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Fix #19907: Skip soft unions in widenSingle of widenInferred #19995

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merged 4 commits into from
Mar 25, 2024
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Fix #19907: Skip soft unions in widenSingle of widenInferred #19995

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7aef415
Skip widenSingletons in widenInferred if the inferred type is too large
noti0na1 Mar 20, 2024
c5c94f0
Make individual tests
noti0na1 Mar 20, 2024
612ba92
Alternative scheme that does not require a size limit
odersky Mar 22, 2024
fd61afb
Refine logic for widenSingle
noti0na1 Mar 25, 2024
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22 changes: 16 additions & 6 deletions compiler/src/dotty/tools/dotc/core/ConstraintHandling.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -648,11 +648,13 @@ trait ConstraintHandling {
* as those could leak the annotation to users (see run/inferred-repeated-result).
*/
def widenInferred(inst: Type, bound: Type, widenUnions: Boolean)(using Context): Type =
def typeSize(tp: Type): Int = tp match
case tp: AndOrType => typeSize(tp.tp1) + typeSize(tp.tp2)
case _ => 1

def widenOr(tp: Type) =
if widenUnions then
val tpw = tp.widenUnion
if (tpw ne tp) && !tpw.isTransparent() && (tpw <:< bound) then tpw else tp
else tp.hardenUnions
val tpw = tp.widenUnion
if (tpw ne tp) && !tpw.isTransparent() && (tpw <:< bound) then tpw else tp

def widenSingle(tp: Type) =
val tpw = tp.widenSingletons
Expand All @@ -665,8 +667,16 @@ trait ConstraintHandling {
val wideInst =
if isSingleton(bound) then inst
else
val widenedFromSingle = widenSingle(inst)
val widenedFromUnion = widenOr(widenedFromSingle)
val widenedFromUnion =
if widenUnions && typeSize(inst) > 64 then
// If the inferred type `inst` is too large, the subtype check for `bound` in `widenSingle`
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Very useful diagnosis! I think we can even go further: If we have widenUnions then we never need to widen singletons in soft unions, since we will do an OrDominator afterwards, and this is the same for original or widen types. We don't even need a size limit for this.

I'll push a commit that tries this.

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Correction: It does not work since we sometimes leave original orType if it is transparent, and then we should ideally widen singleton types.

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I could fix it and account for transparent unions. See latest commit.

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Thanks, I updated the title.

// can be expensive due to comparisons between large union types, so we avoid it by
// `widenUnion` directly here.
// See issue #19907.
widenOr(inst)
else
val widenedFromSingle = widenSingle(inst)
if widenUnions then widenOr(widenedFromSingle) else widenedFromSingle.hardenUnions
val widened = dropTransparentTraits(widenedFromUnion, bound)
widenIrreducible(widened)

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