Fix spurious redundant capability warnings

Author: odersky

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

@@ -936,21 +936,24 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
           if others.accountsFor(ref) then
             report.warning(em"redundant capture: $dom already accounts for $ref", pos)
 
-        if ref.captureSetOfInfo.elems.isEmpty
-            && !ref.coreType.derivesFrom(defn.Caps_Capability)
-            && !ref.coreType.derivesFrom(defn.Caps_CapSet) then
-          val deepStr = if ref.isReach then " deep" else ""
-          report.error(em"$ref cannot be tracked since its$deepStr capture set is empty", pos)
-        check(parent.captureSet, parent)
-
-        val others =
-          for
-            j <- 0 until retained.length if j != i
-            r = retained(j)
-            if !r.isTerminalCapability
-          yield r
-        val remaining = CaptureSet(others*)
-        check(remaining, remaining)
+        if !ref.coreType.derivesFrom(defn.Caps_Capability)
+            // Capability classes don't have their implied capture set yet, so
+            // they would be seen as pure
+            && !ref.coreType.derivesFrom(defn.Caps_CapSet)
+        then
+          if ref.captureSetOfInfo.elems.isEmpty then
+            val deepStr = if ref.isReach then " deep" else ""
+            report.error(em"$ref cannot be tracked since its$deepStr capture set is empty", pos)
+          check(parent.captureSet, parent)
+
+          val others =
+            for
+              j <- 0 until retained.length if j != i
+              r = retained(j)
+              if !r.isTerminalCapability
+            yield r
+          val remaining = CaptureSet(others*)
+          check(remaining, remaining)
       end for
     catch case ex: IllegalCaptureRef =>
       report.error(em"Illegal capture reference: ${ex.getMessage}", tpt.srcPos)

tests/neg-custom-args/captures/filevar.check

@@ -12,7 +12,3 @@
 18 |    o.log
    |
    | longer explanation available when compiling with `-explain`
--- Warning: tests/neg-custom-args/captures/filevar.scala:11:55 ---------------------------------------------------------
-11 |def withFile[T](op: (l: caps.Capability) ?-> (f: File^{l}) => T): T =
-   |                                                       ^
-   |                                        redundant capture: File already accounts for (l : scala.caps.Capability)

tests/neg-custom-args/captures/i15923.check

@@ -20,11 +20,3 @@
    |where:    => refers to a fresh root capability created in anonymous function of type (using lcap: scala.caps.Capability): Cap^{lcap} -> Id[Cap] when instantiating expected result type Cap^{lcap} ->{cap²} Id[Cap^'s13]^'s14 of function literal
    |
    | longer explanation available when compiling with `-explain`
--- Warning: tests/neg-custom-args/captures/i15923.scala:21:56 ----------------------------------------------------------
-21 |    def withCap[X](op: (lcap: caps.Capability) ?-> Cap^{lcap} => X): X = {
-   |                                                        ^^^^
-   |                                redundant capture: test2.Cap already accounts for (lcap : scala.caps.Capability)
--- Warning: tests/neg-custom-args/captures/i15923.scala:6:54 -----------------------------------------------------------
-6 |  def withCap[X](op: (lcap: caps.Capability) ?-> Cap^{lcap} => X): X = {
-  |                                                      ^^^^
-  |                                        redundant capture: Cap already accounts for (lcap : scala.caps.Capability)

tests/pos-custom-args/captures/no-redundant-capability.scala

@@ -0,0 +1,21 @@
+//> using options -Werror
+import caps.*
+
+trait State[A] extends SharedCapability:
+  def get: A
+  def set(a: A): Unit
+
+def get[A]: State[A] ?-> A          = s ?=> s.get
+def set[A](a: A): State[A] ?-> Unit = s ?=> s.set(a)
+
+trait Rand:
+  def range(min: Int, max: Int): Int
+
+object Rand:
+  def fromState: (s: State[Long]) ?-> Rand^{s} =
+    new Rand:
+      override def range(min: Int, max: Int): Int =
+        val seed             = get
+        val (nextSeed, next) = (seed + 1, seed.toInt) // obviously wrong, but not the point...
+        set(nextSeed)
+        next