Disallow taking a class tag of Nothing or Null.

It seems in most cases this leads to weird behavior and cause
confusing error messages later.

It also means we cannot create an Array[Nothing], except by
passing the classtag explicitly.

Author: odersky

compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -521,6 +521,8 @@ trait Implicits { self: Typer =>
                 val etag = inferImplicitArg(defn.ClassTagType.appliedTo(elemTp), error, pos)
                 if (etag.isEmpty) etag else etag.select(nme.wrap)
               case tp if hasStableErasure(tp) =>
+                if (defn.isBottomClass(tp.typeSymbol))
+                  error(where => i"attempt to take ClassTag of undetermined type for $where")
                 ref(defn.ClassTagModule)
                   .select(nme.apply)
                   .appliedToType(tp)

tests/neg/i1802.scala

@@ -14,8 +14,8 @@ object Exception {
     def apply(x: Throwable): T = f(downcast(x).get)
   }
 
-  def mkThrowableCatcher[T](isDef: Throwable => Boolean, f: Throwable => T) = mkCatcher(isDef, f)
+  def mkThrowableCatcher[T](isDef: Throwable => Boolean, f: Throwable => T) = mkCatcher(isDef, f) // error: undetermined ClassTag
 
-  implicit def throwableSubtypeToCatcher[Ex <: Throwable: ClassTag, T](pf: PartialFunction[Ex, T]) = // error: cyclic reference
+  implicit def throwableSubtypeToCatcher[Ex <: Throwable: ClassTag, T](pf: PartialFunction[Ex, T]) =
     mkCatcher(pf.isDefinedAt _, pf.apply _)
 }

tests/neg/i1907.scala

@@ -0,0 +1,7 @@
+import java.io.File
+
+object Test {
+  Some(new File("."))
+    .map(_.listFiles).getOrElse(Array.empty) // error: undetermined ClassTag
+    .map(_.listFiles)
+}

tests/neg/undet-classtag.scala

@@ -0,0 +1,5 @@
+object Test {
+  def f[T: reflect.ClassTag](x: T) = ???
+
+  f(???) // error: undetermined ClassTag
+}

tests/pos/t3859.scala

@@ -1,4 +1,4 @@
 class Test {
-  def foo: Unit = bar(Array(): _*)
+  def foo: Unit = bar(Array[AnyRef](): _*)
   def bar(values: AnyRef*): Unit = ()
 }

tests/pos/t5859.scala

@@ -7,9 +7,9 @@ class A {
   f(List[AnyRef](): _*)
   f(List(): _*)
   f(Nil: _*)
-  f(Array(): _*)
+  // f(Array(): _*)  // undetermined ClassTag
   f(Array[AnyRef](): _*)
   f(List(1))
   f(List(1), Nil: _*)
-  f(List(1), Array(): _*)
+  // f(List(1), Array(): _*)  // undetermined ClassTag
 }

tests/run/array-addition.scala

@@ -4,8 +4,8 @@ object Test {
   def main(args: Array[String]): Unit = {
     prettyPrintArray(Array(1,2,3) :+ 4)
     prettyPrintArray(1 +: Array(2,3,4))
-    prettyPrintArray(Array() :+ 1)
-    prettyPrintArray(1 +: Array())
+    prettyPrintArray(Array[Int]() :+ 1)
+    prettyPrintArray(1 +: Array[Int]())
   }
 }