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More improvements to error messages #23721

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8ed4527
Simplify ParamRef disambiguation
odersky Aug 12, 2025
21562b6
Perform subscript disambiguation also for error message addenda
odersky Aug 12, 2025
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24 changes: 11 additions & 13 deletions compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
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Expand Up @@ -1079,16 +1079,15 @@ class CheckCaptures extends Recheck, SymTransformer:
| Externally visible type: $expected""",
tree.srcPos)

def addenda(expected: Type) = new Addenda:
override def toAdd(using Context) =
def result = if tree.isInstanceOf[ValDef] then"" else " result"
i"""
|
|Note that the expected type $expected
|is the previously inferred$result type of $sym
|which is also the type seen in separately compiled sources.
|The new inferred type $tp
|must conform to this type.""" :: Nil
def addenda(expected: Type) = Addenda:
def result = if tree.isInstanceOf[ValDef] then"" else " result"
i"""
|
|Note that the expected type $expected
|is the previously inferred$result type of $sym
|which is also the type seen in separately compiled sources.
|The new inferred type $tp
|must conform to this type."""

tree.tpt match
case tpt: InferredTypeTree if !canUseInferred =>
Expand Down Expand Up @@ -1276,6 +1275,7 @@ class CheckCaptures extends Recheck, SymTransformer:
else new Addenda:
override def toAdd(using Context) = notes.map: note =>
i"""
|
|Note that ${note.description}."""

/** Addendas for error messages that show where we have under-approximated by
Expand All @@ -1291,13 +1291,11 @@ class CheckCaptures extends Recheck, SymTransformer:
(parent, " deep")
case _ =>
(original, "")*/
add ++ new Addenda:
override def toAdd(using Context): List[String] =
add ++ Addenda:
i"""
|
|Note that a capability $ref in a capture set appearing in contravariant position
|was mapped to $mapped which is not a capability. Therefore, it was under-approximated to the empty set."""
:: Nil
case _ =>
foldOver(add, t)

Expand Down
28 changes: 6 additions & 22 deletions compiler/src/dotty/tools/dotc/reporting/Message.scala
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Expand Up @@ -62,13 +62,6 @@ object Message:
*/
private class Seen(disambiguate: Disambiguation):

/** The set of lambdas that were opened at some point during printing. */
private val openedLambdas = new collection.mutable.HashSet[LambdaType]

/** Register that `tp` was opened during printing. */
def openLambda(tp: LambdaType): Unit =
openedLambdas += tp

val seen = new collection.mutable.HashMap[SeenKey, List[Recorded]].withDefaultValue(Nil)

var nonSensical = false
Expand Down Expand Up @@ -109,16 +102,17 @@ object Message:
lazy val dealiased = followAlias(entry)

/** All lambda parameters with the same name are given the same superscript as
* long as their corresponding binder has been printed.
* See tests/neg/lambda-rename.scala for test cases.
* long as their corresponding binders have the same parameter name lists.
* This avoids spurious distinctions between parameters of mapped lambdas at
* the risk that sometimes we cannot distinguish parameters of distinct functions
* that have the same parameter names. See tests/neg/lambda-rename.scala for test cases.
*/
def sameSuperscript(cur: Recorded, existing: Recorded) =
(cur eq existing) ||
(cur, existing).match
case (cur: ParamRef, existing: ParamRef) =>
(cur.paramName eq existing.paramName) &&
openedLambdas.contains(cur.binder) &&
openedLambdas.contains(existing.binder)
(cur.paramName eq existing.paramName)
&& cur.binder.paramNames == existing.binder.paramNames
case _ =>
false

Expand Down Expand Up @@ -266,20 +260,10 @@ object Message:
case _ =>
super.funMiddleText(isContextual, isPure, refs)

override def toTextMethodAsFunction(info: Type, isPure: Boolean, refs: GeneralCaptureSet | Null): Text =
info match
case info: LambdaType =>
seen.openLambda(info)
case _ =>
super.toTextMethodAsFunction(info, isPure, refs)

override def toText(tp: Type): Text =
if !tp.exists || tp.isErroneous then seen.nonSensical = true
tp match
case tp: TypeRef if useSourceModule(tp.symbol) => Str("object ") ~ super.toText(tp)
case tp: LambdaType =>
seen.openLambda(tp)
super.toText(tp)
case _ => super.toText(tp)

override def toText(sym: Symbol): Text =
Expand Down
10 changes: 5 additions & 5 deletions compiler/src/dotty/tools/dotc/reporting/messages.scala
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Expand Up @@ -17,7 +17,7 @@ import ast.Trees
import config.{Feature, MigrationVersion, ScalaVersion}
import transform.patmat.Space
import transform.patmat.SpaceEngine
import typer.ErrorReporting.{err, matchReductionAddendum, substitutableTypeSymbolsInScope}
import typer.ErrorReporting.{err, matchReductionAddendum, substitutableTypeSymbolsInScope, Addenda, NothingToAdd}
import typer.ProtoTypes.{ViewProto, SelectionProto, FunProto}
import typer.Implicits.*
import typer.Inferencing
Expand Down Expand Up @@ -297,7 +297,7 @@ extends NotFoundMsg(MissingIdentID) {
}
}

class TypeMismatch(val found: Type, expected: Type, val inTree: Option[untpd.Tree], addenda: => String*)(using Context)
class TypeMismatch(val found: Type, expected: Type, val inTree: Option[untpd.Tree], addenda: Addenda = NothingToAdd)(using Context)
extends TypeMismatchMsg(found, expected)(TypeMismatchID):

def msg(using Context) =
Expand Down Expand Up @@ -349,12 +349,12 @@ class TypeMismatch(val found: Type, expected: Type, val inTree: Option[untpd.Tre
|Required: $expectedStr${reported.notes}"""
end msg

override def msgPostscript(using Context) =
override def msgPostscript(using Context): String =
def importSuggestions =
if expected.isTopType || found.isBottomType then ""
else ctx.typer.importSuggestionAddendum(ViewProto(found.widen, expected))
super.msgPostscript
++ addenda.dropWhile(_.isEmpty).headOption.getOrElse(importSuggestions)

addenda.toAdd.mkString ++ super.msgPostscript ++ importSuggestions

override def explain(using Context) =
val treeStr = inTree.map(x => s"\nTree:\n\n${x.show}\n").getOrElse("")
Expand Down
21 changes: 13 additions & 8 deletions compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala
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Expand Up @@ -73,12 +73,16 @@ object ErrorReporting {

/** A mixin trait that can produce added elements for an error message */
trait Addenda:
self =>
def toAdd(using Context): List[String] = Nil
def ++ (follow: Addenda) = new Addenda:
override def toAdd(using Context) = self.toAdd ++ follow.toAdd
def toAdd(using Context): List[String]
def ++(follow: Addenda) = new Addenda:
def toAdd(using Context) = Addenda.this.toAdd ++ follow.toAdd

object Addenda:
def apply(msg: Context ?=> String): Addenda = new Addenda:
def toAdd(using Context) = msg :: Nil

object NothingToAdd extends Addenda
object NothingToAdd extends Addenda:
def toAdd(using Context): List[String] = Nil

class Errors(using Context) {

Expand Down Expand Up @@ -195,10 +199,11 @@ object ErrorReporting {

def missingElse = tree match
case If(_, _, elsep @ Literal(Constant(()))) if elsep.span.isSynthetic =>
"\nMaybe you are missing an else part for the conditional?"
case _ => ""
Addenda("\nMaybe you are missing an else part for the conditional?")
case _ =>
NothingToAdd

errorTree(tree, TypeMismatch(treeTp, expectedTp, Some(tree), (addenda.toAdd :+ missingElse)*))
errorTree(tree, TypeMismatch(treeTp, expectedTp, Some(tree), addenda ++ missingElse))
}

/** A subtype log explaining why `found` does not conform to `expected` */
Expand Down
2 changes: 2 additions & 0 deletions compiler/src/dotty/tools/dotc/typer/Implicits.scala
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Expand Up @@ -481,6 +481,8 @@ object Implicits:
if (argument.isEmpty) i"match type ${clarify(expectedType)}"
else i"convert from ${argument.tpe} to ${clarify(expectedType)}"
}

def toAdd(using Context) = Nil
}

class NoMatchingImplicits(val expectedType: Type, val argument: Tree, constraint: Constraint = OrderingConstraint.empty)
Expand Down
2 changes: 2 additions & 0 deletions tests/neg-custom-args/captures/box-adapt-cases.check
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Expand Up @@ -3,6 +3,7 @@
| ^^^^^^^^^^^^^^^^
| Found: (cap: Cap^{io}) ->{io} Int
| Required: Cap^{io} -> Int
|
| Note that capability io is not included in capture set {}.
|
| longer explanation available when compiling with `-explain`
Expand All @@ -11,6 +12,7 @@
| ^^^^^^^^^^^^^^^^
| Found: (cap: Cap^'s1) ->{io, fs} Int
| Required: Cap^{io, fs} ->{io} Int
|
| Note that capability fs is not included in capture set {io}.
|
| longer explanation available when compiling with `-explain`
6 changes: 4 additions & 2 deletions tests/neg-custom-args/captures/byname.check
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Expand Up @@ -10,16 +10,17 @@
|Found: () ?->{cap1} Int ->{cap1} Int
|Required: () ?=> Int ->{cap2} Int
|
|where: ?=> refers to a fresh root capability created in method test when checking argument to parameter ff of method h
|
|Note that capability cap1 is not included in capture set {cap2}.
|
|where: ?=> refers to a fresh root capability created in method test when checking argument to parameter ff of method h
|
| longer explanation available when compiling with `-explain`
-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/byname.scala:19:5 ----------------------------------------
19 | h(g()) // error
| ^^^
| Found: () ?->{cap2} I^'s1
| Required: () ?->{cap1} I
|
| Note that capability cap2 is not included in capture set {cap1}.
|
| longer explanation available when compiling with `-explain`
Expand All @@ -28,6 +29,7 @@
| ^^^^^^^^^
| Found: () ->{cap2} I^'s2
| Required: () ->{cap1} I
|
| Note that capability cap2 is not included in capture set {cap1}.
|
| longer explanation available when compiling with `-explain`
4 changes: 2 additions & 2 deletions tests/neg-custom-args/captures/capt-depfun.check
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Expand Up @@ -4,8 +4,8 @@
| Found: Str^{} ->{ac, y, z} Str^{y, z}
| Required: Str^{y, z} => Str^{y, z}
|
| where: => refers to a fresh root capability in the type of value dc
|
| Note that capability y is not included in capture set {}.
|
| where: => refers to a fresh root capability in the type of value dc
|
| longer explanation available when compiling with `-explain`
25 changes: 17 additions & 8 deletions tests/neg-custom-args/captures/capt1.check
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Expand Up @@ -3,6 +3,7 @@
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| Found: () ->{x} C
| Required: () -> C
|
| Note that capability x is not included in capture set {}.
|
| longer explanation available when compiling with `-explain`
Expand All @@ -11,6 +12,7 @@
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| Found: () ->{x} C^'s1
| Required: Matchable
|
| Note that capability x is not included in capture set {}.
|
| longer explanation available when compiling with `-explain`
Expand All @@ -19,6 +21,7 @@
| ^
| Found: (y: Int) ->{x} Int
| Required: Matchable
|
| Note that capability x is not included in capture set {}.
16 | f
|
Expand All @@ -28,6 +31,7 @@
| ^
| Found: A^{x}
| Required: A
|
| Note that capability x is not included in capture set {}.
23 | def m() = if x == null then y else y
24 | F(22)
Expand All @@ -38,6 +42,7 @@
| ^
| Found: A^{x}
| Required: A
|
| Note that capability x is not included in capture set {}.
28 | def m() = if x == null then y else y
|
Expand All @@ -55,11 +60,13 @@
|Found: () ->'s2 C^
|Required: () -> C^²
|
|where: ^ refers to a root capability associated with the result type of (): C^
| ^² refers to a fresh root capability created in value z2 when checking argument to parameter a of method h
|Note that capability cap is not included in capture set {cap²}
|because cap is not visible from cap² in value z2.
|
|Note that capability <cap of (): C^> is not included in capture set {cap}
|because <cap of (): C^> is not visible from cap in value z2.
|where: ^ refers to a root capability associated with the result type of (): C^
| ^² refers to a fresh root capability created in value z2 when checking argument to parameter a of method h
| cap is a root capability associated with the result type of (): C^
| cap² is a fresh root capability created in value z2 when checking argument to parameter a of method h
|
| longer explanation available when compiling with `-explain`
-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/capt1.scala:37:5 -----------------------------------------
Expand All @@ -68,11 +75,13 @@
|Found: () ->'s3 C^
|Required: () -> C^²
|
|where: ^ refers to a root capability associated with the result type of (): C^
| ^² refers to a fresh root capability created in value z2 when checking argument to parameter b of method h
|Note that capability cap is not included in capture set {cap²}
|because cap is not visible from cap² in value z2.
|
|Note that capability <cap of (): C^> is not included in capture set {cap}
|because <cap of (): C^> is not visible from cap in value z2.
|where: ^ refers to a root capability associated with the result type of (): C^
| ^² refers to a fresh root capability created in value z2 when checking argument to parameter b of method h
| cap is a root capability associated with the result type of (): C^
| cap² is a fresh root capability created in value z2 when checking argument to parameter b of method h
|
| longer explanation available when compiling with `-explain`
-- Error: tests/neg-custom-args/captures/capt1.scala:38:13 -------------------------------------------------------------
Expand Down
50 changes: 32 additions & 18 deletions tests/neg-custom-args/captures/cc-existential-conformance.check
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Expand Up @@ -4,24 +4,31 @@
| Found: (x : A -> (x²: A) -> B^)
| Required: A -> A -> B^²
|
| where: ^ refers to a root capability associated with the result type of (x²: A): B^
| ^² refers to a fresh root capability in the type of value y
| x is a value in method test
| x² is a reference to a value parameter
| Note that capability cap is not included in capture set {cap²}
| because cap is not visible from cap² in value y.
|
| Note that capability <cap of (x: A): B^> is not included in capture set {cap}
| because <cap of (x: A): B^> is not visible from cap in value y.
| where: ^ refers to a root capability associated with the result type of (x²: A): B^
| ^² refers to a fresh root capability in the type of value y
| cap is a root capability associated with the result type of (x²: A): B^
| cap² is a fresh root capability in the type of value y
| x is a value in method test
| x² is a reference to a value parameter
|
| longer explanation available when compiling with `-explain`
-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/cc-existential-conformance.scala:9:29 --------------------
9 | val z: A -> (x: A) -> B^ = y // error
| ^
| Found: A -> A -> B^{y*}
| Required: A -> (x: A) -> B^
| Found: A -> A -> B^{y*}
| Required: A -> (x: A) -> B^
|
| where: ^ refers to a root capability associated with the result type of (x: A): B^
| Note that capability y* is not included in capture set {cap}.
|
| Note that capability y* is not included in capture set {<cap of (x: A): B^>}.
| Note that the existential capture root in B^²
| cannot subsume the capability y* since that capability is not a `Sharable` capability..
|
| where: ^ refers to a root capability associated with the result type of (x: A): B^
| ^² refers to the universal root capability
| cap is a root capability associated with the result type of (x: A): B^
|
| longer explanation available when compiling with `-explain`
-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/cc-existential-conformance.scala:13:19 -------------------
Expand All @@ -30,13 +37,15 @@
| Found: (x : (x²: A) -> B^)
| Required: A -> B^²
|
| where: ^ refers to a root capability associated with the result type of (x²: A): B^
| ^² refers to a fresh root capability in the type of value y
| x is a value in method test2
| x² is a reference to a value parameter
| Note that capability cap is not included in capture set {cap²}
| because cap is not visible from cap² in value y.
|
| Note that capability <cap of (x: A): B^> is not included in capture set {cap}
| because <cap of (x: A): B^> is not visible from cap in value y.
| where: ^ refers to a root capability associated with the result type of (x²: A): B^
| ^² refers to a fresh root capability in the type of value y
| cap is a root capability associated with the result type of (x²: A): B^
| cap² is a fresh root capability in the type of value y
| x is a value in method test2
| x² is a reference to a value parameter
|
| longer explanation available when compiling with `-explain`
-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/cc-existential-conformance.scala:14:24 -------------------
Expand All @@ -45,8 +54,13 @@
| Found: A -> B^{y*}
| Required: (x: A) -> B^
|
| where: ^ refers to a root capability associated with the result type of (x: A): B^
| Note that capability y* is not included in capture set {cap}.
|
| Note that the existential capture root in B^²
| cannot subsume the capability y* since that capability is not a `Sharable` capability..
|
| Note that capability y* is not included in capture set {<cap of (x: A): B^>}.
| where: ^ refers to a root capability associated with the result type of (x: A): B^
| ^² refers to the universal root capability
| cap is a root capability associated with the result type of (x: A): B^
|
| longer explanation available when compiling with `-explain`
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