Better error message for Null and 'sym singleton types.

Null and 'sym are not legal as singleton types because the underlying
values are not stable. They are rejected now outright instead of issuing
a cryptic "X is not stable" error message.

2 files changed, 8 insertions, 1 deletions

diff --git a/src/dotty/tools/dotc/typer/Typer.scala b/src/dotty/tools/dotc/typer/Typer.scala
index 33ec156a1..4441adcfb 100644
--- a/src/dotty/tools/dotc/typer/Typer.scala
+++ b/src/dotty/tools/dotc/typer/Typer.scala
@@ -782,7 +782,10 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
 
   def typedSingletonTypeTree(tree: untpd.SingletonTypeTree)(implicit ctx: Context): SingletonTypeTree = track("typedSingletonTypeTree") {
     val ref1 = typedExpr(tree.ref)
-    checkStable(ref1.tpe, tree.pos)
+    val illegal = Set[Symbol](defn.NullClass, defn.SymbolClass)
+    val refSym = ref1.tpe.widen.typeSymbol
+    if (illegal contains refSym) ctx.error(i"$refSym is not a legal singleton constant type", tree.pos)
+    else checkStable(ref1.tpe, tree.pos)
     assignType(cpy.SingletonTypeTree(tree)(ref1), ref1)
   }
 
diff --git a/tests/neg/singletons.scala b/tests/neg/singletons.scala
index 3bf7ee050..e4c6db060 100644
--- a/tests/neg/singletons.scala
+++ b/tests/neg/singletons.scala
@@ -2,4 +2,8 @@ object Test {
   val a: 42 = 43  // error: different constant
   val x = 42
   val z: 42 = x   // error: x is not final
+
+  val n: null = null // error: Null is not a legal singleton type
+
+  val sym: 'sym = 'sym // error: Symbol is a legal singleton type
 }