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Better comparisons for type projections #17092

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merged 3 commits into from
Mar 25, 2023
Merged

Better comparisons for type projections #17092

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2ac2315
Better comparisons for type projections
odersky Mar 12, 2023
c8efe69
Align tests in TypeComparer and AsSeenFrom
odersky Mar 18, 2023
d17da7a
Simplify hasPrecisePrefix check
smarter Mar 25, 2023
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17 changes: 14 additions & 3 deletions compiler/src/dotty/tools/dotc/core/TypeComparer.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -283,17 +283,28 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
val ctx = comparerContext
given Context = ctx // optimization for performance
val info2 = tp2.info

/** Does `tp2` have a singleton type or NoPrefix as a prefix?
* If that's not the case, following an alias via asSeenFrom could be lossy
* so we should not conclude `false` if comparing aliases fails.
* See pos/i17064.scala for a test case
*/
def hasPrecisePrefix(tp: NamedType) =
tp.prefix.isSingleton || tp.prefix == NoPrefix
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This seems fine, but since asSeenFrom itself uses TypeOps.isLegalPrefix for this purpose, wouldn't it make sense to also use it here to ensure the checks stay aligned?


info2 match
case info2: TypeAlias =>
if recur(tp1, info2.alias) then return true
if tp2.asInstanceOf[TypeRef].canDropAlias then return false
if tp2.asInstanceOf[TypeRef].canDropAlias && hasPrecisePrefix(tp2) then
return false
case _ =>
tp1 match
case tp1: NamedType =>
tp1.info match {
case info1: TypeAlias =>
if recur(info1.alias, tp2) then return true
if tp1.asInstanceOf[TypeRef].canDropAlias then return false
if tp1.asInstanceOf[TypeRef].canDropAlias && hasPrecisePrefix(tp2) then
return false
case _ =>
}
val sym2 = tp2.symbol
Expand All @@ -302,7 +313,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
// For convenience we want X$ <:< X.type
// This is safe because X$ self-type is X.type
sym1 = sym1.companionModule
if ((sym1 ne NoSymbol) && (sym1 eq sym2))
if (sym1 ne NoSymbol) && (sym1 eq sym2) then
ctx.erasedTypes ||
sym1.isStaticOwner ||
isSubPrefix(tp1.prefix, tp2.prefix) ||
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2 changes: 1 addition & 1 deletion tests/neg/i15525.scala
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Expand Up @@ -37,7 +37,7 @@ def element22(
transmittable20.Type / transmittable21.Type
} = ???

def test22 = // error
def test22 =
Resolution(
element22(
Resolution(element0), Resolution(element0), // error // error
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10 changes: 10 additions & 0 deletions tests/pos/i17064.scala
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@@ -0,0 +1,10 @@
class HiddenInner[+O<:Outer](val outer:O){
}

class Outer{
type Inner = HiddenInner[this.type]
}

val o : Outer = new Outer
def a : o.Inner = new o.Inner(o)
val b : Outer#Inner = a // DOES NOT COMPILE