A relaxation concerning exported type aliases

The rules for export forwarders are changed as follows.

Previously, all export forwarders were declared `final`. Now, only term members are declared `final`. Type aliases left aside.

This makes it possible to export the same type member into several traits and then mix these traits in the same class. `typeclass-aggregates.scala` shows why this is essential to be able to combine multiple givens with type members.
The change does not lose safety since different type aliases would in any case lead to uninstantiatable classes.

Author: odersky

compiler/src/dotty/tools/dotc/config/Feature.scala

@@ -33,6 +33,7 @@ object Feature:
   val pureFunctions = experimental("pureFunctions")
   val captureChecking = experimental("captureChecking")
   val into = experimental("into")
+  val modularity = experimental("modularity")
 
   val globalOnlyImports: Set[TermName] = Set(pureFunctions, captureChecking)
 

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

@@ -25,7 +25,7 @@ import Inferencing.*
 import transform.ValueClasses.*
 import TypeErasure.erasure
 import reporting.*
-import config.Feature.sourceVersion
+import config.Feature.{sourceVersion, modularity}
 import config.SourceVersion.*
 
 import scala.compiletime.uninitialized
@@ -1191,7 +1191,7 @@ class Namer { typer: Typer =>
                 target = target.etaExpand(target.typeParams)
               newSymbol(
                 cls, forwarderName,
-                Exported | Final,
+                Exported | (if Feature.enabled(modularity) then EmptyFlags else Final),
                 TypeAlias(target),
                 coord = span)
               // Note: This will always create unparameterzied aliases. So even if the original type is

docs/_docs/reference/other-new-features/export.md

@@ -37,7 +37,12 @@ final def print(bits: BitMap): Unit = printUnit.print(bits)
 final type PrinterType              = printUnit.PrinterType
 ```
 
-They can be accessed inside `Copier` as well as from outside:
+With the experimental `modularity` language import, only exported methods and values are final, whereas the generated `PrinterType` would be a simple type alias
+```scala
+      type PrinterType              = printUnit.PrinterType
+```
+
+These aliases can be accessed inside `Copier` as well as from outside:
 
 ```scala
 val copier = new Copier
@@ -90,12 +95,17 @@ export O.*
 ```
 
 Export aliases copy the type and value parameters of the members they refer to.
-Export aliases are always `final`. Aliases of given instances are again defined as givens (and aliases of old-style implicits are `implicit`). Aliases of extensions are again defined as extensions. Aliases of inline methods or values are again defined `inline`. There are no other modifiers that can be given to an alias. This has the following consequences for overriding:
+Export aliases of term members are always `final`. Aliases of given instances are again defined as givens (and aliases of old-style implicits are `implicit`). Aliases of extensions are again defined as extensions. Aliases of inline methods or values are again defined `inline`. There are no other modifiers that can be given to an alias. This has the following consequences for overriding:
 
- - Export aliases cannot be overridden, since they are final.
+ - Export aliases of methods or fields cannot be overridden, since they are final.
  - Export aliases cannot override concrete members in base classes, since they are
    not marked `override`.
  - However, export aliases can implement deferred members of base classes.
+ - Export type aliases are normally also final, except when the experimental
+   language import `modularity` is present. The general
+   rules for type aliases ensure in any case that if there are several type aliases in a class,
+   they must agree on their right hand sides, or the class could not be instantiated.
+   So dropping the `final` for export type aliases is safe.
 
 Export aliases for public value definitions that are accessed without
 referring to private values in the qualifier path

tests/pos/typeclass-aggregates.scala

@@ -0,0 +1,47 @@
+//> using options -source future -language:experimental.modularity
+trait Ord:
+  type This
+  extension (x: This)
+    def compareTo(y: This): Int
+    def < (y: This): Boolean = compareTo(y) < 0
+    def > (y: This): Boolean = compareTo(y) > 0
+
+  trait OrdProxy extends Ord:
+    export Ord.this.*
+
+trait SemiGroup:
+  type This
+  extension (x: This) def combine(y: This): This
+
+  trait SemiGroupProxy extends SemiGroup:
+    export SemiGroup.this.*
+
+trait Monoid extends SemiGroup:
+  def unit: This
+
+  trait MonoidProxy extends Monoid:
+    export Monoid.this.*
+
+def ordWithMonoid(ord: Ord, monoid: Monoid{ type This = ord.This }): Ord & Monoid =
+  new ord.OrdProxy with monoid.MonoidProxy {}
+
+trait OrdWithMonoid extends Ord, Monoid
+
+def ordWithMonoid2(ord: Ord, monoid: Monoid{ type This = ord.This }) = //: OrdWithMonoid { type This = ord.This} =
+  new OrdWithMonoid with ord.OrdProxy with monoid.MonoidProxy {}
+
+given intOrd: Ord { type This = Int } = ???
+given intMonoid: Monoid { type This = Int } = ???
+
+//given (using ord: Ord, monoid: Monoid{ type This = ord.This }): (Ord & Monoid { type This = ord.This}) =
+//  ordWithMonoid2(ord, monoid)
+
+val x = summon[Ord & Monoid { type This = Int}]
+val y: Int = ??? : x.This
+
+// given [A, B](using ord: A is Ord, monoid: A is Monoid) => A is Ord & Monoid =
+//   new ord.OrdProxy with monoid.MonoidProxy {}
+
+given [A](using ord: Ord { type This = A }, monoid: Monoid { type This = A}): (Ord & Monoid) { type This = A} =
+  new ord.OrdProxy with monoid.MonoidProxy {}
+