Two fixes to capture checking (scala#17524)

- It changes `isParamDependent` to consider `CaptureDependency` as well,
so that when applying a capture dependent function the capture sets in
the types of the remaining parameters will be correctly substituted. For
example:
```scala
def foo[X](x: Foo[X]^, op: () ->{x} Unit): Unit = ???
foo(a, useA)
```
After rechecking `a` in the argument list, the expected type of the
second argument, `useA`, should become `() -> {a} Unit`. This example
previously does not work.

- It fixes scala#17517. It fixes the healing of capture sets, which happens
at the end of the capture checking phase, in which we traverse the
capture checked tree and try to heal the ill-formed capture sets in type
arguments. Here we say a capture set `C` in a type argument is
ill-formed if it contains `TermParamRef` bound by other lambdas. For
example:
```scala
def usingLogger[sealed T](f: OutputStream^)(op: Logger^{f} => T): T = ???
usingLogger[() ->?1 Unit](file)(l => () => l.log("test"))
```
`l` will be propagated to `?1` as a result of capture checking, which
makes `?1` ill-formed and we should widen `l` to `file`. The problem is
that we heal the capture sets one-after-another but the healing of a
later capture set may propagate more `TermParamRef`s to a healed one.
For example:
```
usingFile[() ->?2 Unit](  // should be error
  "foo",
  file => {
    usingLogger[() ->?1 Unit](file)(l => () => l.log("test"))
  }
)
```
After capture checking both `?1` and `?2` will be `{l}`. When traversing
the tree we first heal `?2`, wherein we widen `l` to `file`, but `file`
is a `TermRef` here and we do nothing. Then, only later when we heal
`?1`, we also widen `l` to `file` but this will end up propagating a
`TermParamRef` of `file` to `?2`, which we should have widened as well.
But `?2` is already healed and is not inspected again.

This PR solves this issue by installing a handler when healing the
capture sets which will try to heal the new elements propagated later in
the healing process.

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

@@ -275,6 +275,11 @@ sealed abstract class CaptureSet extends Showable:
     if isUniversal then handler()
     this
 
+  /** Invoke handler on the elements to check wellformedness of the capture set */
+  def ensureWellformed(handler: List[CaptureRef] => Context ?=> Unit)(using Context): this.type =
+    handler(elems.toList)
+    this
+
   /** An upper approximation of this capture set, i.e. a constant set that is
    *  subcaptured by this set. If the current set is a variable
    *  it is the intersection of all upper approximations of known supersets
@@ -375,6 +380,9 @@ object CaptureSet:
     /** A handler to be invoked if the root reference `cap` is added to this set */
     var rootAddedHandler: () => Context ?=> Unit = () => ()
 
+    /** A handler to be invoked when new elems are added to this set */
+    var newElemAddedHandler: List[CaptureRef] => Context ?=> Unit = _ => ()
+
     var description: String = ""
 
     /** Record current elements in given VarState provided it does not yet
@@ -405,7 +413,8 @@ object CaptureSet:
       if !isConst && recordElemsState() then
         elems ++= newElems
         if isUniversal then rootAddedHandler()
-        // assert(id != 2 || elems.size != 2, this)
+        newElemAddedHandler(newElems.toList)
+        // assert(id != 5 || elems.size != 3, this)
         (CompareResult.OK /: deps) { (r, dep) =>
           r.andAlso(dep.tryInclude(newElems, this))
         }
@@ -425,6 +434,10 @@ object CaptureSet:
       rootAddedHandler = handler
       super.disallowRootCapability(handler)
 
+    override def ensureWellformed(handler: List[CaptureRef] => (Context) ?=> Unit)(using Context): this.type =
+      newElemAddedHandler = handler
+      super.ensureWellformed(handler)
+
     private var computingApprox = false
 
     /** Roughly: the intersection of all constant known supersets of this set.

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

@@ -976,8 +976,11 @@ class CheckCaptures extends Recheck, SymTransformer:
           recur(refs, Nil)
 
         private def healCaptureSet(cs: CaptureSet): Unit =
-          val toInclude = widenParamRefs(cs.elems.toList.filter(!isAllowed(_)).asInstanceOf)
-          toInclude.foreach(checkSubset(_, cs, tree.srcPos))
+          def avoidance(elems: List[CaptureRef])(using Context): Unit =
+            val toInclude = widenParamRefs(elems.filter(!isAllowed(_)).asInstanceOf)
+            //println(i"HEAL $cs by widening to $toInclude")
+            toInclude.foreach(checkSubset(_, cs, tree.srcPos))
+          cs.ensureWellformed(avoidance)
 
         private var allowed: SimpleIdentitySet[TermParamRef] = SimpleIdentitySet.empty
 

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -3880,7 +3880,8 @@ object Types {
     /** Does one of the parameter types contain references to earlier parameters
      *  of this method type which cannot be eliminated by de-aliasing?
      */
-    def isParamDependent(using Context): Boolean = paramDependencyStatus == TrueDeps
+    def isParamDependent(using Context): Boolean =
+      paramDependencyStatus == TrueDeps || paramDependencyStatus == CaptureDeps
 
     /** Is there a dependency involving a reference in a capture set, but
      *  otherwise no true result dependency?

tests/neg-custom-args/captures/usingLogFile-alt.check

@@ -0,0 +1,7 @@
+-- Error: tests/neg-custom-args/captures/usingLogFile-alt.scala:18:2 ---------------------------------------------------
+18 |  usingFile(  // error
+   |  ^^^^^^^^^
+   |  Sealed type variable T cannot  be instantiated to box () => Unit since
+   |  that type captures the root capability `cap`.
+   |  This is often caused by a local capability in the body of method usingFile
+   |  leaking as part of its result.

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

@@ -0,0 +1,23 @@
+// Reported in issue #17517
+
+import language.experimental.captureChecking
+import java.io.*
+
+object Test:
+  class Logger(f: OutputStream^):
+    def log(msg: String): Unit = ???
+
+  def usingFile[sealed T](name: String, op: OutputStream^ => T): T =
+    val f = new FileOutputStream(name)
+    val result = op(f)
+    f.close()
+    result
+
+  def usingLogger[sealed T](f: OutputStream^)(op: Logger^{f} => T): T = ???
+
+  usingFile(  // error
+    "foo",
+    file => {
+      usingLogger(file)(l => () => l.log("test"))
+    }
+  )

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

@@ -45,10 +45,3 @@
    |                that type captures the root capability `cap`.
    |                This is often caused by a local capability in the body of method usingFile
    |                leaking as part of its result.
--- Error: tests/neg-custom-args/captures/usingLogFile.scala:72:6 -------------------------------------------------------
-72 |      usingLogger(_, l => () => l.log("test"))) // error
-   |      ^^^^^^^^^^^
-   |      Sealed type variable T cannot  be instantiated to box () => Unit since
-   |      that type captures the root capability `cap`.
-   |      This is often caused by a local capability in the body of method usingLogger
-   |      leaking as part of its result.

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

@@ -69,5 +69,5 @@ object Test4:
 
   def test =
     val later = usingFile("logfile",            // error
-      usingLogger(_, l => () => l.log("test"))) // error
+      usingLogger(_, l => () => l.log("test"))) // ok, since we can widen `l` to `file` instead of to `cap`
     later()

tests/pos-custom-args/captures/cc-dep-param.scala

@@ -0,0 +1,8 @@
+import language.experimental.captureChecking
+
+trait Foo[T]
+def test(): Unit =
+  val a: Foo[Int]^ = ???
+  val useA: () ->{a} Unit = ???
+  def foo[X](x: Foo[X]^, op: () ->{x} Unit): Unit = ???
+  foo(a, useA)