Fix typing closure blocks where result type refers to a closure parameter. (#24299)

Fix typing closure blocks where the expected result type refers to a
closure parameter. A substitution was missing in this case.

Fixes #23727

Author: odersky

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

@@ -1045,18 +1045,28 @@ class CheckCaptures extends Recheck, SymTransformer:
               checkConformsExpr(argType, paramType, param)
                 .showing(i"compared expected closure formal $argType against $param with ${paramTpt.nuType}", capt)
             if resType.isValueType && isFullyDefined(resType, ForceDegree.none) then
-              val localResType = pt match
+
+              def updateTpt(localResType: Type) =
+                mdef.tpt.updNuType(localResType)
+                // Make sure we affect the info of the anonfun by the previous updNuType
+                // unless the info is already defined in a previous phase and does not change.
+                assert(!anonfun.isCompleted || anonfun.denot.validFor.firstPhaseId != thisPhase.id)
+
+              pt match
                 case RefinedType(_, _, mt: MethodType) =>
-                  inContext(ctx.withOwner(anonfun)):
-                    Internalize(mt)(resType)
-                case _ => resType
-              mdef.tpt.updNuType(localResType)
-              // Make sure we affect the info of the anonfun by the previous updNuType
-              // unless the info is already defined in a previous phase and does not change.
-              assert(!anonfun.isCompleted || anonfun.denot.validFor.firstPhaseId != thisPhase.id)
-              //println(i"updating ${mdef.tpt} to $localResType/${mdef.tpt.nuType}")
+                  if !mt.isResultDependent then
+                    // If mt is result dependent we could compensate this by
+                    // internalizing `resType.substParams(mt, params.tpes)`.
+                    // But this tends to give worse error messages, so we refrain
+                    // from doing that and don't update the local result type instead.
+                    val localResType = inContext(ctx.withOwner(anonfun)):
+                      Internalize(mt)(resType)
+                    updateTpt(localResType)
+                case _ =>
+                  updateTpt(resType)
           case _ =>
         case Nil =>
+      end matchParamsAndResult
 
       openClosures = (anonfun, pt) :: openClosures
         // openClosures is needed for errors but currently makes no difference

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

@@ -1,12 +1,13 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/filevar.scala:15:12 --------------------------------------
 15 |  withFile: f =>       // error with level checking, was OK under both schemes before
    |            ^
-   |Found:    (f: File^'s1) ->'s2 Unit
-   |Required: (f: File^{l}) => Unit
+   |Found:    (l: scala.caps.Capability^) ?->'s1 File^'s2 ->'s3 Unit
+   |Required: (l: scala.caps.Capability^) ?-> (f: File^{l}) => Unit
    |
-   |Note that capability l cannot be included in outer capture set 's1 of parameter f.
+   |Note that capability l cannot be included in outer capture set 's4 of parameter f.
    |
-   |where:    => refers to a fresh root capability created in anonymous function of type (using l: scala.caps.Capability): File^{l} -> Unit when instantiating expected result type (f: File^{l}) ->{cap} Unit of function literal
+   |where:    => refers to a root capability associated with the result type of (using l: scala.caps.Capability^): (f: File^{l}) => Unit
+   |          ^  refers to the universal root capability
 16 |    val o = Service()
 17 |    o.file = f
 18 |    o.log

tests/neg-custom-args/captures/i15923.check

@@ -1,22 +1,13 @@
--- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i15923.scala:27:23 ---------------------------------------
-27 |    val leak = withCap(cap => mkId(cap))  // error (was: no error here since type aliases don't box)
-   |                       ^^^^^^^^^^^^^^^^
-   |Found:    (cap: test2.Cap^'s1) ->'s2 [T] => (op: test2.Cap^'s3 ->'s4 T) ->'s5 T
-   |Required: test2.Cap^{lcap} => [T] => (op: test2.Cap^'s6 ->'s7 T) ->'s8 T
-   |
-   |Note that capability lcap cannot be included in outer capture set 's1 of parameter cap.
-   |
-   |where:    => refers to a fresh root capability created in anonymous function of type (using lcap: scala.caps.Capability): test2.Cap^{lcap} -> [T] => (op: test2.Cap^{lcap} => T) -> T when instantiating expected result type test2.Cap^{lcap} ->{cap²} [T] => (op: test2.Cap^'s6 ->'s7 T) ->'s8 T of function literal
-   |
-   | longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i15923.scala:12:21 ---------------------------------------
 12 |  val leak = withCap(cap => mkId(cap)) // error
    |                     ^^^^^^^^^^^^^^^^
-   |Found:    (cap: Cap^'s9) ->'s10 Id[Cap^'s11]^'s12
-   |Required: Cap^{lcap} => Id[Cap^'s13]^'s14
+   |Found:    (lcap: scala.caps.Capability^) ?->'s1 Cap^'s2 ->'s3 Id[Cap^'s4]^'s5
+   |Required: (lcap: scala.caps.Capability^) ?-> Cap^{lcap} => Id[Cap^'s6]^'s7
    |
-   |Note that capability lcap cannot be included in outer capture set 's9 of parameter cap.
+   |Note that capability cap cannot be included in outer capture set 's6.
    |
-   |where:    => refers to a fresh root capability created in anonymous function of type (using lcap: scala.caps.Capability): Cap^{lcap} -> Id[Cap] when instantiating expected result type Cap^{lcap} ->{cap²} Id[Cap^'s13]^'s14 of function literal
+   |where:    =>  refers to a root capability associated with the result type of (using lcap: scala.caps.Capability^): Cap^{lcap} => Id[Cap^'s6]^'s7
+   |          ^   refers to the universal root capability
+   |          cap is a root capability associated with the result type of (x$0: Cap^'s2): Id[Cap^'s4]^'s5
    |
    | longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/i15923.scala

@@ -24,6 +24,6 @@ object test2:
       result
     }
 
-    val leak = withCap(cap => mkId(cap))  // error (was: no error here since type aliases don't box)
+    val leak = withCap(cap => mkId(cap))  // no error here since type aliases don't box
     leak { cap => cap.use() }
   }

tests/pos-custom-args/captures/i23727.scala

@@ -0,0 +1,3 @@
+def test1(): Unit =
+  val t1: (x: () => Unit) -> (y: () ->{x} Unit) -> Unit =
+    x => y => ()  // should ok, but error