Change handling of curried function types in capture checking (#18131)

Implements the proposed treatment in cc-paths. 

- Capture sets of curried functions don't automatically propagate to the
left
- When computing the capture set of a reference, pick up capture sets
along the span of curried functions
- Drop the previous syntactic sugar for propagating captures left to
right in curried function types.

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

@@ -210,10 +210,11 @@ sealed abstract class CaptureSet extends Showable:
    *  any of the elements in the constant capture set `that`
    */
   def -- (that: CaptureSet.Const)(using Context): CaptureSet =
-    val elems1 = elems.filter(!that.accountsFor(_))
-    if elems1.size == elems.size then this
-    else if this.isConst then Const(elems1)
-    else Diff(asVar, that)
+    if this.isConst then
+      val elems1 = elems.filter(!that.accountsFor(_))
+      if elems1.size == elems.size then this else Const(elems1)
+    else
+      if that.isAlwaysEmpty then this else Diff(asVar, that)
 
   /** The largest subset (via <:<) of this capture set that does not account for `ref` */
   def - (ref: CaptureRef)(using Context): CaptureSet =
@@ -845,34 +846,45 @@ object CaptureSet:
   /** The capture set of the type underlying a CaptureRef */
   def ofInfo(ref: CaptureRef)(using Context): CaptureSet = ref match
     case ref: TermRef if ref.isRootCapability => ref.singletonCaptureSet
-    case _ => ofType(ref.underlying)
+    case _ => ofType(ref.underlying, followResult = true)
 
   /** Capture set of a type */
-  def ofType(tp: Type)(using Context): CaptureSet =
-    def recur(tp: Type): CaptureSet = tp.dealias match
-      case tp: TermRef =>
-        tp.captureSet
-      case tp: TermParamRef =>
-        tp.captureSet
-      case _: TypeRef =>
-        if tp.classSymbol.hasAnnotation(defn.CapabilityAnnot) then universal else empty
-      case _: TypeParamRef =>
-        empty
-      case CapturingType(parent, refs) =>
-        recur(parent) ++ refs
-      case AppliedType(tycon, args) =>
-        val cs = recur(tycon)
-        tycon.typeParams match
-          case tparams @ (LambdaParam(tl, _) :: _) => cs.substParams(tl, args)
-          case _ => cs
-      case tp: TypeProxy =>
-        recur(tp.underlying)
-      case AndType(tp1, tp2) =>
-        recur(tp1) ** recur(tp2)
-      case OrType(tp1, tp2) =>
-        recur(tp1) ++ recur(tp2)
-      case _ =>
-        empty
+  def ofType(tp: Type, followResult: Boolean)(using Context): CaptureSet =
+    def recur(tp: Type): CaptureSet = trace(i"ofType $tp, ${tp.getClass} $followResult", show = true):
+      tp.dealias match
+        case tp: TermRef =>
+          tp.captureSet
+        case tp: TermParamRef =>
+          tp.captureSet
+        case _: TypeRef =>
+          if tp.classSymbol.hasAnnotation(defn.CapabilityAnnot) then universal else empty
+        case _: TypeParamRef =>
+          empty
+        case CapturingType(parent, refs) =>
+          recur(parent) ++ refs
+        case tpd @ RefinedType(parent, _, rinfo: MethodType)
+        if followResult && defn.isFunctionType(tpd) =>
+          ofType(parent, followResult = false)                 // pick up capture set from parent type
+          ++ (recur(rinfo.resType)                             // add capture set of result
+          -- CaptureSet(rinfo.paramRefs.filter(_.isTracked)*)) // but disregard bound parameters
+        case tpd @ AppliedType(tycon, args) =>
+          if followResult && defn.isNonRefinedFunction(tpd) then
+            recur(args.last)
+              // must be (pure) FunctionN type since ImpureFunctions have already
+              // been eliminated in selector's dealias. Use capture set of result.
+          else
+            val cs = recur(tycon)
+            tycon.typeParams match
+              case tparams @ (LambdaParam(tl, _) :: _) => cs.substParams(tl, args)
+              case _ => cs
+        case tp: TypeProxy =>
+          recur(tp.underlying)
+        case AndType(tp1, tp2) =>
+          recur(tp1) ** recur(tp2)
+        case OrType(tp1, tp2) =>
+          recur(tp1) ++ recur(tp2)
+        case _ =>
+          empty
     recur(tp)
       .showing(i"capture set of $tp = $result", capt)