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Attempt to fix a potentially missing case in TypeComparer #5914

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Merged
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Mar 27, 2019
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Attempt to fix a potentially missing case in TypeComparer #5914

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6 changes: 5 additions & 1 deletion compiler/src/dotty/tools/dotc/core/TypeComparer.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -451,7 +451,11 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
// widening in `fourthTry` before adding to the constraint.
if (frozenConstraint) isSubType(tp1, bounds(tp2).lo)
else isSubTypeWhenFrozen(tp1, tp2)
alwaysTrue || {
alwaysTrue ||
frozenConstraint && (tp1 match {
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It would be good to have a test case that fails if the case is missing! Failing that, one can try to instrument the code to see whether the new logic makes a difference anywhere in the existing code base.

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@abgruszecki abgruszecki Feb 14, 2019 •
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@odersky the following code fails without this case:

val a = newTypeVar(TypeBounds.empty)
val b = newTypeVar(TypeBounds.empty)
a <:< b
assert(a frozen_<:< b)

I think that seems potentially problematic. I believe I actually ran across this issue half a year ago, when I was trying to make my exhaustivity checker use Constraint as a constraint solver. That being said, I have 0 idea on how to make TypeComparer enter this case w/o manually triggering it.

EDIT: for clarity, I meant to hand off this PR to @smarter. I think it's useful to know that a potential problem exists and that we already have a fix for it, even if it doesn't impact the codebase yet. It seems to me that if (a <:< b) then a frozen_<:< b is a useful invariant to have.

case tp1: TypeParamRef => constraint.isLess(tp1, tp2)
case _ => false
}) || {
if (canConstrain(tp2) && !approx.low)
addConstraint(tp2, tp1.widenExpr, fromBelow = true)
else fourthTry
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22 changes: 22 additions & 0 deletions compiler/test/dotty/tools/dotc/typer/SubtypingInvariantTests.scala
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@@ -0,0 +1,22 @@
package dotty.tools.dotc.typer

import dotty.tools.DottyTest
import dotty.tools.dotc.core.Contexts.Context
import dotty.tools.dotc.core.Types.TypeBounds
import dotty.tools.dotc.typer.ProtoTypes.newTypeVar
import org.junit.Test

class SubtypingInvariantTests extends DottyTest {

@Test
def typeVarInvariant(): Unit = {
checkCompile("frontend", "class A") { (_, ctx0) =>
implicit val ctx: Context = ctx0
val a = newTypeVar(TypeBounds.empty)
val b = newTypeVar(TypeBounds.empty)
assert(a <:< b)
assert(a frozen_<:< b)
}
}

}