /** Widen from TermRef to its underlying non-termref
+     *  base type, while also skipping Expr types.
+     */
+    final def widenTermRefExpr(implicit ctx: Context): Type = stripTypeVar match {
+      case tp: TermRef if !tp.isOverloaded => tp.underlying.widenExpr.widenTermRefExpr
+      case _ => this
+    }
+
     /** Widen from ExprType type to its result type.
      *  (Note: no stripTypeVar needed because TypeVar's can't refer to ExprTypes.)
      */
diff --git a/src/dotty/tools/dotc/transform/Memoize.scala b/src/dotty/tools/dotc/transform/Memoize.scala
index 63466dc46..fbf8ed763 100644
--- a/src/dotty/tools/dotc/transform/Memoize.scala
+++ b/src/dotty/tools/dotc/transform/Memoize.scala
@@ -70,13 +70,26 @@ import Decorators._
     lazy val field = sym.field.orElse(newField).asTerm
     if (sym.is(Accessor, butNot = NoFieldNeeded))
       if (sym.isGetter) {
-          var rhs = tree.rhs.changeOwnerAfter(sym, field, thisTransform)
-          if (isWildcardArg(rhs)) rhs = EmptyTree
-          val fieldDef = transformFollowing(ValDef(field, rhs))
-          val getterDef = cpy.DefDef(tree)(rhs = transformFollowingDeep(ref(field))(ctx.withOwner(sym), info))
-          Thicket(fieldDef, getterDef)
+        def skipBlocks(t: Tree): Tree = t match {
+          case Block(_, t1) => skipBlocks(t1)
+          case _ => t
         }
-      else if (sym.isSetter) {
+        skipBlocks(tree.rhs) match {
+          case lit: Literal if sym.is(Final) && isIdempotentExpr(tree.rhs) =>
+            // duplicating scalac behavior: for final vals that have rhs as constant, we do not create a field
+            // and instead return the value. This seemingly minor optimization has huge effect on initialization
+            // order and the values that can be observed during superconstructor call
+
+            // see remark about idempotency in PostTyper#normalizeTree
+            cpy.DefDef(tree)(rhs = lit)
+          case _ =>
+            var rhs = tree.rhs.changeOwnerAfter(sym, field, thisTransform)
+            if (isWildcardArg(rhs)) rhs = EmptyTree
+            val fieldDef = transformFollowing(ValDef(field, rhs))
+            val getterDef = cpy.DefDef(tree)(rhs = transformFollowingDeep(ref(field))(ctx.withOwner(sym), info))
+            Thicket(fieldDef, getterDef)
+        }
+      } else if (sym.isSetter) {
         if (!sym.is(ParamAccessor)) { val Literal(Constant(())) = tree.rhs } // this is intended as an assertion
         val initializer = Assign(ref(field), ref(tree.vparamss.head.head.symbol))
         cpy.DefDef(tree)(rhs = transformFollowingDeep(initializer)(ctx.withOwner(sym), info))
diff --git a/src/dotty/tools/dotc/transform/PostTyper.scala b/src/dotty/tools/dotc/transform/PostTyper.scala
index a0670bca0..7a25a9870 100644
--- a/src/dotty/tools/dotc/transform/PostTyper.scala
+++ b/src/dotty/tools/dotc/transform/PostTyper.scala
@@ -68,8 +68,43 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer  { thisTran
     // TODO fill in
   }
 
-  private def normalizeTypeTree(tree: Tree)(implicit ctx: Context) = {
-    def norm(tree: Tree) = if (tree.isType) TypeTree(tree.tpe).withPos(tree.pos) else tree
+  /** Check bounds of AppliedTypeTrees.
+   *  Replace type trees with TypeTree nodes.
+   *  Replace constant expressions with Literal nodes.
+   *  Note: Demanding idempotency instead of purityin literalize is strictly speaking too loose.
+   *  Example
+   *
+   *    object O { final val x = 42; println("43") }
+   *    O.x
+   *
+   *  Strictly speaking we can't replace `O.x` with `42`.  But this would make
+   *  most expressions non-constant. Maybe we can change the spec to accept this
+   *  kind of eliding behavior. Or else enforce true purity in the compiler.
+   *  The choice will be affected by what we will do with `inline` and with
+   *  Singleton type bounds (see SIP 23). Presumably
+   *
+   *     object O1 { val x: Singleton = 42; println("43") }
+   *     object O2 { inline val x = 42; println("43") }
+   *
+   *  should behave differently.
+   *
+   *     O1.x  should have the same effect as   { println("43"; 42 }
+   *
+   *  whereas
+   *
+   *     O2.x = 42
+   *
+   *  Revisit this issue once we have implemented `inline`. Then we can demand
+   *  purity of the prefix unless the selection goes to an inline val.
+   */
+  private def normalizeTree(tree: Tree)(implicit ctx: Context): Tree = {
+    def literalize(tp: Type): Tree = tp.widenTermRefExpr match {
+      case ConstantType(value) if isIdempotentExpr(tree) => Literal(value)
+      case _ => tree
+    }
+    def norm(tree: Tree) =
+      if (tree.isType) TypeTree(tree.tpe).withPos(tree.pos)
+      else literalize(tree.tpe)
     tree match {
       case tree: TypeTree =>
         tree
@@ -115,7 +150,7 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer  { thisTran
     }
 
     override def transform(tree: Tree)(implicit ctx: Context): Tree =
-      try normalizeTypeTree(tree) match {
+      try normalizeTree(tree) match {
         case tree: Ident =>
           tree.tpe match {
             case tpe: ThisType => This(tpe.cls).withPos(tree.pos)
diff --git a/src/dotty/tools/dotc/typer/ConstFold.scala b/src/dotty/tools/dotc/typer/ConstFold.scala
index ac1c7260b..68a5d05f5 100644
--- a/src/dotty/tools/dotc/typer/ConstFold.scala
+++ b/src/dotty/tools/dotc/typer/ConstFold.scala
@@ -20,16 +20,16 @@ object ConstFold {
   def apply(tree: Tree)(implicit ctx: Context): Tree = finish(tree) {
     tree match {
       case Apply(Select(xt, op), yt :: Nil) =>
-        xt.tpe match {
+        xt.tpe.widenTermRefExpr match {
           case ConstantType(x) =>
-            yt.tpe match {
+            yt.tpe.widenTermRefExpr match {
               case ConstantType(y) => foldBinop(op, x, y)
               case _ => null
             }
           case _ => null
         }
       case Select(xt, op) =>
-        xt.tpe match {
+        xt.tpe.widenTermRefExpr match {
           case ConstantType(x) => foldUnop(op, x)
           case _ => null
         }
@@ -42,7 +42,7 @@ object ConstFold {
    */
   def apply(tree: Tree, pt: Type)(implicit ctx: Context): Tree =
     finish(apply(tree)) {
-      tree.tpe match {
+      tree.tpe.widenTermRefExpr match {
         case ConstantType(x) => x convertTo pt
         case _ => null
       }
diff --git a/src/dotty/tools/dotc/typer/Namer.scala b/src/dotty/tools/dotc/typer/Namer.scala
index 0e8b4d8cf..99119acb3 100644
--- a/src/dotty/tools/dotc/typer/Namer.scala
+++ b/src/dotty/tools/dotc/typer/Namer.scala
@@ -707,18 +707,19 @@ class Namer { typer: Typer =>
       // println(s"final inherited for $sym: ${inherited.toString}") !!!
       // println(s"owner = ${sym.owner}, decls = ${sym.owner.info.decls.show}")
       def isInline = sym.is(Final, butNot = Method)
-      def widenRhs(tp: Type): Type = tp match {
-        case tp: TermRef => widenRhs(tp.underlying)
-        case tp: ExprType => widenRhs(tp.resultType)
+      def widenRhs(tp: Type): Type = tp.widenTermRefExpr match {
         case tp: ConstantType if isInline => tp
         case _ => tp.widen.approximateUnion
       }
       val rhsCtx = ctx.addMode(Mode.InferringReturnType)
-      def rhsType = typedAheadExpr(mdef.rhs, rhsProto)(rhsCtx).tpe
+      def rhsType = typedAheadExpr(mdef.rhs, inherited orElse rhsProto)(rhsCtx).tpe
       def cookedRhsType = ctx.deskolemize(widenRhs(rhsType))
-      def lhsType = fullyDefinedType(cookedRhsType, "right-hand side", mdef.pos)
+      lazy val lhsType = fullyDefinedType(cookedRhsType, "right-hand side", mdef.pos)
       //if (sym.name.toString == "y") println(i"rhs = $rhsType, cooked = $cookedRhsType")
-      if (inherited.exists) inherited
+      if (inherited.exists)
+        if (sym.is(Final, butNot = Method) && lhsType.isInstanceOf[ConstantType])
+          lhsType // keep constant types that fill in for a non-constant (to be revised when inline has landed).
+        else inherited
       else {
         if (sym is Implicit) {
           val resStr = if (mdef.isInstanceOf[DefDef]) "result " else ""