Handle local type parameters in markFree

These need to be handled like reach capabilities.

Fixes #21347

Author: odersky

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

@@ -388,23 +388,25 @@ class CheckCaptures extends Recheck, SymTransformer:
           // should be included.
           val included = cs.filter: c =>
             c.stripReach match
-              case ref: TermRef =>
-                //if c.isReach then println(i"REACH $c in ${env.owner}")
-                //assert(!env.owner.isAnonymousFunction)
+              case ref: NamedType =>
                 val refSym = ref.symbol
                 val refOwner = refSym.owner
                 val isVisible = isVisibleFromEnv(refOwner)
-                if !isVisible && c.isReach && refSym.is(Param) && refOwner == env.owner then
-                if refSym.hasAnnotation(defn.UnboxAnnot) then
-                  capt.println(i"exempt: $ref in $refOwner")
-                else
-                  // Reach capabilities that go out of scope have to be approximated
-                  // by their underlying capture set, which cannot be universal.
-                  // Reach capabilities of @unboxed parameters are exempted.
-                  val cs = CaptureSet.ofInfo(c)
-                  cs.disallowRootCapability: () =>
-                    report.error(em"Local reach capability $c leaks into capture scope of ${env.ownerString}", pos)
-                  checkSubset(cs, env.captured, pos, provenance(env))
+                if !isVisible
+                    && (c.isReach || ref.isType)
+                    && refSym.is(Param)
+                    && refOwner == env.owner
+                then
+                  if refSym.hasAnnotation(defn.UnboxAnnot) then
+                    capt.println(i"exempt: $ref in $refOwner")
+                  else
+                    // Reach capabilities that go out of scope have to be approximated
+                    // by their underlying capture set, which cannot be universal.
+                    // Reach capabilities of @unboxed parameters are exempted.
+                    val cs = CaptureSet.ofInfo(c)
+                    cs.disallowRootCapability: () =>
+                      report.error(em"Local reach capability $c leaks into capture scope of ${env.ownerString}", pos)
+                    checkSubset(cs, env.captured, pos, provenance(env))
                 isVisible
               case ref: ThisType => isVisibleFromEnv(ref.cls)
               case _ => false

tests/neg-custom-args/captures/i21347.check

@@ -0,0 +1,15 @@
+-- Error: tests/neg-custom-args/captures/i21347.scala:4:15 -------------------------------------------------------------
+4 |  ops.foreach: op => // error
+  |               ^
+  |               Local reach capability C leaks into capture scope of method runOps
+5 |    op()
+-- Error: tests/neg-custom-args/captures/i21347.scala:8:14 -------------------------------------------------------------
+8 |  () => runOps(f :: Nil) // error
+  |        ^^^^^^^^^^^^^^^^
+  |        reference (caps.cap : caps.Capability) is not included in the allowed capture set {}
+  |        of an enclosing function literal with expected type () -> Unit
+-- Error: tests/neg-custom-args/captures/i21347.scala:11:15 ------------------------------------------------------------
+11 |  ops.foreach: op => // error
+   |               ^
+   |               Local reach capability ops* leaks into capture scope of method runOpsAlt
+12 |    op()

tests/neg-custom-args/captures/i21347.scala

@@ -0,0 +1,12 @@
+import language.experimental.captureChecking
+
+def runOps[C^](ops: List[() ->{C^} Unit]): Unit =
+  ops.foreach: op => // error
+    op()
+
+def boom(f: () => Unit): () -> Unit =
+  () => runOps(f :: Nil) // error
+
+def runOpsAlt(ops: List[() => Unit]): Unit =
+  ops.foreach: op => // error
+    op()