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fix: Instantiate type parameters for parametric extension methods in signature help #25041

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fix: Instantiate type parameters for parametric extension methods in signature help #25041

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e78c5de
Apply review feedback: instantiate type parameters consistently
dorodb-web22 Jan 22, 2026
1238df7
fix: use asSeenFrom to correctly instantiate type parameters in Signa…
dorodb-web22 Jan 23, 2026
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102 changes: 101 additions & 1 deletion presentation-compiler/src/main/dotty/tools/pc/SignatureHelpProvider.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -12,6 +12,10 @@ import dotty.tools.pc.printer.ShortenedTypePrinter.IncludeDefaultParam
import dotty.tools.pc.utils.InteractiveEnrichments.*
import org.eclipse.lsp4j as l

import dotty.tools.dotc.ast.tpd
import dotty.tools.dotc.core.Types.{MethodType, TypeVar}
import dotty.tools.dotc.util.Spans.Span

import scala.jdk.CollectionConverters.*
import scala.meta.pc.reports.ReportContext
import scala.meta.pc.OffsetParams
Expand Down Expand Up @@ -44,8 +48,9 @@ object SignatureHelpProvider:
.setPrinterFn(_ => ShortenedTypePrinter(search, IncludeDefaultParam.Never)(using indexedContext))

val (paramN, callableN, alternatives) = Signatures.signatureHelp(path, pos.span)
val refinedAlternatives = refineSignatures(alternatives, path, pos.span)(using driver.currentCtx)

val infos = alternatives.flatMap: signature =>
val infos = refinedAlternatives.flatMap: signature =>
signature.denot.map(signature -> _)

val signatureInfos = infos.map { case (signature, denot) =>
Expand Down Expand Up @@ -163,4 +168,99 @@ object SignatureHelpProvider:
markup.setValue(content.trim())
markup

private def refineSignatures(
help: List[Signatures.Signature],
path: List[tpd.Tree],
span: Span
)(using Context): List[Signatures.Signature] =
val enclosingApply = path.find {
case tpd.Apply(fun, _) => !fun.span.contains(span)
case _ => false
}

enclosingApply match
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@tgodzik tgodzik Jan 22, 2026

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I suggested using asSeenFrom methods and alike. This approach seems very manual and fragile. We should instead of fixing the created signature, fix the code that creates it.

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@dorodb-web22 dorodb-web22 Jan 23, 2026

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Thanks for the pointers.

I refactored the logic into Signatures.scala
using asSeenFrom and appliedTo as suggested. I also added the regression test for the parametric extension case.

The diff looks large mostly because I had to update the existing test expectations in SignatureHelpSuite to match the corrected signatures (e.g. showing [Int] instead of [T])

case Some(tpd.Apply(fun, _)) =>
help.map { signature =>
val matches = signature.denot.exists(_.symbol == fun.symbol)
if matches then
val (head, actions) = unwind(fun)
var currType = head.tpe.widenTermRefExpr
var actionIndex = 0

val newParamss = signature.paramss.map { paramList =>
paramList match
case (p: Signatures.MethodParam) :: _ =>
// Handle PolyTypes if they were skipped in signature but exist in type
while currType.isInstanceOf[dotty.tools.dotc.core.Types.PolyType] do
if actionIndex < actions.size && actions(actionIndex).isInstanceOf[tpd.TypeApply] then
val targs = actions(actionIndex).asInstanceOf[tpd.TypeApply].args.map(_.tpe)
currType = currType.appliedTo(targs)
actionIndex += 1
else
// Fallback: strip aliases or stop if strict matching fails
currType = currType.resultType

if currType.isInstanceOf[MethodType] then
val mt = currType.asInstanceOf[MethodType]
val paramNames = mt.paramNames.map(_.show)
val res = paramList.map {
case p: Signatures.MethodParam =>
val idx = paramNames.indexOf(p.name)
val shown = if idx >= 0 then mt.paramInfos(idx).widenTermRefExpr.show else p.tpe
if shown.contains("Any") || shown.contains("Nothing") || shown.contains("error") then p
else p.copy(tpe = shown)
case other => other
}
currType = mt.resultType
if actionIndex < actions.size && actions(actionIndex).isInstanceOf[tpd.Apply] then
actionIndex += 1
res
else paramList

case (p: Signatures.TypeParam) :: _ =>
if currType.isInstanceOf[dotty.tools.dotc.core.Types.PolyType] then
if actionIndex < actions.size && actions(actionIndex).isInstanceOf[tpd.TypeApply] then
val targs = actions(actionIndex).asInstanceOf[tpd.TypeApply].args.map(_.tpe)
val newParams = paramList.zip(targs).map {
case (p: Signatures.TypeParam, targ) =>
val shown = targ.show
if shown.contains("Any") || shown.contains("Nothing") || shown.contains("error") then p
else p.copy(tpe = shown)
case (p, _) => p
}
currType = currType.appliedTo(targs)
actionIndex += 1
newParams
else paramList
else paramList

case _ => paramList
}
// Update return type if we have consumed all actions and ended up with a result type
val finalType = currType.widen.show
val returnTypeLabel =
if actionIndex == actions.size && !finalType.contains("error") && !finalType.contains("Any") && !finalType.contains("Nothing") then Some(finalType)
else signature.returnType

signature.copy(paramss = newParamss, returnType = returnTypeLabel)
else signature
}
case _ => help

private def unwind(tree: tpd.Tree): (tpd.Tree, List[tpd.Tree]) =
tree match
case tpd.Apply(fn, _) =>
val (head, actions) = unwind(fn)
(head, actions :+ tree)
case tpd.TypeApply(fn, _) =>
val (head, actions) = unwind(fn)
(head, actions :+ tree)
case _ => (tree, Nil)

private def countParams(tree: tpd.Tree): Int =
tree match
case tpd.Apply(fun, _) => 1 + countParams(fun)
case tpd.TypeApply(fun, _) => 1 + countParams(fun)
case _ => 0

end SignatureHelpProvider
30 changes: 15 additions & 15 deletions presentation-compiler/test/dotty/tools/pc/tests/signaturehelp/SignatureHelpDocSuite.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -43,9 +43,9 @@ class SignatureHelpDocSuite extends BaseSignatureHelpSuite:
|}
""".stripMargin,
s"""Found documentation for scala/Option#fold().
|fold[B](ifEmpty: => B)(f: Int => B): B
| ^^^^^^^^^^^
| @param B Found documentation for type param B
|fold[String](ifEmpty: => String)(f: Int => String): String
| ^^^^^^^^^^^^^^^^
| @param String Found documentation for type param B
| @param ifEmpty Found documentation for param ifEmpty
| @param f Found documentation for param f
""".stripMargin
Expand All @@ -59,9 +59,9 @@ class SignatureHelpDocSuite extends BaseSignatureHelpSuite:
|}
""".stripMargin,
s"""|Found documentation for scala/Option#fold().
|fold[B](ifEmpty: => B)(f: Int => B): B
| ^^^^^^^^^^^^^
| @param B Found documentation for type param B
|fold[String](ifEmpty: => String)(f: Int => String): String
| ^^^^^^^^^^^^^^^^^^
| @param String Found documentation for type param B
| @param ifEmpty Found documentation for param ifEmpty
| @param f Found documentation for param f
|""".stripMargin
Expand All @@ -77,9 +77,9 @@ class SignatureHelpDocSuite extends BaseSignatureHelpSuite:
|}
""".stripMargin,
"""|Found documentation for scala/collection/LinearSeqOps#foldLeft().
|foldLeft[B](z: B)(op: (B, Int) => B): B
| ^^^^^^^^^^^^^^^^^
| @param B Found documentation for type param B
|foldLeft[Int](z: Int)(op: (Int, Int) => Int): Int
| ^^^^^^^^^^^^^^^^^^^^^
| @param Int Found documentation for type param B
| @param z Found documentation for param z
| @param op Found documentation for param op
|""".stripMargin
Expand Down Expand Up @@ -136,9 +136,9 @@ class SignatureHelpDocSuite extends BaseSignatureHelpSuite:
|}
""".stripMargin,
"""|Found documentation for java/util/Collections#singleton().
|singleton[T](o: T): java.util.Set[T]
| ^^^^
| @param T Found documentation for type param T
|singleton[Object](o: Object): java.util.Set[Object]
| ^^^^^^^^^
| @param Object Found documentation for type param T
| @param o Found documentation for param o
|""".stripMargin
)
Expand All @@ -151,7 +151,7 @@ class SignatureHelpDocSuite extends BaseSignatureHelpSuite:
|}
""".stripMargin,
"""|Found documentation for scala/util/control/Exception.Catch#
|Catch[T](pf: Catcher[T], fin: Option[Finally], rethrow: Throwable => Boolean)
|Catch[T](pf: Catcher[T], fin: Option[scala.util.control.Exception.Finally], rethrow: Throwable => Boolean)
| ^^^^^^^^^^^^^^
| @param pf Found documentation for param pf
| @param fin Found documentation for param fin
Expand Down Expand Up @@ -235,7 +235,7 @@ class SignatureHelpDocSuite extends BaseSignatureHelpSuite:
|}
""".stripMargin,
"""|Found documentation for scala/Some#
|Some[A](value: A)
| ^^^^^^^^
|Some[Int](value: Int): Some[Int]
| ^^^^^^^^^^
|""".stripMargin
)
73 changes: 36 additions & 37 deletions ...ion-compiler/test/dotty/tools/pc/tests/signaturehelp/SignatureHelpInterleavingSuite.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -30,8 +30,8 @@ class SignatureHelpInterleavingSuite extends BaseSignatureHelpSuite:
| pair[Int](@@1)[String]("1")
""".stripMargin,
"""
|pair[A](a: A)[B](b: B): (A, B)
| ^^^^
|pair[Int](a: Int)[B](b: B): (Int, B)
| ^^^^^^
|""".stripMargin
)

Expand All @@ -56,8 +56,8 @@ class SignatureHelpInterleavingSuite extends BaseSignatureHelpSuite:
| pair[Int](1)[String](@@"1")
""".stripMargin,
"""
|pair[A](a: A)[B](b: B): (A, B)
| ^^^^
|pair[Int](a: Int)[String](b: String): (Int, String)
| ^^^^^^^^^
|""".stripMargin
)

Expand All @@ -83,8 +83,8 @@ class SignatureHelpInterleavingSuite extends BaseSignatureHelpSuite:
| pair[Int](@@1)
""".stripMargin,
"""
|pair[A](a: A)[B](b: B): (A, B)
| ^^^^
|pair[Int](a: Int)[B](b: B): (Int, B)
| ^^^^^^
|""".stripMargin
)

Expand Down Expand Up @@ -121,8 +121,8 @@ class SignatureHelpInterleavingSuite extends BaseSignatureHelpSuite:
| pair(1@@)
""".stripMargin,
"""
|pair[A](a: A)[B](b: B): (A, B)
| ^^^^
|pair[Int](a: Int)[B](b: B): (Int, B)
| ^^^^^^
|""".stripMargin
)

Expand All @@ -147,8 +147,8 @@ class SignatureHelpInterleavingSuite extends BaseSignatureHelpSuite:
| pair(1)[String]("1"@@)
""".stripMargin,
"""
|pair[A](a: A)[B](b: B): (A, B)
| ^^^^
|pair[Int](a: Int)[String](b: String): (Int, String)
| ^^^^^^^^^
|""".stripMargin
)

Expand All @@ -160,8 +160,8 @@ class SignatureHelpInterleavingSuite extends BaseSignatureHelpSuite:
| pair(1)("1"@@)
""".stripMargin,
"""
|pair[A](a: A)[B](b: B): (A, B)
| ^^^^
|pair[Int](a: Int)[String](b: String): (Int, String)
| ^^^^^^^^^
|""".stripMargin
)

Expand All @@ -186,8 +186,8 @@ class SignatureHelpInterleavingSuite extends BaseSignatureHelpSuite:
| pair[Int](@@)
""".stripMargin,
"""
|pair[A](a: A)[B](b: B): (A, B)
| ^^^^
|pair[Int](a: Int)[B](b: B): (Int, B)
| ^^^^^^
|""".stripMargin
)

Expand All @@ -212,8 +212,8 @@ class SignatureHelpInterleavingSuite extends BaseSignatureHelpSuite:
| pair[Int](1)[String](@@)
""".stripMargin,
"""
|pair[A](a: A)[B](b: B): (A, B)
| ^^^^
|pair[Int](a: Int)[String](b: String): (Int, String)
| ^^^^^^^^^
|""".stripMargin
)

Expand All @@ -238,8 +238,8 @@ class SignatureHelpInterleavingSuite extends BaseSignatureHelpSuite:
| pair(1)(@@)
""".stripMargin,
"""
|pair[A](a: A)[B](b: B): (A, B)
| ^^^^
|pair[Int](a: Int)[B](b: B): (A, B)
| ^^^^
|""".stripMargin
)

Expand Down Expand Up @@ -329,8 +329,8 @@ class SignatureHelpInterleavingSuite extends BaseSignatureHelpSuite:
| pair[String]()[Int](@@)
""".stripMargin,
"""
|pair[A](a: A)[B](b: B): (A, B)
| ^^^^
|pair[String](a: String)[Int](b: Int): (String, Int)
| ^^^^^^
|""".stripMargin
)

Expand All @@ -355,8 +355,8 @@ class SignatureHelpInterleavingSuite extends BaseSignatureHelpSuite:
| pair[String](52)[Int](""@@)
""".stripMargin,
"""
|pair[A](a: A)[B](b: B): (A, B)
| ^^^^
|pair[String](a: String)[Int](b: Int): (String, Int)
| ^^^^^^
|""".stripMargin
)

Expand All @@ -368,8 +368,8 @@ class SignatureHelpInterleavingSuite extends BaseSignatureHelpSuite:
| foo[Int](using 1)(List(1, 2, 3))(@@)
""".stripMargin,
"""
|foo[A](using a: A)(b: List[A])[C <: a.type, D](cd: (C, D))[E]: Foo[A, B, C, D, E]
| ^^^^^^^^^^
|foo[Int](using a: Int)(b: List[Int])[(1 : Int), D](cd: (C, D))[E]: Foo[A, B, C, D, E]
| ^^^^^^^^^^
|""".stripMargin
)

Expand Down Expand Up @@ -418,8 +418,8 @@ class SignatureHelpInterleavingSuite extends BaseSignatureHelpSuite:
| def test[A](a: A)[B](b: B)[C](c: C)[D](d: D): Int = ???
| test[Int](@@)
|""".stripMargin,
"""|test[A](a: A)[B](b: B)[C](c: C)[D](d: D): Int
| ^^^^
"""|test[Int](a: Int)[B](b: B)[C](c: C)[D](d: D): Int
| ^^^^^^
|""".stripMargin
)

Expand All @@ -442,8 +442,8 @@ class SignatureHelpInterleavingSuite extends BaseSignatureHelpSuite:
| def test[A](a: A)[B](b: B)[C](c: C)[D](d: D): Int = ???
| test[Int](1)[String](@@)
|""".stripMargin,
"""|test[A](a: A)[B](b: B)[C](c: C)[D](d: D): Int
| ^^^^
"""|test[Int](a: Int)[String](b: String)[C](c: C)[D](d: D): Int
| ^^^^^^^^^
|""".stripMargin
)

Expand All @@ -466,8 +466,8 @@ class SignatureHelpInterleavingSuite extends BaseSignatureHelpSuite:
| def test[A](a: A)[B](b: B)[C](c: C)[D](d: D): Int = ???
| test[Int](1)[String]("1")[Int](@@)
|""".stripMargin,
"""|test[A](a: A)[B](b: B)[C](c: C)[D](d: D): Int
| ^^^^
"""|test[Int](a: Int)[String](b: String)[Int](c: Int)[D](d: D): Int
| ^^^^^^
|""".stripMargin
)

Expand All @@ -488,8 +488,8 @@ class SignatureHelpInterleavingSuite extends BaseSignatureHelpSuite:
| def test[A](a: A)[B](b: B)[C](c: C)[D](d: D): Int = ???
| test[Int](1)[String]("1")[Int](2)[String](@@)
|""".stripMargin,
"""|test[A](a: A)[B](b: B)[C](c: C)[D](d: D): Int
| ^^^^
"""|test[Int](a: Int)[String](b: String)[Int](c: Int)[String](d: String): Int
| ^^^^^^^^^
|""".stripMargin
)

Expand All @@ -499,8 +499,8 @@ class SignatureHelpInterleavingSuite extends BaseSignatureHelpSuite:
| def test[A](a: A)(using Int)[B](b: B)[C](c: C)[D](d: D): Int = ???
| test[Int](1)(using 5)[String]("1")[Int](@@)
|""".stripMargin,
"""|test[A](a: A)(using Int)[B](b: B)[C](c: C)[D](d: D): Int
| ^^^^
"""|test[Int](a: Int)(using Int)[String](b: String)[Int](c: Int)[D](d: D): Int
| ^^^^^^
|""".stripMargin
)

Expand All @@ -510,8 +510,7 @@ class SignatureHelpInterleavingSuite extends BaseSignatureHelpSuite:
| def test[A](a: A)(using Int)[B](b: B)[C](c: C)[D](d: D): Int = ???
| test[Int](1)(5)[String]("1")[Int](@@)
|""".stripMargin,
"""|test[A](a: A)(using Int)[B](b: B)[C](c: C)[D](d: D): Int
| ^^^^
"""|test[Int](a: Int)(using Int)[B](b: B)[String](c: String)[Int](d: Int): Int
| ^^^^^^^^^
|""".stripMargin
)

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