Treat bounds of type variables as boxed

Author: odersky

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

@@ -67,6 +67,10 @@ extension (tp: Type)
     case _ =>
       tp
 
+  /** If `sym` is a type parameter, the boxed version of `tp`, otherwise `tp` */
+  def boxedIfTypeParam(sym: Symbol)(using Context) =
+    if sym.is(TypeParam) then tp.boxed else tp
+
   /** The boxed version of `tp`, unless `tycon` is a function symbol */
   def boxedUnlessFun(tycon: Type)(using Context) =
     if ctx.phase != Phases.checkCapturesPhase || defn.isFunctionSymbol(tycon.typeSymbol)

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

@@ -23,7 +23,7 @@ import typer.ProtoTypes.constrained
 import typer.Applications.productSelectorTypes
 import reporting.trace
 import annotation.constructorOnly
-import cc.{CapturingType, derivedCapturingType, CaptureSet, stripCapturing, isBoxedCapturing, boxedUnlessFun}
+import cc.{CapturingType, derivedCapturingType, CaptureSet, stripCapturing, isBoxedCapturing, boxed, boxedUnlessFun, boxedIfTypeParam}
 
 /** Provides methods to compare types.
  */
@@ -426,7 +426,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
             case tp2: TypeParamRef => constraint.isLess(tp1, tp2)
             case _ => false
           } ||
-          isSubTypeWhenFrozen(bounds(tp1).hi, tp2) || {
+          isSubTypeWhenFrozen(bounds(tp1).hi.boxed, tp2) || {
             if (canConstrain(tp1) && !approx.high)
               addConstraint(tp1, tp2, fromBelow = false) && flagNothingBound
             else thirdTry
@@ -547,7 +547,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
             || narrowGADTBounds(tp2, tp1, approx, isUpper = false))
           && (isBottom(tp1) || GADTusage(tp2.symbol))
 
-        isSubApproxHi(tp1, info2.lo) && (trustBounds || isSubApproxHi(tp1, info2.hi))
+        isSubApproxHi(tp1, info2.lo.boxedIfTypeParam(tp2.symbol)) && (trustBounds || isSubApproxHi(tp1, info2.hi))
         || compareGADT
         || tryLiftedToThis2
         || fourthTry
@@ -586,7 +586,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
           // So if the constraint is not yet frozen, we do the same comparison again
           // with a frozen constraint, which means that we get a chance to do the
           // widening in `fourthTry` before adding to the constraint.
-          if (frozenConstraint) recur(tp1, bounds(tp2).lo)
+          if (frozenConstraint) recur(tp1, bounds(tp2).lo.boxed)
           else isSubTypeWhenFrozen(tp1, tp2)
         alwaysTrue || {
           if (canConstrain(tp2) && !approx.low)
@@ -832,7 +832,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
     }
 
     def tryBaseType(cls2: Symbol) = {
-      val base = nonExprBaseType(tp1, cls2)
+      val base = nonExprBaseType(tp1, cls2).boxedIfTypeParam(tp1.typeSymbol)
       if base.exists && (base ne tp1)
         && (!caseLambda.exists || canWidenAbstract || tp1.widen.underlyingClassRef(refinementOK = true).exists)
       then
@@ -856,7 +856,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
               && (tp2.isAny || GADTusage(tp1.symbol))
 
             (!caseLambda.exists || canWidenAbstract)
-                && isSubType(hi1, tp2, approx.addLow) && (trustBounds || isSubType(lo1, tp2, approx.addLow))
+                && isSubType(hi1.boxedIfTypeParam(tp1.symbol), tp2, approx.addLow) && (trustBounds || isSubType(lo1, tp2, approx.addLow))
             || compareGADT
             || tryLiftedToThis1
           case _ =>

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

@@ -0,0 +1,14 @@
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/eta.scala:4:9 --------------------------------------------
+4 |         g  // error
+  |         ^
+  |         Found:    (g : () -> A)
+  |         Required: () -> {f} Proc
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/eta.scala:6:14 -------------------------------------------
+6 |         bar( () => f )  // error
+  |              ^^^^^^^
+  |              Found:    {f} () -> box {f} () -> Unit
+  |              Required: () -> box ? () -> Unit
+  |
+  | longer explanation available when compiling with `-explain`

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

@@ -1,7 +1,7 @@
     type Proc = (() -> Unit)
      def foo(f: {*} Proc): {} Proc =
        def bar[A <: {f} Proc](g: () -> A): () -> {f} Proc =
-         g
+         g  // error
        val stowaway: () -> {f} Proc =
          bar( () => f )  // error
        () => { stowaway.apply().apply() }

tests/neg-custom-args/captures/override-boxed.scala

@@ -0,0 +1,7 @@
+class A
+
+def test(x: {*} Any) =
+  abstract class Getter:
+    def get(): {x} A
+  class PolyGetter[T <: {x} A] extends Getter:
+    override def get(): T = ??? // error