Enforce restriction that constructor proxies cannot be values

Author: odersky

compiler/src/dotty/tools/dotc/transform/PostTyper.scala

@@ -250,9 +250,14 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
       }
     }
 
+    def checkNoConstructorProxy(tree: Tree)(using Context): Unit =
+      if tree.symbol.is(ConstructorProxy) then
+        report.error(em"constructor proxy ${tree.symbol} cannot be used as a value", tree.srcPos)
+
     override def transform(tree: Tree)(using Context): Tree =
       try tree match {
         case tree: Ident if !tree.isType =>
+          checkNoConstructorProxy(tree)
           tree.tpe match {
             case tpe: ThisType => This(tpe.cls).withSpan(tree.span)
             case _ => tree
@@ -263,6 +268,7 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
             withMode(Mode.Type)(super.transform(tree))
           }
           else
+            checkNoConstructorProxy(tree)
             transformSelect(tree, Nil)
         case tree: Apply =>
           val methType = tree.fun.tpe.widen

tests/neg/constructor-proxy-values.scala

@@ -0,0 +1,16 @@
+
+object Test extends App {
+  class A {
+    class A22(s: String) {
+      def run = s
+    }
+  }
+  val a = A()
+  val x = a.A22("x")        // OK
+  val x2 = a.A22.apply("X") // OK
+  val x3 = a.A22.apply(_)   // OK
+  val y = a.A22           // error: Cannot be used as value
+  val z = a.A22.toString  // error: Cannot be used as value
+  val u = A               // error: Cannot be used as value
+  println(y)
+}