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Fix NotNullInfo in multi-case match #23864

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Merged
merged 1 commit into from
Sep 2, 2025
Merged

Fix NotNullInfo in multi-case match #23864

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6 changes: 4 additions & 2 deletions compiler/src/dotty/tools/dotc/typer/Typer.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -2216,7 +2216,8 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
var alreadyStripped = false
val cases1 = tree.cases.zip(pt.cases)
.map { case (cas, tpe) =>
val case1 = typedCase(cas, sel, wideSelType, tpe)(using caseCtx)
given Context = caseCtx
val case1 = typedCase(cas, sel, wideSelType, tpe)
caseCtx = Nullables.afterPatternContext(sel, case1.pat)
Comment on lines +2219 to 2221
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Copilot AI Sep 2, 2025

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The caseCtx is being updated after typedCase is called, but afterPatternContext should use the context from before typing the case, not the caseCtx that was passed to typedCase. This could lead to incorrect null flow analysis.

Suggested change
given Context = caseCtx
val case1 = typedCase(cas, sel, wideSelType, tpe)
caseCtx = Nullables.afterPatternContext(sel, case1.pat)
val beforeCaseCtx = caseCtx
given Context = caseCtx
val case1 = typedCase(cas, sel, wideSelType, tpe)
caseCtx = Nullables.afterPatternContext(sel, case1.pat)(using beforeCaseCtx)

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if !alreadyStripped && Nullables.matchesNull(case1) then
wideSelType = wideSelType.stripNull()
Expand Down Expand Up @@ -2248,7 +2249,8 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
var wideSelType = wideSelType0
var alreadyStripped = false
cases.mapconserve { cas =>
val case1 = typedCase(cas, sel, wideSelType, pt)(using caseCtx)
given Context = caseCtx
val case1 = typedCase(cas, sel, wideSelType, pt)
caseCtx = Nullables.afterPatternContext(sel, case1.pat)
Comment on lines +2252 to 2254
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Copilot AI Sep 2, 2025

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Same issue as in typedMatchFinish: the caseCtx update should use the original context, not the caseCtx that was used for typing the case.

Suggested change
given Context = caseCtx
val case1 = typedCase(cas, sel, wideSelType, pt)
caseCtx = Nullables.afterPatternContext(sel, case1.pat)
caseCtx = Nullables.afterPatternContext(sel, case1.pat)(using ctx)

Copilot uses AI. Check for mistakes.

if !alreadyStripped && Nullables.matchesNull(case1) then
wideSelType = wideSelType.stripNull()
Expand Down
16 changes: 16 additions & 0 deletions tests/explicit-nulls/pos/flow-match.scala
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Expand Up @@ -55,4 +55,20 @@ object MatchTest {
case s2 => s2.nn
case s3 => s3
}

def f8(a: AnyRef | Null): String = a match {
case null => "null"
case s: String => s
case _ =>
val a2: AnyRef = a
a.toString
}

def f9(a: AnyRef | Null): String = a match {
case null => "null"
case s: String => s
case a1 =>
val a2: AnyRef = a1
a1.toString
}
}
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