Widen type parameters before box adaptation (#23809)

Fixes #23746 and #19076.

Widen type parameters to their upper bounds in certain cases before box
adaptation. This helps revealing a boxed type inside the upper bound of
a type parameter.

Author: odersky

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

@@ -15,7 +15,7 @@ import typer.Inferencing.isFullyDefined
 import typer.RefChecks.{checkAllOverrides, checkSelfAgainstParents, OverridingPairsChecker}
 import typer.Checking.{checkBounds, checkAppliedTypesIn}
 import typer.ErrorReporting.{Addenda, NothingToAdd, err}
-import typer.ProtoTypes.{LhsProto, WildcardSelectionProto}
+import typer.ProtoTypes.{LhsProto, WildcardSelectionProto, SelectionProto}
 import util.{SimpleIdentitySet, EqHashMap, EqHashSet, SrcPos, Property}
 import transform.{Recheck, PreRecheck, CapturedVars}
 import Recheck.*
@@ -1309,17 +1309,64 @@ class CheckCaptures extends Recheck, SymTransformer:
     override def checkConformsExpr(actual: Type, expected: Type, tree: Tree, addenda: Addenda)(using Context): Type =
       testAdapted(actual, expected, tree, addenda)(err.typeMismatch)
 
+    @annotation.tailrec
+    private def findImpureUpperBound(tp: Type)(using Context): Type = tp match
+      case _: SingletonType => findImpureUpperBound(tp.widen)
+      case tp: TypeRef if tp.symbol.isAbstractOrParamType =>
+        tp.info match
+          case TypeBounds(_, hi) if hi.isBoxedCapturing => hi
+          case TypeBounds(_, hi) => findImpureUpperBound(hi)
+          case _ => NoType
+      case _ => NoType
+
     inline def testAdapted(actual: Type, expected: Type, tree: Tree, addenda: Addenda)
         (fail: (Tree, Type, Addenda) => Unit)(using Context): Type =
+
       var expected1 = alignDependentFunction(expected, actual.stripCapturing)
       val falseDeps = expected1 ne expected
-      val actualBoxed = adapt(actual, expected1, tree)
+      val actual1 =
+        if expected.stripCapturing.isInstanceOf[SelectionProto] then
+          // If the expected type is a `SelectionProto`, we should be careful about cases when
+          // the actual type is a type parameter (for instance, `X <: box IO^`).
+          // If `X` were not widen to reveal the boxed type, both sides are unboxed and thus
+          // no box adaptation happens. But it is unsound: selecting a member from `X` implicitly
+          // unboxes the value.
+          //
+          // Therefore, when the expected type is a selection proto, we conservatively widen
+          // the actual type to strip type parameters.
+          val hi = findImpureUpperBound(actual)
+          if !hi.exists then actual else hi
+        else actual
+      val actualBoxed = adapt(actual1, expected1, tree)
       //println(i"check conforms $actualBoxed <<< $expected1")
 
       if actualBoxed eq actual then
         // Only `addOuterRefs` when there is no box adaptation
         expected1 = addOuterRefs(expected1, actual, tree.srcPos)
-      TypeComparer.compareResult(isCompatible(actualBoxed, expected1)) match
+
+      def tryCurrentType: Boolean =
+        isCompatible(actualBoxed, expected1)
+
+      /** When the actual type is a named type, and the previous attempt failed, try to widen the named type
+       * and try another time.
+       *
+       * This is useful for cases like:
+       *
+       *   def id[X <: box IO^{a}](x: X): IO^{a} = x
+       *
+       * When typechecking the body, we need to show that `(x: X)` can be typed at `IO^{a}`.
+       * In the first attempt, since `X` is simply a parameter reference, we treat it as non-boxed and perform
+       * no box adptation. But its upper bound is in fact boxed, and adaptation is needed for typechecking the body.
+       * In those cases, we widen such types and try box adaptation another time.
+       */
+      def tryWidenNamed: Boolean =
+        val actual1 = findImpureUpperBound(actual)
+        actual1.exists && {
+          val actualBoxed1 = adapt(actual1, expected1, tree)
+          isCompatible(actualBoxed1, expected1)
+        }
+
+      TypeComparer.compareResult(tryCurrentType || tryWidenNamed) match
         case TypeComparer.CompareResult.Fail(notes) =>
           capt.println(i"conforms failed for ${tree}: $actual vs $expected")
           if falseDeps then expected1 = unalignFunction(expected1)
@@ -1477,7 +1524,8 @@ class CheckCaptures extends Recheck, SymTransformer:
             (actualShape, CaptureSet())
         end adaptShape
 
-        def adaptStr = i"adapting $actual ${if covariant then "~~>" else "<~~"} $expected"
+        //val adaptStr = i"adapting $actual ${if covariant then "~~>" else "<~~"} $expected"
+        //println(adaptStr)
 
         // Get wildcards out of the way
         expected match

tests/neg-custom-args/captures/i23746.check

@@ -0,0 +1,21 @@
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i23746.scala:5:2 -----------------------------------------
+5 |  () => op.run()  // error
+  |  ^^^^^^^^^^^^^^
+  |  Found:    () => Unit
+  |  Required: () -> Unit
+  |
+  |  Note that capability cap is not included in capture set {}.
+  |
+  |  where:    =>  refers to a fresh root capability in the type of type X
+  |            cap is a fresh root capability in the type of type X
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i23746.scala:10:4 ----------------------------------------
+10 |    () => op.run()  // error
+   |    ^^^^^^^^^^^^^^
+   |    Found:    () ->{a} Unit
+   |    Required: () -> Unit
+   |
+   |    Note that capability a is not included in capture set {}.
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/i23746.scala

@@ -0,0 +1,10 @@
+import language.experimental.captureChecking
+trait Op:
+  def run(): Unit
+def helper[X <: Op^](op: X): () -> Unit =
+  () => op.run()  // error
+def test1(a: Op^): Unit =
+  def foo[X <: Op^{a}](op: X): () ->{a} Unit =
+    () => op.run()  // ok
+  def bar[X <: Op^{a}](op: X): () ->{} Unit =
+    () => op.run()  // error

tests/pos-custom-args/captures/i19076.scala

@@ -0,0 +1,4 @@
+import language.experimental.captureChecking
+trait IO
+def main(a: IO^): Unit =
+  def foo[X <: IO^{a}](x: X): IO^{a} = x  // now ok