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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 stable 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 hasStablePrefix(tp: NamedType) =
tp.prefix.isStable
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I dropped the NoPrefix check since it's already implied by isStable

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Ah yes, I overlooked that.


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 && hasStablePrefix(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 && hasStablePrefix(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