Add missing case to TypeComparer (#23550)

We now have:

  x.type^{x} <: x.type

Author: noti0na1

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -547,6 +547,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
           if tp2.isAny then true
           else if compareCaptures(tp1, refs1, tp2, tp2.captureSet)
             || !ctx.mode.is(Mode.CheckBoundsOrSelfType) && tp1.isAlwaysPure
+            || parent1.isSingleton && refs1.elems.forall(parent1 eq _)
           then
             val tp2a =
               if tp1.isBoxedCapturing && !parent1.isBoxedCapturing

tests/neg-custom-args/captures/i23207.check

@@ -5,10 +5,21 @@
    |             Required: A
    |
    | longer explanation available when compiling with `-explain`
--- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i23207.scala:16:11 ---------------------------------------
-16 |  b.getBox.x // error
-   |  ^^^^^^^^^^
-   |  Found:    (Box[(b : B^{io})^{b}]#x : (b : B^{io})^{b})
-   |  Required: A^?
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i23207.scala:18:13 ---------------------------------------
+18 |  val _: A = c // error
+   |             ^
+   |             Found:    (c : B^{b})
+   |             Required: A
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i23207.scala:23:2 ----------------------------------------
+23 |  class B extends A:   // error, now we see the error for the whole block since there are no nested errors
+   |  ^
+   |  Found:    A^{io}
+   |  Required: A
+24 |    val hide: AnyRef^{io} = io
+25 |  val b = new B
+26 |  val c = b.getBox.x
+27 |  c
    |
    | longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/i23207.scala

@@ -13,4 +13,16 @@ def leak(io: AnyRef^): A =
   val b = new B
   val box = b.getBox
   val a: A = box.x // error
-  b.getBox.x // error
+  val c = b.getBox.x
+  val _: B^{b} = c // ok
+  val _: A = c // error
+  c // no error here since we don't propagate expected type into the last expression of a block
+    // and the whole block's span overlaps with previous errors
+
+def leak2(io: AnyRef^): A =
+  class B extends A:   // error, now we see the error for the whole block since there are no nested errors
+    val hide: AnyRef^{io} = io
+
+  val b = new B
+  val c = b.getBox.x
+  c

tests/neg/no-patches.check

@@ -3,9 +3,9 @@
   |        ^^^^^^^^^^^^^^^^^^^^
   |        value 3.4 is not a member of object language
 -- [E008] Not Found Error: tests/neg/no-patches.scala:4:36 -------------------------------------------------------------
-4 |val _ = scala.language.experimental.captureChecking // error: we do not patch `scala.language.experimental`
-  |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-  |        value captureChecking is not a member of object language.experimental
+4 |val _ = scala.language.experimental.modularity // error: we do not patch `scala.language.experimental`
+  |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |        value modularity is not a member of object language.experimental
 -- [E008] Not Found Error: tests/neg/no-patches.scala:5:15 -------------------------------------------------------------
 5 |val _ = Predef.summon[DummyImplicit] // error: we do not patch `scala.Predef`
   |        ^^^^^^^^^^^^^

tests/neg/no-patches.scala

@@ -1,5 +1,5 @@
 //> using options -Yno-stdlib-patches
 
 val _ = scala.language.`3.4` // error: we do not patch `scala.language`
-val _ = scala.language.experimental.captureChecking // error: we do not patch `scala.language.experimental`
+val _ = scala.language.experimental.modularity // error: we do not patch `scala.language.experimental`
 val _ = Predef.summon[DummyImplicit] // error: we do not patch `scala.Predef`

tests/pos-custom-args/captures/singleton-conformance.scala

@@ -0,0 +1,6 @@
+import language.experimental.captureChecking
+
+def id[T](x: T): T = x
+
+trait A:
+  def t(): this.type = id(this)