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WojciechMazurWojciechMazur
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Improve message for discarded pure non-Unit values
Fixes #18408 Fixes #18722 [Cherry-picked 7451a18][modified]
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+150
-19
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13 files changed

+150
-19
lines changed

‎compiler/src/dotty/tools/dotc/reporting/messages.scala

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@@ -2386,6 +2386,16 @@ class PureExpressionInStatementPosition(stat: untpd.Tree, val exprOwner: Symbol)
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|It can be removed without changing the semantics of the program. This may indicate an error."""
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}
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class PureUnitExpression(stat: untpd.Tree, tpe: Type)(using Context)
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extends Message(PureUnitExpressionID) {
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def kind = MessageKind.PotentialIssue
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def msg(using Context) = i"Discarded non-Unit value of type ${tpe.widen}. You may want to use `()`."
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def explain(using Context) =
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i"""As this expression is not of type Unit, it is desugared into `{ $stat; () }`.
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|Here the `$stat` expression is a pure statement that can be discarded.
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|Therefore the expression is effectively equivalent to `()`."""
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}
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class UnqualifiedCallToAnyRefMethod(stat: untpd.Tree, method: Symbol)(using Context)
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extends Message(UnqualifiedCallToAnyRefMethodID) {
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def kind = MessageKind.PotentialIssue

‎compiler/src/dotty/tools/dotc/typer/Typer.scala

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@@ -4127,7 +4127,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
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// local adaptation makes sure every adapted tree conforms to its pt
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// so will take the code path that decides on inlining
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val tree1 = adapt(tree, WildcardType, locked)
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checkStatementPurity(tree1)(tree, ctx.owner)
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checkStatementPurity(tree1)(tree, ctx.owner, isUnitExpr = true)
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if (!ctx.isAfterTyper && !tree.isInstanceOf[Inlined] && ctx.settings.WvalueDiscard.value && !isThisTypeResult(tree)) {
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report.warning(ValueDiscarding(tree.tpe), tree.srcPos)
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}
@@ -4424,7 +4424,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
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else false
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}
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private def checkStatementPurity(tree: tpd.Tree)(original: untpd.Tree, exprOwner: Symbol)(using Context): Unit =
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private def checkStatementPurity(tree: tpd.Tree)(original: untpd.Tree, exprOwner: Symbol, isUnitExpr: Boolean = false)(using Context): Unit =
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if !tree.tpe.isErroneous
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&& !ctx.isAfterTyper
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&& !tree.isInstanceOf[Inlined]
@@ -4442,6 +4442,8 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
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// sometimes we do not have the original anymore and use the transformed tree instead.
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// But taken together, the two criteria are quite accurate.
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missingArgs(tree, tree.tpe.widen)
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case _ if isUnitExpr =>
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report.warning(PureUnitExpression(original, tree.tpe), original.srcPos)
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case _ =>
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report.warning(PureExpressionInStatementPosition(original, exprOwner), original.srcPos)
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‎tests/neg-custom-args/captures/real-try.check

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@@ -1,7 +1,7 @@
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-- [E129] Potential Issue Warning: tests/neg-custom-args/captures/real-try.scala:30:4 ----------------------------------
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30 | b.x
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-- [E190] Potential Issue Warning: tests/neg-custom-args/captures/real-try.scala:36:4 ----------------------------------
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36 | b.x
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| ^^^
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| A pure expression does nothing in statement position; you may be omitting necessary parentheses
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| Discarded non-Unit value of type () -> Unit. You may want to use `()`.
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|
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| longer explanation available when compiling with `-explain`
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-- Error: tests/neg-custom-args/captures/real-try.scala:12:2 -----------------------------------------------------------

‎tests/neg/i18408a.check

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@@ -0,0 +1,18 @@
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-- [E103] Syntax Error: tests/neg/i18408a.scala:2:0 --------------------------------------------------------------------
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2 |fa(42) // error
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|^^
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|Illegal start of toplevel definition
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|
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| longer explanation available when compiling with `-explain`
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-- [E190] Potential Issue Warning: tests/neg/i18408a.scala:3:15 --------------------------------------------------------
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3 |def test1 = fa(42)
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| ^^
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| Discarded non-Unit value of type Int. You may want to use `()`.
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|
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| longer explanation available when compiling with `-explain`
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-- [E129] Potential Issue Warning: tests/neg/i18408a.scala:4:16 --------------------------------------------------------
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4 |def test2 = fa({42; ()})
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| ^^
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| A pure expression does nothing in statement position; you may be omitting necessary parentheses
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|
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| longer explanation available when compiling with `-explain`

‎tests/neg/i18408a.scala

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@@ -0,0 +1,4 @@
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def fa(f: String ?=> Unit): Unit = ???
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fa(42) // error
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def test1 = fa(42)
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def test2 = fa({42; ()})

‎tests/neg/i18408b.check

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@@ -0,0 +1,18 @@
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-- [E103] Syntax Error: tests/neg/i18408b.scala:2:0 --------------------------------------------------------------------
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2 |fa(42) // error
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|^^
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|Illegal start of toplevel definition
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|
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| longer explanation available when compiling with `-explain`
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-- [E190] Potential Issue Warning: tests/neg/i18408b.scala:3:15 --------------------------------------------------------
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3 |def test1 = fa(42)
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| ^^
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| Discarded non-Unit value of type Int. You may want to use `()`.
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|
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| longer explanation available when compiling with `-explain`
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-- [E129] Potential Issue Warning: tests/neg/i18408b.scala:4:16 --------------------------------------------------------
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4 |def test2 = fa({42; ()})
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| ^^
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| A pure expression does nothing in statement position; you may be omitting necessary parentheses
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|
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| longer explanation available when compiling with `-explain`

‎tests/neg/i18408b.scala

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@@ -0,0 +1,4 @@
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def fa(f: => Unit): Unit = ???
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fa(42) // error
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def test1 = fa(42)
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def test2 = fa({42; ()})

‎tests/neg/i18408c.check

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@@ -0,0 +1,18 @@
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-- [E103] Syntax Error: tests/neg/i18408c.scala:2:0 --------------------------------------------------------------------
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2 |fa(42) // error
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|^^
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|Illegal start of toplevel definition
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|
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| longer explanation available when compiling with `-explain`
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-- [E190] Potential Issue Warning: tests/neg/i18408c.scala:3:15 --------------------------------------------------------
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3 |def test1 = fa(42)
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| ^^
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| Discarded non-Unit value of type Int. You may want to use `()`.
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|
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| longer explanation available when compiling with `-explain`
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-- [E129] Potential Issue Warning: tests/neg/i18408c.scala:4:16 --------------------------------------------------------
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4 |def test2 = fa({42; ()})
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| ^^
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| A pure expression does nothing in statement position; you may be omitting necessary parentheses
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|
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| longer explanation available when compiling with `-explain`

‎tests/neg/i18408c.scala

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@@ -0,0 +1,4 @@
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def fa(f: Unit): Unit = ???
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fa(42) // error
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def test1 = fa(42)
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def test2 = fa({42; ()})

‎tests/neg/i18722.check

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@@ -0,0 +1,44 @@
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-- [E190] Potential Issue Error: tests/neg/i18722.scala:3:15 -----------------------------------------------------------
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3 |def f1: Unit = null // error
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| ^^^^
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| Discarded non-Unit value of type Null. You may want to use `()`.
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|---------------------------------------------------------------------------------------------------------------------
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| Explanation (enabled by `-explain`)
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|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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| As this expression is not of type Unit, it is desugared into `{ null; () }`.
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| Here the `null` expression is a pure statement that can be discarded.
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| Therefore the expression is effectively equivalent to `()`.
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---------------------------------------------------------------------------------------------------------------------
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-- [E190] Potential Issue Error: tests/neg/i18722.scala:4:15 -----------------------------------------------------------
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4 |def f2: Unit = 1 // error
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| ^
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| Discarded non-Unit value of type Int. You may want to use `()`.
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|---------------------------------------------------------------------------------------------------------------------
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| Explanation (enabled by `-explain`)
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|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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| As this expression is not of type Unit, it is desugared into `{ 1; () }`.
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| Here the `1` expression is a pure statement that can be discarded.
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| Therefore the expression is effectively equivalent to `()`.
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---------------------------------------------------------------------------------------------------------------------
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-- [E190] Potential Issue Error: tests/neg/i18722.scala:5:15 -----------------------------------------------------------
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5 |def f3: Unit = "a" // error
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| ^^^
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| Discarded non-Unit value of type String. You may want to use `()`.
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|---------------------------------------------------------------------------------------------------------------------
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| Explanation (enabled by `-explain`)
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|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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| As this expression is not of type Unit, it is desugared into `{ "a"; () }`.
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| Here the `"a"` expression is a pure statement that can be discarded.
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| Therefore the expression is effectively equivalent to `()`.
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---------------------------------------------------------------------------------------------------------------------
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-- [E190] Potential Issue Error: tests/neg/i18722.scala:7:15 -----------------------------------------------------------
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7 |def f4: Unit = i // error
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| ^
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| Discarded non-Unit value of type Int. You may want to use `()`.
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|---------------------------------------------------------------------------------------------------------------------
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| Explanation (enabled by `-explain`)
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|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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| As this expression is not of type Unit, it is desugared into `{ i; () }`.
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| Here the `i` expression is a pure statement that can be discarded.
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| Therefore the expression is effectively equivalent to `()`.
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---------------------------------------------------------------------------------------------------------------------

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