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Explain no expansion of ContextFunction0 #23844

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Merged
odersky merged 1 commit into scala:main from som-snytt:issue/21321-CF0
Sep 10, 2025
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10 changes: 8 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 @@ -3749,12 +3749,18 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
val ifpt = defn.asContextFunctionType(pt)
val result =
if ifpt.exists
&& defn.functionArity(ifpt) > 0 // ContextFunction0 is only used after ElimByName
&& !ctx.isAfterTyper
&& {
// ContextFunction0 is only used after ElimByName
val arity = defn.functionArity(ifpt)
if arity == 0 then
report.error(em"context function types require at least one parameter", xtree.srcPos)
arity > 0
}
&& xtree.isTerm
&& !untpd.isContextualClosure(xtree)
&& !ctx.mode.is(Mode.Pattern)
&& !xtree.isInstanceOf[SplicePattern]
&& !ctx.isAfterTyper
&& !ctx.isInlineContext
then
makeContextualFunction(xtree, ifpt)
Expand Down
2 changes: 1 addition & 1 deletion tests/neg/context-function-syntax.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -2,7 +2,7 @@ val test =
(using x: Int) => x // error // error // error

val f = () ?=> 23 // error
val g: ContextFunction0[Int] = ??? // ok
val g: ContextFunction0[Int] = ??? // error at typer for RHS not expanded
val h: () ?=> Int = ??? // error

object Example3 extends App {
Expand Down
19 changes: 19 additions & 0 deletions tests/neg/i21321.check
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Original file line number Diff line number Diff line change
@@ -0,0 +1,19 @@
-- Error: tests/neg/i21321.scala:3:42 ----------------------------------------------------------------------------------
3 |val v1b: scala.ContextFunction0[String] = () ?=> "x" // error
| ^^
| context function literals require at least one formal parameter
-- Error: tests/neg/i21321.scala:4:8 -----------------------------------------------------------------------------------
4 |val v2: () ?=> String = "y" // error // error in parser
| ^^
| context function types require at least one parameter
-- Error: tests/neg/i21321.scala:2:41 ----------------------------------------------------------------------------------
2 |val v1: scala.ContextFunction0[String] = "x" // error
| ^^^
| context function types require at least one parameter
-- [E007] Type Mismatch Error: tests/neg/i21321.scala:4:24 -------------------------------------------------------------
4 |val v2: () ?=> String = "y" // error // error in parser
| ^^^
| Found: ("y" : String)
| Required: () => String
|
| longer explanation available when compiling with `-explain`
4 changes: 4 additions & 0 deletions tests/neg/i21321.scala
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Original file line number Diff line number Diff line change
@@ -0,0 +1,4 @@

val v1: scala.ContextFunction0[String] = "x" // error
val v1b: scala.ContextFunction0[String] = () ?=> "x" // error
val v2: () ?=> String = "y" // error // error in parser
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