Document toResultInResults and add test cases

Author: Linyxus

compiler/src/dotty/tools/dotc/cc/Capability.scala

@@ -851,7 +851,8 @@ object Capabilities:
   end toResult
 
   /** Map global roots in function results to result roots. Also,
-   *  map roots in the types of parameterless def methods.
+   *  map roots in the types of def methods that are parameterless
+   *  or have only type parameters.
    */
   def toResultInResults(sym: Symbol, fail: Message => Unit, keepAliases: Boolean = false)(tp: Type)(using Context): Type =
     val m = new TypeMap with FollowAliasesMap:
@@ -880,8 +881,17 @@ object Capabilities:
             throw ex
     m(tp) match
       case tp1: ExprType if sym.is(Method, butNot = Accessor) =>
+        // Map the result of parameterless `def` methods.
         tp1.derivedExprType(toResult(tp1.resType, tp1, fail))
       case tp1: PolyType if !tp1.resType.isInstanceOf[MethodicType] =>
+        // Map also the result type of method with only type parameters.
+        // This way, the `^` in the following method will be mapped to a `ResultCap`:
+        // ```
+        // object Buffer:
+        //   def empty[T]: Buffer[T]^
+        // ```
+        // This is more desirable than interpreting `^` as a `Fresh` at the level of `Buffer.empty`
+        // in most cases.
         tp1.derivedLambdaType(resType = toResult(tp1.resType, tp1, fail))
       case tp1 => tp1
   end toResultInResults

tests/neg-custom-args/captures/cc-fresh-levels.check

@@ -0,0 +1,19 @@
+Flag -source set repeatedly
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/cc-fresh-levels.scala:14:10 ------------------------------
+14 |    r.put(x)  // error
+   |          ^
+   |          Found:    IO^{x}
+   |          Required: IO^
+   |
+   |          where:    ^ refers to a fresh root capability in the type of value r
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/cc-fresh-levels.scala:17:10 ------------------------------
+17 |    r.put(innerIO)  // error
+   |          ^^^^^^^
+   |          Found:    IO^{innerIO}
+   |          Required: IO^
+   |
+   |          where:    ^ refers to a fresh root capability in the type of value r
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/cc-fresh-levels.scala

@@ -0,0 +1,19 @@
+//> using options -source 3.7
+import language.experimental.captureChecking
+import caps.*
+class IO
+class Ref[X](init: X):
+  private var _data = init
+  def get: X = _data
+  def put(y: X): Unit = _data = y
+def runIO(op: IO^ => Unit): Unit = ()
+def test1(a: IO^, b: IO^, c: IO^): Unit =
+  val r: Ref[IO^] = Ref(a)
+  r.put(b) // ok
+  def outer(x: IO^): Unit =
+    r.put(x)  // error
+  r.put(c) // ok
+  runIO: (innerIO: IO^) =>
+    r.put(innerIO)  // error
+  runIO: innerIO =>
+    r.put(innerIO)  // should be error, but ok // unsound

tests/pos-custom-args/captures/cc-def-fresh.scala

@@ -0,0 +1,10 @@
+import language.experimental.captureChecking
+trait Collection[T]
+trait IO
+def empty[T]: Collection[T]^ = ???
+def emptyAlt[T](): Collection[T]^ = ???
+def newIO: IO^ = ???
+def test1(): Unit =
+  val t1: Collection[Int]^ = empty[Int]  // ok
+  val t2: IO^ = newIO  // ok
+  val t3: Collection[Int]^ = emptyAlt[Int]() // ok

tests/pos-custom-args/captures/i23421.scala

@@ -0,0 +1,16 @@
+import language.experimental.captureChecking
+import caps.*
+
+trait Collection[T] extends Mutable:
+  update def add(elem: T): Unit
+  update def remove(elem: T): Unit
+  def get(index: Int): Option[T]
+
+object Collection:
+  def empty[T]: Collection[T] = ???
+
+trait Foo:
+  val thunks: Collection[() => Unit] // that's fine
+
+object FooImpl1 extends Foo:
+  val thunks: Collection[() => Unit] = Collection.empty // was error, now ok