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Better error message for polytypes wrapping capturing types #21843

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Oct 27, 2024
+33 −20
Merged

Better error message for polytypes wrapping capturing types #21843

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38 changes: 18 additions & 20 deletions compiler/src/dotty/tools/dotc/parsing/Parsers.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -518,6 +518,22 @@ object Parsers {
tree
}

def makePolyFunction(tparams: List[Tree], body: Tree,
kind: String, errorTree: => Tree,
start: Offset, arrowOffset: Offset): Tree =
atSpan(start, arrowOffset):
getFunction(body) match
case None =>
syntaxError(em"Implementation restriction: polymorphic function ${kind}s must have a value parameter", arrowOffset)
errorTree
Comment on lines +526 to +528
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Maybe we should add a test for this case too

case Some(Function(_, _: CapturesAndResult)) =>
// A function tree like this will be desugared
// into a capturing type in the typer.
syntaxError(em"Implementation restriction: polymorphic function types cannot wrap function types that have capture sets", arrowOffset)
errorTree
case Some(f) =>
PolyFunction(tparams, body)

/* --------------- PLACEHOLDERS ------------------------------------------- */

/** The implicit parameters introduced by `_` in the current expression.
Expand Down Expand Up @@ -1539,11 +1555,6 @@ object Parsers {
private def getFunction(tree: Tree): Option[Function] = tree match {
case Parens(tree1) => getFunction(tree1)
case Block(Nil, tree1) => getFunction(tree1)
case Function(_, _: CapturesAndResult) =>
// A function tree like this will be desugared
// into a capturing type in the typer,
// so None is returned.
None
case t: Function => Some(t)
case _ => None
}
Expand Down Expand Up @@ -1757,13 +1768,7 @@ object Parsers {
else if in.token == ARROW || isPureArrow(nme.PUREARROW) then
val arrowOffset = in.skipToken()
val body = toplevelTyp(nestedIntoOK(in.token))
atSpan(start, arrowOffset):
getFunction(body) match
case Some(f) =>
PolyFunction(tparams, body)
case None =>
syntaxError(em"Implementation restriction: polymorphic function types must have a value parameter", arrowOffset)
Ident(nme.ERROR.toTypeName)
makePolyFunction(tparams, body, "type", Ident(nme.ERROR.toTypeName), start, arrowOffset)
else
accept(TLARROW)
typ()
Expand Down Expand Up @@ -2360,14 +2365,7 @@ object Parsers {
val tparams = typeParamClause(ParamOwner.Type)
val arrowOffset = accept(ARROW)
val body = expr(location)
atSpan(start, arrowOffset) {
getFunction(body) match
case Some(f) =>
PolyFunction(tparams, f)
case None =>
syntaxError(em"Implementation restriction: polymorphic function literals must have a value parameter", arrowOffset)
errorTermTree(arrowOffset)
}
makePolyFunction(tparams, body, "literal", errorTermTree(arrowOffset), start, arrowOffset)
case _ =>
val saved = placeholderParams
placeholderParams = Nil
Expand Down
8 changes: 8 additions & 0 deletions tests/neg-custom-args/captures/polyCaptures.check
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Original file line number Diff line number Diff line change
@@ -0,0 +1,8 @@
-- Error: tests/neg-custom-args/captures/polyCaptures.scala:4:22 -------------------------------------------------------
4 |val runOpsCheck: [C^] -> (ops: List[() ->{C^} Unit]) ->{C^} Unit = runOps // error
| ^
| Implementation restriction: polymorphic function types cannot wrap function types that have capture sets
-- Error: tests/neg-custom-args/captures/polyCaptures.scala:5:23 -------------------------------------------------------
5 |val runOpsCheck2: [C^] => (ops: List[() ->{C^} Unit]) ->{C^} Unit = runOps // error
| ^
| Implementation restriction: polymorphic function types cannot wrap function types that have capture sets
7 changes: 7 additions & 0 deletions tests/neg-custom-args/captures/polyCaptures.scala
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Original file line number Diff line number Diff line change
@@ -0,0 +1,7 @@
class Box[X](val elem: X)

val runOps = [C^] => (b: Box[() ->{C^} Unit]) => b.elem()
val runOpsCheck: [C^] -> (ops: List[() ->{C^} Unit]) ->{C^} Unit = runOps // error
val runOpsCheck2: [C^] => (ops: List[() ->{C^} Unit]) ->{C^} Unit = runOps // error


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