Allow transparent classes

Author: odersky

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

@@ -357,7 +357,7 @@ object Flags {
   val (_, DefaultMethod @ _, _) = newFlags(38, "<defaultmethod>")
 
   /** Symbol is a transparent inline method or trait */
-  val (Transparent @ _, _, _) = newFlags(39, "transparent")
+  val (Transparent @ _, _, TransparentType @ _) = newFlags(39, "transparent")
 
   /** Symbol is an enum class or enum case (if used with case) */
   val (Enum @ _, EnumVal @ _, _) = newFlags(40, "enum")
@@ -609,5 +609,4 @@ object Flags {
   val SyntheticParam: FlagSet                = Synthetic | Param
   val SyntheticTermParam: FlagSet            = Synthetic | TermParam
   val SyntheticTypeParam: FlagSet            = Synthetic | TypeParam
-  val TransparentTrait: FlagSet              = Trait | Transparent
 }

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

@@ -1152,7 +1152,7 @@ object SymDenotations {
       isOneOf(FinalOrSealed) || isClass && !isOneOf(EffectivelyOpenFlags)
 
     final def isTransparentTrait(using Context): Boolean =
-      isAllOf(TransparentTrait)
+      is(TransparentType)
       || defn.assumedTransparentTraits.contains(symbol)
       || isClass && hasAnnotation(defn.TransparentTraitAnnot)
 

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

@@ -495,7 +495,7 @@ object Checking {
     }
     if sym.is(Transparent) then
       if sym.isType then
-        if !sym.is(Trait) then fail(em"`transparent` can only be used for traits".toMessage)
+        if !sym.isExtensibleClass then fail(em"`transparent` can only be used for extensible classes and traits".toMessage)
       else
         if !sym.isInlineMethod then fail(em"`transparent` can only be used for inline methods".toMessage)
     if (!sym.isClass && sym.is(Abstract))

tests/neg/transparent.scala

@@ -1,7 +1,8 @@
 transparent def foo = 1  // error
 transparent inline def bar = 2 // ok
 transparent inline val x = 2 // error
-transparent class c // error
+transparent class c // ok
+transparent final class d // error
 transparent object y // error
 transparent trait t // ok
 transparent type T = c  // error

tests/pos/unions.scala

@@ -4,4 +4,20 @@ object Test:
     val x = if ??? then "" else 1
     val _: String | Int = x
 
+object Test2:
+  transparent class A
+  class B extends A
+  class C extends A
+  val x = if ??? then B() else C()
+  val _: B | C = x
+
+object Test3:
+  class A
+  class B extends A
+  class C extends A
+  val x = if ??? then B() else C()
+  val _: A = x
+
+
+