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Fix Java generic signatures for context functions #25632

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Fix Java generic signatures for context functions #25632

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8f81c72
Regression test
SolalPirelli Mar 26, 2026
b1d733c
Fix
SolalPirelli Mar 26, 2026
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10 changes: 9 additions & 1 deletion compiler/src/dotty/tools/dotc/transform/GenericSignatures.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -16,6 +16,7 @@ import core.Types.*
import core.classfile.ClassfileConstants

import config.Printers.transforms
import dotty.tools.dotc.core.NameOps.isContextFunction
import reporting.trace
import java.lang.StringBuilder

Expand Down Expand Up @@ -529,7 +530,14 @@ object GenericSignatures {
@tailrec def recur(tpe: Type): Type = tpe match
case mtd: MethodType =>
vparams ++= mtd.paramInfos.filterNot(_.hasAnnotation(defn.ErasedParamAnnot))
recur(mtd.resType)
mtd.resType match
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@tanishiking tanishiking Apr 3, 2026

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Suggested change
mtd.resType match
mtd.resType.dealias match

otherwise, it doesn't catch the following case?

object Test:
   trait Ctx
   type Handler = Ctx ?=> String
   def foo(x: Int): Handler = "hello"

// Returned context functions are erased by putting their parameters into the method's parameters,
// so we must duplicate that logic here
case AppliedType(tycon, args) if tycon.typeSymbol.name.isContextFunction =>
vparams ++= args.take(args.length - 1)
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@tanishiking tanishiking Apr 3, 2026

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We might want to filter out .filterNot(_.hasAnnotation(defn.ErasedParamAnnot)) similar to for normal parameters, otherwise, erased type parameter will remain in generic signature, while it is removed in descriptor ?

import language.experimental.erasedDefinitions

class CanSerialize extends compiletime.Erased

object Test:
  trait Ctx
  def foo(x: Int): CanSerialize ?=> Ctx ?=> String = "foo"

(I didn't know about this feature until I see the vparams ++= mtd.paramInfos.filterNot(_.hasAnnotation(defn.ErasedParamAnnot)) line above :p
https://docs.scala-lang.org/scala3/reference/experimental/erased-defs.html

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@tanishiking tanishiking Apr 3, 2026

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it seems that for method that contains Erased parameter, the result type will be RefinedType instead of AppliedType.

We'd like to pattern match against

case defn.FunctionTypeOfMethod(mt: MethodType) if mt.isContextualMethod

?

recur(args.last)
case _ =>
recur(mtd.resType)
case PolyType(tps, tpe) =>
tparams ++= tps
recur(tpe)
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6 changes: 6 additions & 0 deletions tests/run/returned-context-function-signature.check
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@@ -0,0 +1,6 @@
public void CtxFns$.puts(java.lang.Object,CtxFns$Context)
class java.lang.Object,class CtxFns$Context -> void
T,class CtxFns$Context -> void
private java.lang.Object CtxFns$.writeReplace()
-> class java.lang.Object
-> class java.lang.Object
17 changes: 17 additions & 0 deletions tests/run/returned-context-function-signature.scala
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@@ -0,0 +1,17 @@
// scalajs: --skip
// (this is a JVM-only test)

object CtxFns {
abstract class Context:
def puts[T](t: T): Unit

def puts[T](t: T): Context ?=> Unit = summon[Context].puts(t)
}

object Test:
def main(args: Array[String]): Unit =
classOf[CtxFns.type].getDeclaredMethods.sortBy(_.getName).foreach(m =>
println(m)
println(m.getParameterTypes().mkString(",") + " -> " + m.getReturnType.toString)
println(m.getGenericParameterTypes().mkString(",") + " -> " + m.getGenericReturnType.toString)
)
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