Special rule for {this} in capture sets of class members

Consider the lazylists.scala test in pos-custom-args/captures:
```scala
class CC
type Cap = {*} CC

trait LazyList[+A]:
  this: ({*} LazyList[A]) =>

  def isEmpty: Boolean
  def head: A
  def tail: {this} LazyList[A]

object LazyNil extends LazyList[Nothing]:
  def isEmpty: Boolean = true
  def head = ???
  def tail = ???

extension [A](xs: {*} LazyList[A])
  def map[B](f: {*} A => B): {xs, f} LazyList[B] =
    class Mapped extends LazyList[B]:
      this: ({xs, f} Mapped) =>

      def isEmpty = false
      def head: B = f(xs.head)
      def tail: {this} LazyList[B] = xs.tail.map(f)  // OK
    new Mapped
```
Without this commit, the second to last line is an error since the right hand side
has capture set `{xs, f}` but the required capture set is `this`.

To fix this, we widen the expected type of the rhs `xs.tail.map(f)` from `{this}` to
`{this, f, xs}`. That is, we add the declared captures of the self type to the expected
type. The soundness argument for doing this is as follows:

Since `tail` does not have parameters, the only thing it could capture are references that the
receiver `this` captures as well. So `xs` and `f` must come via `this`. For instance, if
the receiver `xs` of `xs.tail` happens to be pure, then `xs.tail` is pure as well.

On the other hand, in the neg test `lazylists1.scala` we add the following line to `Mapped`:
```scala
      def concat(other: {f} LazyList[A]): {this} LazyList[A] = ??? : ({xs, f} LazyList[A]) // error
```
Here, we cannot widen the expected type from `{this}` to `{this, xs, f}` since the result of concat
refers to `f` independently of `this`, namely through its parameter `other`. Hence, if `ys: {f} LazyList[String]`
then
```
   LazyNil.concat(ys)
```
still refers to `f` even though `LazyNil` is pure. But if we would accept the definition of `concat`
above, the type of `LazyNil.concat(ys)` would be `LazyList[String]`, which is unsound.

The current implementation widens the expected type of class members if the class member does not
have tracked parameters. We could potentially refine this to say we widen with all references in
the expected type that are not subsumed by one of the parameter types.

Author: odersky

compiler/src/dotty/tools/dotc/transform/Recheck.scala

@@ -196,12 +196,15 @@ abstract class Recheck extends Phase, IdentityDenotTransformer:
         bindType
 
     def recheckValDef(tree: ValDef, sym: Symbol)(using Context): Unit =
-      if !tree.rhs.isEmpty then recheck(tree.rhs, sym.info)
+      if !tree.rhs.isEmpty then recheckRHS(tree.rhs, sym.info, sym)
 
     def recheckDefDef(tree: DefDef, sym: Symbol)(using Context): Unit =
       val rhsCtx = linkConstructorParams(sym).withOwner(sym)
       if !tree.rhs.isEmpty && !sym.isInlineMethod && !sym.isEffectivelyErased then
-        inContext(rhsCtx) { recheck(tree.rhs, recheck(tree.tpt)) }
+        inContext(rhsCtx) { recheckRHS(tree.rhs, recheck(tree.tpt), sym) }
+
+    def recheckRHS(tree: Tree, pt: Type, sym: Symbol)(using Context): Type =
+      recheck(tree, pt)
 
     def recheckTypeDef(tree: TypeDef, sym: Symbol)(using Context): Type =
       recheck(tree.rhs)

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

@@ -322,6 +322,17 @@ class CheckCaptures extends Recheck:
         interpolateVarsIn(tree.tpt)
         curEnv = saved
 
+    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)
+        case _ =>
+          pt
+      recheck(tree, pt1)
+
     override def recheckClassDef(tree: TypeDef, impl: Template, cls: ClassSymbol)(using Context): Type =
       for param <- cls.paramGetters do
         if param.is(Private) && !param.info.captureSet.isAlwaysEmpty then

tests/neg-custom-args/captures/lazylists1.check

@@ -0,0 +1,7 @@
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/lazylists1.scala:25:63 -----------------------------------
+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]
+
+longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/lazylists1.scala

@@ -0,0 +1,27 @@
+class CC
+type Cap = {*} CC
+
+trait LazyList[+A]:
+  this: ({*} LazyList[A]) =>
+
+  def isEmpty: Boolean
+  def head: A
+  def tail: {this} LazyList[A]
+
+object LazyNil extends LazyList[Nothing]:
+  def isEmpty: Boolean = true
+  def head = ???
+  def tail = ???
+
+extension [A](xs: {*} LazyList[A])
+  def map[B](f: {*} A => B): {xs, f} LazyList[B] =
+    class Mapped extends LazyList[B]:
+      this: ({xs, f} Mapped) =>
+
+      def isEmpty = false
+      def head: B = f(xs.head)
+      def tail: {this} LazyList[B] = xs.tail.map(f)  // OK
+      def drop(n: Int): {this} LazyList[B] = ??? : ({xs, f} LazyList[B]) // OK
+      def concat(other: {f} LazyList[A]): {this} LazyList[A] = ??? : ({xs, f} LazyList[A]) // error
+    new Mapped
+

tests/neg-custom-args/captures/lazylists2.check

@@ -0,0 +1,38 @@
+-- [E163] Declaration Error: tests/neg-custom-args/captures/lazylists2.scala:50:10 -------------------------------------
+50 |      def tail: {xs, f} LazyList[B] = xs.tail.map(f) // error
+   |          ^
+   |          error overriding method tail in trait LazyList of type => {Mapped.this} LazyList[B];
+   |            method tail of type => {xs, f} LazyList[B] has incompatible type
+
+longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/lazylists2.scala:18:4 ------------------------------------
+18 |    class Mapped extends LazyList[B]:  // error
+   |    ^
+   |    Found:    {f, xs} LazyList[B]
+   |    Required: {f} LazyList[B]
+19 |      this: ({xs, f} Mapped) =>
+20 |      def isEmpty = false
+21 |      def head: B = f(xs.head)
+22 |      def tail: {this} LazyList[B] = xs.tail.map(f)
+23 |    new Mapped
+
+longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/lazylists2.scala:27:4 ------------------------------------
+27 |    class Mapped extends LazyList[B]:  // error
+   |    ^
+   |    Found:    {f, xs} LazyList[B]
+   |    Required: {xs} LazyList[B]
+28 |      this: ({xs, f} Mapped) =>
+29 |      def isEmpty = false
+30 |      def head: B = f(xs.head)
+31 |      def tail: {this} LazyList[B] = xs.tail.map(f)
+32 |    new Mapped
+
+longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/lazylists2.scala:41:48 -----------------------------------
+41 |      def tail: {this} LazyList[B] = xs.tail.map(f) // error
+   |                                     ^^^^^^^^^^^^^^
+   |                                     Found:    {f} LazyList[B]
+   |                                     Required: {xs} LazyList[B]
+
+longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/lazylists2.scala

@@ -0,0 +1,52 @@
+class CC
+type Cap = {*} CC
+
+trait LazyList[+A]:
+  this: ({*} LazyList[A]) =>
+
+  def isEmpty: Boolean
+  def head: A
+  def tail: {this} LazyList[A]
+
+object LazyNil extends LazyList[Nothing]:
+  def isEmpty: Boolean = true
+  def head = ???
+  def tail = ???
+
+extension [A](xs: {*} LazyList[A])
+  def map[B](f: {*} A => B): {f} LazyList[B] =
+    class Mapped extends LazyList[B]:  // error
+      this: ({xs, f} Mapped) =>
+
+      def isEmpty = false
+      def head: B = f(xs.head)
+      def tail: {this} LazyList[B] = xs.tail.map(f)
+    new Mapped
+
+  def map2[B](f: {*} A => B): {xs} LazyList[B] =
+    class Mapped extends LazyList[B]:  // error
+      this: ({xs, f} Mapped) =>
+
+      def isEmpty = false
+      def head: B = f(xs.head)
+      def tail: {this} LazyList[B] = xs.tail.map(f)
+    new Mapped
+
+  def map3[B](f: {*} A => B): {xs} LazyList[B] =
+    class Mapped extends LazyList[B]:
+      this: ({xs} Mapped) =>
+
+      def isEmpty = false
+      def head: B = f(xs.head)
+      def tail: {this} LazyList[B] = xs.tail.map(f) // error
+    new Mapped
+
+  def map4[B](f: {*} A => B): {xs} LazyList[B] =
+    class Mapped extends LazyList[B]:
+      this: ({xs, f} Mapped) =>
+
+      def isEmpty = false
+      def head: B = f(xs.head)
+      def tail: {xs, f} LazyList[B] = xs.tail.map(f) // error
+    new Mapped
+

tests/pos-custom-args/captures/lazylists-mono.scala

@@ -0,0 +1,27 @@
+class CC
+type Cap = {*} CC
+
+//-------------------------------------------------
+
+def test(E: Cap) =
+
+  trait LazyList[+A]:
+    protected def contents: {E} () => (A, {E} LazyList[A])
+    def isEmpty: Boolean
+    def head: A = contents()._1
+    def tail: {E} LazyList[A] = contents()._2
+
+  class LazyCons[+A](override val contents: {E} () => (A, {E} LazyList[A]))
+  extends LazyList[A]:
+    def isEmpty: Boolean = false
+
+  object LazyNil extends LazyList[Nothing]:
+    def contents: {E} () => (Nothing, LazyList[Nothing]) = ???
+    def isEmpty: Boolean = true
+
+  extension [A](xs: {E} LazyList[A])
+    def map[B](f: {E} A => B): {E} LazyList[B] =
+      if xs.isEmpty then LazyNil
+      else
+        val cons = () => (f(xs.head), xs.tail.map(f))
+        LazyCons(cons)

tests/pos-custom-args/captures/lazylists.scala

@@ -0,0 +1,37 @@
+class CC
+type Cap = {*} CC
+
+trait LazyList[+A]:
+  this: ({*} LazyList[A]) =>
+
+  def isEmpty: Boolean
+  def head: A
+  def tail: {this} LazyList[A]
+
+object LazyNil extends LazyList[Nothing]:
+  def isEmpty: Boolean = true
+  def head = ???
+  def tail = ???
+
+extension [A](xs: {*} LazyList[A])
+  def map[B](f: {*} A => B): {xs, f} LazyList[B] =
+    class Mapped extends LazyList[B]:
+      this: ({xs, f} Mapped) =>
+
+      def isEmpty = false
+      def head: B = f(xs.head)
+      def tail: {this} LazyList[B] = xs.tail.map(f)  // OK
+    new Mapped
+
+def test(cap1: Cap, cap2: Cap) =
+  def f(x: String): String = if cap1 == cap1 then "" else "a"
+  def g(x: String): String = if cap2 == cap2 then "" else "a"
+
+  val xs =
+    class Initial extends LazyList[String]:
+      this: ({cap1} Initial) =>
+
+      def isEmpty = false
+      def head = f("")
+      def tail = LazyNil
+    new Initial