Disallow covariant caps in the lower bound of type members

Author: Linyxus

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

@@ -168,6 +168,13 @@ object CheckCaptures:
             if !seen.contains(t) then
               seen += t
               traverseChildren(t)
+
+              // Check the lower bound of path dependent types.
+              // See issue #19330.
+              val isTypeParam = t.prefix eq NoPrefix
+              t.info match
+                case TypeBounds(lo, hi) if !isTypeParam => traverse(lo)
+                case _ =>
           case AnnotatedType(_, ann) if ann.symbol == defn.UncheckedCapturesAnnot =>
             ()
           case t =>

tests/neg-custom-args/captures/i19330-alt.scala

@@ -0,0 +1,14 @@
+import language.experimental.captureChecking
+
+trait Logger
+def usingLogger[T](op: Logger^ => T): T = ???
+
+def foo[T >: () => Logger^](): T =
+  val leaked = usingLogger[T]: l =>  // ok
+    val t: () => Logger^ = () => l
+    t: T
+  leaked
+
+def test(): Unit =
+  val bad: () => Logger^ = foo[() => Logger^]  // error
+  val leaked: Logger^ = bad()  // leaked scoped capability!

tests/neg-custom-args/captures/i19330-alt2.scala

@@ -0,0 +1,13 @@
+import language.experimental.captureChecking
+
+trait Logger
+def usingLogger[T](op: Logger^ => T): T = ???
+
+trait Foo:
+  type T >: () => Logger^
+
+  def foo: this.T =
+    val leaked = usingLogger[T]: l =>  // error
+      val t: () => Logger^ = () => l
+      t: T
+    leaked

tests/neg-custom-args/captures/i19330.scala

@@ -0,0 +1,21 @@
+import language.experimental.captureChecking
+
+trait Logger
+def usingLogger[T](op: Logger^ => T): T = ???
+
+trait Foo:
+  type T >: () => Logger^
+
+class Bar extends Foo:
+  type T = () => Logger^
+
+def foo(x: Foo): x.T =
+  val leaked = usingLogger[x.T]: l =>  // error
+    val t: () => Logger^ = () => l
+    t: x.T
+  leaked
+
+def test(): Unit =
+  val bar = new Bar
+  val bad: bar.T = foo(bar)
+  val leaked: Logger^ = bad()  // leaked scoped capability!