Turn nested environment capture sets into constants at the end of box adaptation

This change lets more ref trees with underlying function types keep their singleton
types.

Author: odersky

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

@@ -1080,7 +1080,7 @@ object CaptureSet:
         case _ =>
           empty
     recur(tp)
-      .showing(i"capture set of $tp = $result", captDebug)
+      //.showing(i"capture set of $tp = $result", captDebug)
 
   private def deepCaptureSet(tp: Type)(using Context): CaptureSet =
     val collect = new TypeAccumulator[CaptureSet]:

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

@@ -1006,7 +1006,7 @@ class CheckCaptures extends Recheck, SymTransformer:
             if (ares1 eq ares) && (aargs1 eq aargs) then actual
             else reconstruct(aargs1, ares1)
 
-          (resTp, curEnv.captured)
+          (resTp, CaptureSet(curEnv.captured.elems))
       end adaptFun
 
       /** Adapt type function type `actual` to the expected type.
@@ -1028,7 +1028,7 @@ class CheckCaptures extends Recheck, SymTransformer:
             if ares1 eq ares then actual
             else reconstruct(ares1)
 
-          (resTp, curEnv.captured)
+          (resTp, CaptureSet(curEnv.captured.elems))
       end adaptTypeFun
 
       def adaptInfo(actual: Type, expected: Type, covariant: Boolean): String =

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

@@ -842,13 +842,27 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
           val refs1 = tp1.captureSet
           try
             if refs1.isAlwaysEmpty then recur(tp1, parent2)
-            else subCaptures(refs1, refs2, frozenConstraint).isOK
-              && sameBoxed(tp1, tp2, refs1)
-              && (recur(tp1.widen.stripCapturing, parent2)
-                 || tp1.isInstanceOf[SingletonType] && recur(tp1, parent2)
-                    // this alternative is needed in case the right hand side is a
-                    // capturing type that contains the lhs as an alternative of a union type.
-                 )
+            else
+              // The singletonOK branch is because we sometimes have a larger capture set in a singleton
+              // than in its underlying type. An example is `f: () -> () ->{x} T`, which might be
+              // the type of a closure. In that case the capture set of `f.type` is `{x}` but the
+              // capture set of the underlying type is `{}`. So without the `singletonOK` test, a singleton
+              // might not be a subtype of its underlying type. Examples where this arises is
+              // capt-capibility.scala and function-combinators.scala
+              val singletonOK = tp1 match
+                case tp1: SingletonType
+                if subCaptures(tp1.underlying.captureSet, refs2, frozen = true).isOK =>
+                  recur(tp1.widen, tp2)
+                case _ =>
+                  false
+              singletonOK
+              || subCaptures(refs1, refs2, frozenConstraint).isOK
+                  && sameBoxed(tp1, tp2, refs1)
+                  && (recur(tp1.widen.stripCapturing, parent2)
+                     || tp1.isInstanceOf[SingletonType] && recur(tp1, parent2)
+                        // this alternative is needed in case the right hand side is a
+                        // capturing type that contains the lhs as an alternative of a union type.
+                    )
           catch case ex: AssertionError =>
             println(i"assertion failed while compare captured $tp1 <:< $tp2")
             throw ex

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

@@ -6,13 +6,13 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/byname.scala:4:2 -----------------------------------------
  4 |  def f() = if cap1 == cap1 then g else g // error
    |  ^
-   |  Found:    (x$0: Int) ->{cap2} Int
-   |  Required: (x$0: Int) -> Int
+   |  Found:    ((x$0: Int) ->{cap2} Int)^{}
+   |  Required: Int -> Int
    |
    |  Note that the expected type Int ->{} Int
    |  is the previously inferred result type of method test
    |  which is also the type seen in separately compiled sources.
-   |  The new inferred type (x$0: Int) ->{cap2} Int
+   |  The new inferred type ((x$0: Int) ->{cap2} Int)^{}
    |  must conform to this type.
  5 |  def g(x: Int) = if cap2 == cap2 then 1 else x
  6 |  def g2(x: Int) = if cap1 == cap1 then 1 else x

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

@@ -1,7 +1,7 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/eta.scala:4:9 --------------------------------------------
 4 |         g  // error
   |         ^
-  |         Found:    () ->? A
+  |         Found:    (g : () -> A)
   |         Required: () -> Proc^{f}
   |
   | longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/i15772.check

@@ -35,7 +35,7 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i15772.scala:44:2 ----------------------------------------
 44 |  x: (() -> Unit) // error
    |  ^
-   |  Found:    () ->{x} Unit
+   |  Found:    (x : () ->{filesList, sayHello} Unit)
    |  Required: () -> Unit
    |
    | longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/outer-var.check

@@ -1,7 +1,7 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/outer-var.scala:11:8 -------------------------------------
 11 |    x = q           // error
    |        ^
-   |        Found:    () ->{q} Unit
+   |        Found:    (q : Proc)
    |        Required: () ->{p, q²} Unit
    |
    |        where:    q  is a parameter in method inner
@@ -28,7 +28,7 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/outer-var.scala:14:8 -------------------------------------
 14 |    y = q           // error
    |        ^
-   |        Found:    () ->{q} Unit
+   |        Found:    (q : Proc)
    |        Required: () ->{p} Unit
    |
    |        Note that reference (q : Proc), defined in method inner