@publicInBinary has now effect on secondary constructors

compiler/src/dotty/tools/dotc/inlines/PrepareInlineable.scala

@@ -105,8 +105,8 @@ object PrepareInlineable {
       def preTransform(tree: Tree)(using Context): Tree = tree match {
         case tree: RefTree if needsAccessor(tree.symbol) =>
           if (tree.symbol.isConstructor) {
-            report.error("Implementation restriction: cannot use private constructors in inline methods", tree.srcPos)
-            tree // TODO: create a proper accessor for the private constructor
+            report.error("Private constructors used in inline methods require @publicInBinary", tree.srcPos)
+            tree
           }
           else
             val accessor = useAccessor(tree)

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

@@ -20,11 +20,14 @@ import ValueClasses.*
  *  Make private accessor in value class not-private. This is necessary to unbox
  *  the value class when accessing it from separate compilation units
  *
- *  Also, make non-private any private parameter forwarders that forward to an inherited
+ *  Make non-private any private parameter forwarders that forward to an inherited
  *  public or protected parameter accessor with the same name as the forwarder.
  *  This is necessary since private methods are not allowed to have the same name
  *  as inherited public ones.
  *
+ * Also, make non-private any private constructor that is annotated with `@publicInBinary`.
+ * (See SIP-52)
+ *
  *  See discussion in https://github.com/scala/scala3/pull/784
  *  and https://github.com/scala/scala3/issues/783
  */
@@ -102,6 +105,8 @@ class ExpandPrivate extends MiniPhase with IdentityDenotTransformer { thisPhase
   override def transformDefDef(tree: DefDef)(using Context): DefDef = {
     val sym = tree.symbol
     tree.rhs match {
+      case _ if sym.isConstructor && sym.hasPublicInBinary =>
+        sym.ensureNotPrivate.installAfter(thisPhase)
       case Apply(sel @ Select(_: Super, _), _)
       if sym.isAllOf(PrivateParamAccessor) && sel.symbol.is(ParamAccessor) && sym.name == sel.symbol.name =>
         sym.ensureNotPrivate.installAfter(thisPhase)

tests/neg/i22498.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/i22498.scala:7:32 ----------------------------------------------------------------------------------
+7 |    inline def proxy: Foo = new Foo(0) // error
+  |                                ^^^
+  |                                Private constructors used in inline methods require @publicInBinary

tests/neg/i22498.scala

@@ -0,0 +1,7 @@
+//> using options -experimental
+
+import scala.annotation.publicInBinary
+
+class Foo:
+    private def this(x: Int) = this()
+    inline def proxy: Foo = new Foo(0) // error

tests/run/i22497.check

@@ -0,0 +1,3 @@
+public Foo()
+public Foo(int)
+public Foo(java.lang.String)

tests/run/i22497.scala

@@ -0,0 +1,13 @@
+// scalajs: --skip
+
+import scala.annotation.publicInBinary
+import scala.annotation.experimental
+
+@experimental
+class Foo:
+  @publicInBinary private def this(i: Int) = this()
+  @publicInBinary protected def this(i: String) = this()
+
+@experimental
+@main def Test =
+  println(classOf[Foo].getConstructors().mkString("\n"))

tests/run/i22498.scala

@@ -0,0 +1,10 @@
+//> using options -experimental
+
+import scala.annotation.publicInBinary
+
+class Foo:
+    @publicInBinary private def this(x: Int) = this()
+    inline def proxy: Foo = new Foo(0)
+
+@main def Test = 
+    val x = (new Foo).proxy