Avoid in ConstantTypes

Author: dwijnand

compiler/src/dotty/tools/dotc/core/TypeOps.scala

@@ -2,7 +2,7 @@ package dotty.tools
 package dotc
 package core
 
-import Contexts._, Types._, Symbols._, Names._, NameKinds.*, Flags._
+import Contexts._, Types._, Symbols._, Names._, NameKinds.*, Flags._, Constants.*
 import SymDenotations._
 import util.Spans._
 import util.Stats
@@ -560,6 +560,9 @@ object TypeOps:
             widenUnions = tp.widenUnions)(using mapCtx)
           val lo1 = apply(lo)
           if (lo1 ne lo) lo1 else tp
+        case tp @ ConstantType(Constant(tp1: Type)) =>
+          val tp2 = apply(tp1)
+          if tp2 eq tp1 then tp else ConstantType(Constant(tp2))
         case _ =>
           super.apply(tp)
       end apply

tests/neg/i16954.check

@@ -0,0 +1,4 @@
+-- [E170] Type Error: tests/neg/i16954.scala:4:12 ----------------------------------------------------------------------
+4 |    classOf[y] // error
+  |            ^
+  |            y is not a class type

tests/neg/i16954.scala

@@ -0,0 +1,5 @@
+class Test:
+  def test =
+    type y
+    classOf[y] // error: y is not a class type
+// was: "assertion failed: leak: y in { [..] }" crash after that error