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merged 2 commits into from
Sep 5, 2025
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Mention named givens in double def explainer #23833

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912fce1
Mention named givens in double def explainer
som-snytt Aug 28, 2025
6fe3abc
Naming anon givens is a fix by renaming
som-snytt Sep 3, 2025
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38 changes: 28 additions & 10 deletions compiler/src/dotty/tools/dotc/reporting/messages.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -2364,7 +2364,7 @@ class SymbolIsNotAValue(symbol: Symbol)(using Context) extends TypeMsg(SymbolIsN
}

class DoubleDefinition(decl: Symbol, previousDecl: Symbol, base: Symbol)(using Context)
extends NamingMsg(DoubleDefinitionID) {
extends NamingMsg(DoubleDefinitionID):
import Signature.MatchDegree.*

private def erasedType: Type =
Expand Down Expand Up @@ -2426,6 +2426,25 @@ extends NamingMsg(DoubleDefinitionID) {
} + details
}
def explain(using Context) =
def givenAddendum =
def isGivenName(sym: Symbol) = sym.name.startsWith("given_") // Desugar.inventGivenName
def print(tpe: Type): String =
def addParams(tpe: Type): List[String] = tpe match
case tpe: MethodType =>
val s = if tpe.isContextualMethod then i"(${tpe.paramInfos}%, %) =>" else ""
s :: addParams(tpe.resType)
case tpe: PolyType =>
i"[${tpe.paramNames}%, %] =>" :: addParams(tpe.resType)
case tpe =>
i"$tpe" :: Nil
addParams(tpe).mkString(" ")
if decl.is(Given) && previousDecl.is(Given) && isGivenName(decl) && isGivenName(previousDecl) then
i"""| Provide an explicit, unique name to given definitions,
| since the names assigned to anonymous givens may clash. For example:
|
| given myGiven: ${print(atPhase(typerPhase)(decl.info))}
|"""
else ""
decl.signature.matchDegree(previousDecl.signature) match
case FullMatch =>
i"""
Expand All @@ -2439,30 +2458,29 @@ extends NamingMsg(DoubleDefinitionID) {
|
|In your code the two declarations
|
| ${previousDecl.showDcl}
| ${decl.showDcl}
| ${atPhase(typerPhase)(previousDecl.showDcl)}
| ${atPhase(typerPhase)(decl.showDcl)}
|
|erase to the identical signature
|
| ${erasedType}
|
|so the compiler cannot keep both: the generated bytecode symbols would collide.
|
|To fix this error, you need to disambiguate the two definitions. You can either:
|To fix this error, you must disambiguate the two definitions by doing one of the following:
|
|1. Rename one of the definitions, or
|1. Rename one of the definitions.$givenAddendum
|2. Keep the same names in source but give one definition a distinct
| bytecode-level name via `@targetName` for example:
| bytecode-level name via `@targetName`; for example:
|
| @targetName("${decl.name.show}_2")
| ${decl.showDcl}
| ${atPhase(typerPhase)(decl.showDcl)}
|
|Choose the `@targetName` argument carefully: it is the name that will be used
|when calling the method externally, so it should be unique and descriptive.
"""
|"""
case _ => ""

}
end DoubleDefinition

class ImportedTwice(sel: Name)(using Context) extends SyntaxMsg(ImportedTwiceID) {
def msg(using Context) = s"${sel.show} is imported twice on the same import line."
Expand Down
7 changes: 3 additions & 4 deletions tests/neg/i23350.check
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Expand Up @@ -31,16 +31,15 @@
|
| so the compiler cannot keep both: the generated bytecode symbols would collide.
|
| To fix this error, you need to disambiguate the two definitions. You can either:
| To fix this error, you must disambiguate the two definitions by doing one of the following:
|
| 1. Rename one of the definitions, or
| 1. Rename one of the definitions.
| 2. Keep the same names in source but give one definition a distinct
| bytecode-level name via `@targetName` for example:
| bytecode-level name via `@targetName`; for example:
|
| @targetName("apply_2")
| def apply(a: UndefOr2[String]): Unit
|
| Choose the `@targetName` argument carefully: it is the name that will be used
| when calling the method externally, so it should be unique and descriptive.
|
---------------------------------------------------------------------------------------------------------------------
7 changes: 3 additions & 4 deletions tests/neg/i23402.check
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Original file line number Diff line number Diff line change
Expand Up @@ -31,16 +31,15 @@
|
| so the compiler cannot keep both: the generated bytecode symbols would collide.
|
| To fix this error, you need to disambiguate the two definitions. You can either:
| To fix this error, you must disambiguate the two definitions by doing one of the following:
|
| 1. Rename one of the definitions, or
| 1. Rename one of the definitions.
| 2. Keep the same names in source but give one definition a distinct
| bytecode-level name via `@targetName` for example:
| bytecode-level name via `@targetName`; for example:
|
| @targetName("apply_2")
| def apply(p1: String)(p2: Int): A
|
| Choose the `@targetName` argument carefully: it is the name that will be used
| when calling the method externally, so it should be unique and descriptive.
|
---------------------------------------------------------------------------------------------------------------------
45 changes: 45 additions & 0 deletions tests/neg/i23832a.check
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@@ -0,0 +1,45 @@
-- [E120] Naming Error: tests/neg/i23832a.scala:9:8 --------------------------------------------------------------------
9 | given Special[Option[Int]] = ??? // error
| ^
| Conflicting definitions:
| final lazy given val given_Special_Option: Special[Option[Long]] in object syntax at line 8 and
| final lazy given val given_Special_Option: Special[Option[Int]] in object syntax at line 9
|---------------------------------------------------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
|
| As part of the Scala compilation pipeline every type is reduced to its erased
| (runtime) form. In this phase, among other transformations, generic parameters
| disappear and separate parameter-list boundaries are flattened.
|
| For example, both `f[T](x: T)(y: String): Unit` and `f(x: Any, z: String): Unit`
| erase to the same runtime signature `f(x: Object, y: String): Unit`. Note that
| parameter names are irrelevant.
|
| In your code the two declarations
|
| final lazy given val given_Special_Option: Special[Option[Long]]
| final lazy given val given_Special_Option: Special[Option[Int]]
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shouldn't it be something like:

final lazy given Special[Option[Long]] = ???
final lazy given Special[Option[Int]] = ???

which leads to 2 identical signatures: given_Special_Option?

When Developer names the vals, it is very obvious which names the Developer gives them - problems usually start with non-obvious auto-naming.

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The declarations are not printed with "fidelity". That could be fixed as a follow-up. (I assume but did not check that the syntax changed faster than the printer.)

|
| erase to the identical signature
|
| Special
|
| so the compiler cannot keep both: the generated bytecode symbols would collide.
|
| To fix this error, you must disambiguate the two definitions by doing one of the following:
|
| 1. Rename one of the definitions. Provide an explicit, unique name to given definitions,
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maybe Rename one or all the definitions.

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All these errors fail at the first pair that is noticed; and "already defined" in particular works with just minimal info; it does not try to enumerate all problematic definitions and their types. The spec will sometimes say "pairwise distinct", and the user only needs to fix one pair at a time.

| since the names assigned to anonymous givens may clash. For example:
|
| given myGiven: Special[Option[Int]]
|
| 2. Keep the same names in source but give one definition a distinct
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and ..., but give one or to all of them a distinct - like we should not suggest to rename only one of them - sometimes renaming them all makes more sense and improves consistency

| bytecode-level name via `@targetName`; for example:
|
| @targetName("given_Special_Option_2")
| final lazy given val given_Special_Option: Special[Option[Int]]
|
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there is 3rd option on how to name them, specifically in for-comprehension (it could shadow previous one):

for {
  // ...
  givenName @ given Type <- ??? 
  // ...

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@mr-git that is a different error ID, though a similar explainer about "how to name givens" may be warranted (since it is not obvious); presumably it is obvious how to rename other things?

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Will you create a ticket and PR for that? I struggled with this case the most.

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@mr-git I added some explain text on the other PR. There is no pattern syntax for given here because there is no equivalent to val Extract(x, y) = expr.

| Choose the `@targetName` argument carefully: it is the name that will be used
| when calling the method externally, so it should be unique and descriptive.
---------------------------------------------------------------------------------------------------------------------
9 changes: 9 additions & 0 deletions tests/neg/i23832a.scala
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@@ -0,0 +1,9 @@
//> using options -explain

// follow-up to neg/i23402*.scala

trait Special[A]

object syntax:
given Special[Option[Long]] = ???
given Special[Option[Int]] = ??? // error
45 changes: 45 additions & 0 deletions tests/neg/i23832b.check
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@@ -0,0 +1,45 @@
-- [E120] Naming Error: tests/neg/i23832b.scala:9:8 --------------------------------------------------------------------
9 | given [A] => Special[Option[A]] = ??? // error
| ^
| Conflicting definitions:
| final lazy given val given_Special_Option: Special[Option[Long]] in object syntax at line 8 and
| final given def given_Special_Option[A]: Special[Option[A]] in object syntax at line 9
|---------------------------------------------------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
|
| As part of the Scala compilation pipeline every type is reduced to its erased
| (runtime) form. In this phase, among other transformations, generic parameters
| disappear and separate parameter-list boundaries are flattened.
|
| For example, both `f[T](x: T)(y: String): Unit` and `f(x: Any, z: String): Unit`
| erase to the same runtime signature `f(x: Object, y: String): Unit`. Note that
| parameter names are irrelevant.
|
| In your code the two declarations
|
| final lazy given val given_Special_Option: Special[Option[Long]]
| final given def given_Special_Option[A]: Special[Option[A]]
|
| erase to the identical signature
|
| (): Special
|
| so the compiler cannot keep both: the generated bytecode symbols would collide.
|
| To fix this error, you must disambiguate the two definitions by doing one of the following:
|
| 1. Rename one of the definitions. Provide an explicit, unique name to given definitions,
| since the names assigned to anonymous givens may clash. For example:
|
| given myGiven: [A] => Special[Option[A]]
|
| 2. Keep the same names in source but give one definition a distinct
| bytecode-level name via `@targetName`; for example:
|
| @targetName("given_Special_Option_2")
| final given def given_Special_Option[A]: Special[Option[A]]
|
| Choose the `@targetName` argument carefully: it is the name that will be used
| when calling the method externally, so it should be unique and descriptive.
---------------------------------------------------------------------------------------------------------------------
9 changes: 9 additions & 0 deletions tests/neg/i23832b.scala
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Original file line number Diff line number Diff line change
@@ -0,0 +1,9 @@
//> using options -explain

// follow-up to neg/i23402*.scala

trait Special[A]

object syntax:
given Special[Option[Long]] = ???
given [A] => Special[Option[A]] = ??? // error
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