Interpolate type variables when symbols are completed

Author: odersky

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

@@ -173,6 +173,15 @@ sealed abstract class CaptureSet extends Showable:
   def substParams(tl: BindingType, to: List[Type])(using Context) =
     map(Substituters.SubstParamsMap(tl, to))
 
+  /** An upper approximation of this capture set. This is the set itself
+   *  except for real (non-mapped, non-filtered) capture set variables, where
+   *  it is the intersection of all upper approximations of known supersets
+   *  of the variable.
+   *  The upper approximation is meaningful only if it is constant. If not,
+   *  `upperApprox` can return an arbitrary capture set variable.
+   */
+  def upperApprox(using Context): CaptureSet
+
   def toRetainsTypeArg(using Context): Type =
     assert(isConst)
     ((NoType: Type) /: elems) ((tp, ref) =>
@@ -221,6 +230,8 @@ object CaptureSet:
 
     def addSuper(cs: CaptureSet)(using Context, VarState) = CompareResult.OK
 
+    def upperApprox(using Context): CaptureSet = this
+
     override def toString = elems.toString
   end Const
 
@@ -265,14 +276,31 @@ object CaptureSet:
         CompareResult.fail(this)
 
     def addSuper(cs: CaptureSet)(using Context, VarState): CompareResult =
-      if (cs eq this) || cs.elems.contains(defn.captureRoot.termRef) then
+      if (cs eq this) || cs.elems.contains(defn.captureRoot.termRef) || isConst then
         CompareResult.OK
       else if recordDepsState() then
         deps += cs
         CompareResult.OK
       else
         CompareResult.fail(this)
 
+    def upperApprox(using Context): CaptureSet =
+      if isConst then this
+      else (universal /: deps) { (acc, sup) =>
+        if acc.isConst then
+          val supApprox = sup.upperApprox
+          if supApprox.isConst then acc ** supApprox else supApprox
+        else acc
+      }
+
+    def solve(variance: Int)(using Context): Unit =
+      if variance > 0 then isSolved = true
+      else if variance < 0 then
+        val approx = upperApprox
+        if approx.isConst then
+          elems = approx.elems
+          isSolved = true
+
     override def toString = s"Var$id$elems"
   end Var
 
@@ -307,6 +335,8 @@ object CaptureSet:
         else CompareResult.fail(this)
       else result
 
+    override def upperApprox(using Context): CaptureSet = this
+
     override def toString = s"Mapped$id($cv, elems = $elems)"
   end Mapped
 
@@ -323,6 +353,8 @@ object CaptureSet:
             .showing(i"propagating new elems $newElems backward from $this to $cv", capt)
           else r
 
+    override def upperApprox(using Context): CaptureSet = this
+
     override def toString = s"BiMapped$id($cv, elems = $elems)"
   end BiMapped
 
@@ -334,6 +366,8 @@ object CaptureSet:
     override def addNewElems(newElems: Refs, origin: CaptureSet)(using Context, VarState): CompareResult =
       super.addNewElems(newElems.filter(p), origin)
 
+    override def upperApprox(using Context): CaptureSet = this
+
     override def toString = s"${getClass.getSimpleName}$id($cv, elems = $elems)"
   end Filtered
 

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

@@ -203,6 +203,14 @@ class CheckCaptures extends Recheck:
         tp
     end transformType
 
+    def interpolateVars(using Context) = new TypeTraverser:
+      override def traverse(t: Type) =
+        t match
+          case CapturingType(_, refs: CaptureSet.Var, _) if !refs.isConst =>
+          	refs.solve(variance)
+          case _ =>
+        traverseChildren(t)
+
     private var curEnv: Env = Env(NoSymbol, CaptureSet.empty, false, null)
 
     private val myCapturedVars: util.EqHashMap[Symbol, CaptureSet] = EqHashMap()
@@ -268,14 +276,17 @@ class CheckCaptures extends Recheck:
       super.recheckIdent(tree)
 
     override def recheckValDef(tree: ValDef, sym: Symbol)(using Context): Unit =
-      super.recheckValDef(tree, sym)
+      try super.recheckValDef(tree, sym)
+      finally interpolateVars.traverse(sym.info)
 
     override def recheckDefDef(tree: DefDef, sym: Symbol)(using Context): Unit =
       val saved = curEnv
       val localSet = capturedVars(sym)
       if !localSet.isAlwaysEmpty then curEnv = Env(sym, localSet, false, curEnv)
       try super.recheckDefDef(tree, sym)
-      finally curEnv = saved
+      finally
+        interpolateVars.traverse(sym.info)
+        curEnv = saved
 
     override def recheckClassDef(tree: TypeDef, impl: Template, cls: ClassSymbol)(using Context): Type =
       for param <- cls.paramGetters do

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

@@ -1,7 +1,7 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/boxmap.scala:14:2 ----------------------------------------
 14 |  () => b[Box[B]]((x: A) => box(f(x)))  // error
    |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |  Found:    {f} () => ? Box[B]
+   |  Found:    {f} () => Box[B]
    |  Required: () => Box[B]
 
 longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/capt1.check

@@ -1,14 +1,14 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/capt1.scala:3:2 ------------------------------------------
 3 |  () => if x == null then y else y  // error
   |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-  |  Found:    {x} () => ? C
+  |  Found:    {x} () => C
   |  Required: () => C
 
 longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/capt1.scala:6:2 ------------------------------------------
 6 |  () => if x == null then y else y  // error
   |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-  |  Found:    {x} () => ? C
+  |  Found:    {x} () => C
   |  Required: Matchable
 
 longer explanation available when compiling with `-explain`
@@ -40,7 +40,7 @@ longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/capt1.scala:31:24 ----------------------------------------
 31 |  val z2 = h[() => Cap](() => x)(() => C())  // error
    |                        ^^^^^^^
-   |                        Found:    {x} () => ? Cap
+   |                        Found:    {x} () => Cap
    |                        Required: () => Cap
 
 longer explanation available when compiling with `-explain`

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

@@ -1,7 +1,7 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/try.scala:28:43 ------------------------------------------
 28 |  val b = handle[Exception, () => Nothing] {  // error
    |                                           ^
-   |                                           Found:    ? (x: CanThrow[Exception]) => {x} () => ? Nothing
+   |                                           Found:    ? (x: CanThrow[Exception]) => {x} () => Nothing
    |                                           Required: CanThrow[Exception] => () => Nothing
 29 |    (x: CanThrow[Exception]) => () => raise(new Exception)(using x)
 30 |  } {