Refine rule for this widening

We now widen the expected type of the right hand side of a class member as follows: Add all references of the declared type of this that are not subsumed by a capture set of a parameter type.
scala · Oct 2, 2021 · c568038 · c568038
1 parent ca932ca
commit c568038
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Showing 4 changed files with 18 additions and 6 deletions.
diff --git a/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala b/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
@@ -49,7 +49,14 @@ sealed abstract class CaptureSet extends Showable:
   /** Is this capture set definitely non-empty? */
   final def isNotEmpty: Boolean = !elems.isEmpty
 
-  /** Cast to variable. @pre: @isConst */
+  /** Cast to Const. @pre: isConst */
+  def asConst: Const = this match
+    case c: Const => c
+    case v: Var =>
+      assert(v.isConst)
+      Const(v.elems)
+
+  /** Cast to variable. @pre: !isConst */
   def asVar: Var =
     assert(!isConst)
     asInstanceOf[Var]

diff --git a/compiler/src/dotty/tools/dotc/typer/CheckCaptures.scala b/compiler/src/dotty/tools/dotc/typer/CheckCaptures.scala
@@ -325,10 +325,14 @@ class CheckCaptures extends Recheck:
     override def recheckRHS(tree: Tree, pt: Type, sym: Symbol)(using Context): Type =
       val pt1 = pt match
         case CapturingType(core, refs, _)
-        if sym.owner.isClass
-            && refs.elems.contains(sym.owner.thisType)
-            && sym.paramSymss.forall(_.forall(p => p.isType || p.info.captureSet.isAlwaysEmpty)) =>
-          pt.derivedCapturingType(core, refs ++ sym.owner.asClass.givenSelfType.captureSet)
+        if sym.owner.isClass && refs.elems.contains(sym.owner.thisType) =>
+          val paramCaptures =
+            sym.paramSymss.flatten.foldLeft(CaptureSet.empty) { (cs, p) =>
+              val pcs = p.info.captureSet
+              (cs ++ (if pcs.isConst then pcs else CaptureSet.universal)).asConst
+            }
+          val declaredCaptures = sym.owner.asClass.givenSelfType.captureSet
+          pt.derivedCapturingType(core, refs ++ (declaredCaptures -- paramCaptures))
         case _ =>
           pt
       recheck(tree, pt1)

diff --git a/tests/neg-custom-args/captures/lazylists1.check b/tests/neg-custom-args/captures/lazylists1.check
@@ -2,6 +2,6 @@
 25 |      def concat(other: {f} LazyList[A]): {this} LazyList[A] = ??? : ({xs, f} LazyList[A]) // error
    |                                                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |                                                               Found:    {xs, f} LazyList[A]
-   |                                                               Required: {Mapped.this} LazyList[A]
+   |                                                               Required: {Mapped.this, xs} LazyList[A]
 
 longer explanation available when compiling with `-explain`
diff --git a/tests/pos-custom-args/captures/lazylists.scala b/tests/pos-custom-args/captures/lazylists.scala
@@ -21,6 +21,7 @@ extension [A](xs: {*} LazyList[A])
       def isEmpty = false
       def head: B = f(xs.head)
       def tail: {this} LazyList[B] = xs.tail.map(f)  // OK
+      def concat(other: {f} LazyList[A]): {this, f} LazyList[A] = ??? : ({xs, f} LazyList[A]) // OK
     new Mapped
 
 def test(cap1: Cap, cap2: Cap) =