Do expected type widening only in final classes

Alex found a counter-example why this is required. See map5 in
neg-customargs/captures/lazylists2.scala

Author: odersky

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

@@ -325,7 +325,8 @@ 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) =>
+        if sym.owner.isClass && !sym.owner.isExtensibleClass
+            && refs.elems.contains(sym.owner.thisType) =>
           val paramCaptures =
             sym.paramSymss.flatten.foldLeft(CaptureSet.empty) { (cs, p) =>
               val pcs = p.info.captureSet

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

@@ -6,7 +6,7 @@
 
 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
+18 |    final class Mapped extends LazyList[B]:  // error
    |    ^
    |    Found:    {f, xs} LazyList[B]
    |    Required: {f} LazyList[B]
@@ -18,7 +18,7 @@ longer explanation available when compiling with `-explain`
 
 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
+27 |    final class Mapped extends LazyList[B]:  // error
    |    ^
    |    Found:    {f, xs} LazyList[B]
    |    Required: {xs} LazyList[B]
@@ -36,3 +36,10 @@ longer explanation available when compiling with `-explain`
    |                                     Required: {xs} LazyList[B]
 
 longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/lazylists2.scala:59:48 -----------------------------------
+59 |      def tail: {this} LazyList[B] = xs.tail.map(f) // error
+   |                                     ^^^^^^^^^^^^^^
+   |                                     Found:    {f} LazyList[B]
+   |                                     Required: {Mapped.this} LazyList[B]
+
+longer explanation available when compiling with `-explain`

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

@@ -15,7 +15,7 @@ object LazyNil extends LazyList[Nothing]:
 
 extension [A](xs: {*} LazyList[A])
   def map[B](f: {*} A => B): {f} LazyList[B] =
-    class Mapped extends LazyList[B]:  // error
+    final class Mapped extends LazyList[B]:  // error
       this: ({xs, f} Mapped) =>
 
       def isEmpty = false
@@ -24,7 +24,7 @@ extension [A](xs: {*} LazyList[A])
     new Mapped
 
   def map2[B](f: {*} A => B): {xs} LazyList[B] =
-    class Mapped extends LazyList[B]:  // error
+    final class Mapped extends LazyList[B]:  // error
       this: ({xs, f} Mapped) =>
 
       def isEmpty = false
@@ -33,7 +33,7 @@ extension [A](xs: {*} LazyList[A])
     new Mapped
 
   def map3[B](f: {*} A => B): {xs} LazyList[B] =
-    class Mapped extends LazyList[B]:
+    final class Mapped extends LazyList[B]:
       this: ({xs} Mapped) =>
 
       def isEmpty = false
@@ -42,11 +42,23 @@ extension [A](xs: {*} LazyList[A])
     new Mapped
 
   def map4[B](f: {*} A => B): {xs} LazyList[B] =
-    class Mapped extends LazyList[B]:
+    final 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
 
+  def map5[B](f: {*} A => B): 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) // error
+    class Mapped2 extends Mapped:
+      this: Mapped =>
+    new Mapped2
+
+

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

@@ -15,7 +15,7 @@ object LazyNil extends LazyList[Nothing]:
 
 extension [A](xs: {*} LazyList[A])
   def map[B](f: {*} A => B): {xs, f} LazyList[B] =
-    class Mapped extends LazyList[B]:
+    final class Mapped extends LazyList[B]:
       this: ({xs, f} Mapped) =>
 
       def isEmpty = false

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

@@ -13,7 +13,7 @@ object LazyNil extends LazyList[Nothing]:
   def head = ???
   def tail = ???
 
-class LazyCons[+T](val x: T, val xs: {*} () => {*} LazyList[T]) extends LazyList[T]:
+final class LazyCons[+T](val x: T, val xs: {*} () => {*} LazyList[T]) extends LazyList[T]:
   this: ({*} LazyList[T]) =>
 
   def isEmpty = false