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Fix beta-reduction with Nothing and null args #16938

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Feb 21, 2023
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Fix beta-reduction with Nothing and null args #16938

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5 changes: 4 additions & 1 deletion compiler/src/dotty/tools/dotc/transform/BetaReduce.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -128,7 +128,10 @@ object BetaReduce:
ref.symbol
case _ =>
val flags = Synthetic | (param.symbol.flags & Erased)
val tpe = if arg.tpe.dealias.isInstanceOf[ConstantType] then arg.tpe.dealias else arg.tpe.widen
val tpe =
if arg.tpe.isBottomType then param.tpe.widenTermRefExpr
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I'm not sure why we use widenTermRefExpr here (and not widen for example), just curious.

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@nicolasstucki nicolasstucki Feb 21, 2023 •
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To keep the type precise. For example, if we have a singleton String type we do not want to widen it to String. For example ((x: "foo") => x: "foo").apply(???) should become val x: "foo" = ???; x: "foo" and not val x: String = ???; x: "foo". The second one would not type check after beta-reduction.

else if arg.tpe.dealias.isInstanceOf[ConstantType] then arg.tpe.dealias
else arg.tpe.widen
val binding = ValDef(newSymbol(ctx.owner, param.name, flags, tpe, coord = arg.span), arg).withSpan(arg.span)
if !(tpe.isInstanceOf[ConstantType] && isPureExpr(arg)) then
bindings += binding
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6 changes: 6 additions & 0 deletions tests/pos/i15165.scala
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Original file line number Diff line number Diff line change
@@ -0,0 +1,6 @@
def test1 = { (y: Int) => y + 1 }.apply(???)

class C:
def x: Int = 8

def test2 = { (c: C) => c.x }.apply(null)