Treat new Array(0) as immutable (#19192)

An array of size 0 is immutable, thus we can safely abstract them with
the bottom value.

For the rules to be simple and understandable, we usually want to avoid
such fine-tuning. However, given that we expect such code patterns to be
rare and we want to avoid changes in the standard library, we fine-tune
the analysis as a compromise.

compiler/src/dotty/tools/dotc/transform/init/Objects.scala

@@ -565,7 +565,7 @@ object Objects:
 
   // --------------------------- domain operations -----------------------------
 
-  type ArgInfo = TraceValue[Value]
+  case class ArgInfo(value: Value, trace: Trace, tree: Tree)
 
   extension (a: Value)
     def join(b: Value): Value =
@@ -884,9 +884,14 @@ object Objects:
 
     case outer: (Ref | Cold.type | Bottom.type) =>
       if klass == defn.ArrayClass then
-        val arr = OfArray(State.currentObject, summon[Regions.Data])
-        Heap.writeJoin(arr.addr, Bottom)
-        arr
+        args.head.tree.tpe match
+          case ConstantType(Constants.Constant(0)) =>
+            // new Array(0)
+            Bottom
+          case _ =>
+            val arr = OfArray(State.currentObject, summon[Regions.Data])
+            Heap.writeJoin(arr.addr, Bottom)
+            arr
       else
         // Widen the outer to finitize the domain. Arguments already widened in `evalArgs`.
         val (outerWidened, envWidened) =
@@ -1328,7 +1333,7 @@ object Objects:
           case _ => List()
 
         val implicitArgsAfterScrutinee = evalArgs(implicits.map(Arg.apply), thisV, klass)
-        val args = implicitArgsBeforeScrutinee(fun) ++ (TraceValue(scrutinee, summon[Trace]) :: implicitArgsAfterScrutinee)
+        val args = implicitArgsBeforeScrutinee(fun) ++ (ArgInfo(scrutinee, summon[Trace], EmptyTree) :: implicitArgsAfterScrutinee)
         val unapplyRes = call(receiver, funRef.symbol, args, funRef.prefix, superType = NoType, needResolve = true)
 
         if fun.symbol.name == nme.unapplySeq then
@@ -1425,15 +1430,15 @@ object Objects:
       // call .lengthCompare or .length
       val lengthCompareDenot = getMemberMethod(scrutineeType, nme.lengthCompare, lengthCompareType)
       if lengthCompareDenot.exists then
-        call(scrutinee, lengthCompareDenot.symbol, TraceValue(Bottom, summon[Trace]) :: Nil, scrutineeType, superType = NoType, needResolve = true)
+        call(scrutinee, lengthCompareDenot.symbol, ArgInfo(Bottom, summon[Trace], EmptyTree) :: Nil, scrutineeType, superType = NoType, needResolve = true)
       else
         val lengthDenot = getMemberMethod(scrutineeType, nme.length, lengthType)
         call(scrutinee, lengthDenot.symbol, Nil, scrutineeType, superType = NoType, needResolve = true)
       end if
 
       // call .apply
       val applyDenot = getMemberMethod(scrutineeType, nme.apply, applyType(elemType))
-      val applyRes = call(scrutinee, applyDenot.symbol, TraceValue(Bottom, summon[Trace]) :: Nil, scrutineeType, superType = NoType, needResolve = true)
+      val applyRes = call(scrutinee, applyDenot.symbol, ArgInfo(Bottom, summon[Trace], EmptyTree) :: Nil, scrutineeType, superType = NoType, needResolve = true)
 
       if isWildcardStarArgList(pats) then
         if pats.size == 1 then
@@ -1444,7 +1449,7 @@ object Objects:
         else
           // call .drop
           val dropDenot = getMemberMethod(scrutineeType, nme.drop, applyType(elemType))
-          val dropRes = call(scrutinee, dropDenot.symbol, TraceValue(Bottom, summon[Trace]) :: Nil, scrutineeType, superType = NoType, needResolve = true)
+          val dropRes = call(scrutinee, dropDenot.symbol, ArgInfo(Bottom, summon[Trace], EmptyTree) :: Nil, scrutineeType, superType = NoType, needResolve = true)
           for pat <- pats.init do evalPattern(applyRes, pat)
           evalPattern(dropRes, pats.last)
         end if
@@ -1546,7 +1551,7 @@ object Objects:
         case _ =>
           res.widen(1)
 
-      argInfos += TraceValue(widened, trace.add(arg.tree))
+      argInfos += ArgInfo(widened, trace.add(arg.tree), arg.tree)
     }
     argInfos.toList
 
@@ -1644,7 +1649,7 @@ object Objects:
             // The parameter check of traits comes late in the mixin phase.
             // To avoid crash we supply hot values for erroneous parent calls.
             // See tests/neg/i16438.scala.
-            val args: List[ArgInfo] = ctor.info.paramInfoss.flatten.map(_ => new ArgInfo(Bottom, Trace.empty))
+            val args: List[ArgInfo] = ctor.info.paramInfoss.flatten.map(_ => new ArgInfo(Bottom, Trace.empty, EmptyTree))
             extendTrace(superParent) {
               superCall(tref, ctor, args, tasks)
             }

tests/init-global/pos/array-size-zero.scala

@@ -0,0 +1,10 @@
+object A:
+  val emptyArray = new Array(0)
+
+object B:
+  def build(data: Int*) =
+    if data.size == 0 then A.emptyArray else Array(data)
+
+  val arr = build(5, 6)
+  val first = arr(0)
+