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SIP-56: Better foundations for match types #18262

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Dec 18, 2023
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c96d847
Introduce MatchTypeCaseSpec to categorize match type cases.
sjrd Jul 20, 2023
5260c60
Use new specced match types for class type constructors.
sjrd Nov 24, 2023
cc41d48
Use new specced match types for `scala.compiletime.int.S[n]`.
sjrd Aug 4, 2023
067b828
Short-circuit match type cases with missing captures in their patterns.
sjrd Nov 24, 2023
aa8d348
Use the specced match types for abstract tycons in patterns.
sjrd Aug 4, 2023
97725d7
Handle type member extractors as specced match types.
sjrd Aug 8, 2023
7ab7b0f
Report a compile error on illegal match types.
sjrd Nov 24, 2023
ec94ff5
Allow to reduce type member extractors when the member is a class.
sjrd Nov 24, 2023
1b2a16e
New implementation of `provablyDisjoint` to match SIP-56.
sjrd Aug 17, 2023
f432d08
Be more specific about higher-kinded types in provablyDisjoint.
sjrd Aug 29, 2023
c653793
Do not use provablyEmpty anymore; use S <: T + provablyDisjoint(S, T)…
sjrd Nov 24, 2023
16cf4f2
Add regression tests for old issues fixed with the new match types.
sjrd Aug 30, 2023
c8b4da8
Under -source:3.3 and below, always use the legacy match type algorithm.
sjrd Nov 24, 2023
2dd9f45
In type assigner for Apply, carry ErrorType's from the `fn`.
sjrd Nov 29, 2023
ec097a9
Fix 1 cause of infinite recursion in the new provablyDisjoint.
sjrd Dec 18, 2023
b5a4ca5
Demonstrate more potential unsoundness with a CCE reproducer.
sjrd Dec 8, 2023
c3b9d9b
Add comments that link to relevant parts of the spec of SIP-56.
sjrd Dec 15, 2023
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20 changes: 14 additions & 6 deletions compiler/src/dotty/tools/dotc/core/MatchTypeTrace.scala
Original file line number Diff line number Diff line change
Expand Up @@ -12,8 +12,8 @@ object MatchTypeTrace:

private enum TraceEntry:
case TryReduce(scrut: Type)
case Stuck(scrut: Type, stuckCase: Type, otherCases: List[Type])
case NoInstance(scrut: Type, stuckCase: Type, fails: List[(Name, TypeBounds)])
case Stuck(scrut: Type, stuckCase: MatchTypeCaseSpec, otherCases: List[MatchTypeCaseSpec])
case NoInstance(scrut: Type, stuckCase: MatchTypeCaseSpec, fails: List[(Name, TypeBounds)])
case EmptyScrutinee(scrut: Type)
import TraceEntry.*

Expand Down Expand Up @@ -54,10 +54,10 @@ object MatchTypeTrace:
* not disjoint from it either, which means that the remaining cases `otherCases`
* cannot be visited. Only the first failure is recorded.
*/
def stuck(scrut: Type, stuckCase: Type, otherCases: List[Type])(using Context) =
def stuck(scrut: Type, stuckCase: MatchTypeCaseSpec, otherCases: List[MatchTypeCaseSpec])(using Context) =
matchTypeFail(Stuck(scrut, stuckCase, otherCases))

def noInstance(scrut: Type, stuckCase: Type, fails: List[(Name, TypeBounds)])(using Context) =
def noInstance(scrut: Type, stuckCase: MatchTypeCaseSpec, fails: List[(Name, TypeBounds)])(using Context) =
matchTypeFail(NoInstance(scrut, stuckCase, fails))

/** Record a failure that scrutinee `scrut` is provably empty.
Expand All @@ -80,13 +80,16 @@ object MatchTypeTrace:
case _ =>
op

def caseText(spec: MatchTypeCaseSpec)(using Context): String =
caseText(spec.origMatchCase)

def caseText(tp: Type)(using Context): String = tp match
case tp: HKTypeLambda => caseText(tp.resultType)
case defn.MatchCase(any, body) if any eq defn.AnyType => i"case _ => $body"
case defn.MatchCase(pat, body) => i"case $pat => $body"
case _ => i"case $tp"

private def casesText(cases: List[Type])(using Context) =
private def casesText(cases: List[MatchTypeCaseSpec])(using Context) =
i"${cases.map(caseText)}%\n %"

private def explainEntry(entry: TraceEntry)(using Context): String = entry match
Expand Down Expand Up @@ -116,10 +119,15 @@ object MatchTypeTrace:
| ${fails.map((name, bounds) => i"$name$bounds")}%\n %"""

/** The failure message when the scrutinee `scrut` does not match any case in `cases`. */
def noMatchesText(scrut: Type, cases: List[Type])(using Context): String =
def noMatchesText(scrut: Type, cases: List[MatchTypeCaseSpec])(using Context): String =
i"""failed since selector $scrut
|matches none of the cases
|
| ${casesText(cases)}"""

def illegalPatternText(scrut: Type, cas: MatchTypeCaseSpec.LegacyPatMat)(using Context): String =
i"""The match type contains an illegal case:
| ${caseText(cas)}
|(this error can be ignored for now with `-source:3.3`)"""

end MatchTypeTrace
709 changes: 493 additions & 216 deletions compiler/src/dotty/tools/dotc/core/TypeComparer.scala

Large diffs are not rendered by default.

190 changes: 188 additions & 2 deletions compiler/src/dotty/tools/dotc/core/Types.scala
Original file line number Diff line number Diff line change
Expand Up @@ -7,7 +7,7 @@ import Flags.*
import Names.*
import StdNames.*, NameOps.*
import NullOpsDecorator.*
import NameKinds.SkolemName
import NameKinds.{SkolemName, WildcardParamName}
import Scopes.*
import Constants.*
import Contexts.*
Expand All @@ -30,6 +30,8 @@ import Hashable.*
import Uniques.*
import collection.mutable
import config.Config
import config.Feature.sourceVersion
import config.SourceVersion
import annotation.{tailrec, constructorOnly}
import scala.util.hashing.{ MurmurHash3 => hashing }
import config.Printers.{core, typr, matchTypes}
Expand Down Expand Up @@ -5036,7 +5038,7 @@ object Types extends TypeUtils {
trace(i"reduce match type $this $hashCode", matchTypes, show = true)(inMode(Mode.Type) {
def matchCases(cmp: TrackingTypeComparer): Type =
val saved = ctx.typerState.snapshot()
try cmp.matchCases(scrutinee.normalized, cases)
try cmp.matchCases(scrutinee.normalized, cases.map(MatchTypeCaseSpec.analyze(_)))
catch case ex: Throwable =>
handleRecursive("reduce type ", i"$scrutinee match ...", ex)
finally
Expand Down Expand Up @@ -5088,6 +5090,190 @@ object Types extends TypeUtils {
case _ => None
}

enum MatchTypeCasePattern:
case Capture(num: Int, isWildcard: Boolean)
case TypeTest(tpe: Type)
case BaseTypeTest(classType: TypeRef, argPatterns: List[MatchTypeCasePattern], needsConcreteScrut: Boolean)
case CompileTimeS(argPattern: MatchTypeCasePattern)
case AbstractTypeConstructor(tycon: Type, argPatterns: List[MatchTypeCasePattern])
case TypeMemberExtractor(typeMemberName: TypeName, capture: Capture)

def isTypeTest: Boolean =
this.isInstanceOf[TypeTest]

def needsConcreteScrutInVariantPos: Boolean = this match
case Capture(_, isWildcard) => !isWildcard
case TypeTest(_) => false
case _ => true
end MatchTypeCasePattern

enum MatchTypeCaseSpec:
case SubTypeTest(origMatchCase: Type, pattern: Type, body: Type)
case SpeccedPatMat(origMatchCase: HKTypeLambda, captureCount: Int, pattern: MatchTypeCasePattern, body: Type)
case LegacyPatMat(origMatchCase: HKTypeLambda)
case MissingCaptures(origMatchCase: HKTypeLambda, missing: collection.BitSet)

def origMatchCase: Type
end MatchTypeCaseSpec

object MatchTypeCaseSpec:
def analyze(cas: Type)(using Context): MatchTypeCaseSpec =
cas match
case cas: HKTypeLambda if !sourceVersion.isAtLeast(SourceVersion.`3.4`) =>
// Always apply the legacy algorithm under -source:3.3 and below
LegacyPatMat(cas)
case cas: HKTypeLambda =>
val defn.MatchCase(pat, body) = cas.resultType: @unchecked
val missing = checkCapturesPresent(cas, pat)
if !missing.isEmpty then
MissingCaptures(cas, missing)
else
val specPattern = tryConvertToSpecPattern(cas, pat)
if specPattern != null then
SpeccedPatMat(cas, cas.paramNames.size, specPattern, body)
else
LegacyPatMat(cas)
case _ =>
val defn.MatchCase(pat, body) = cas: @unchecked
SubTypeTest(cas, pat, body)
end analyze

/** Checks that all the captures of the case are present in the case.
*
* Sometimes, because of earlier substitutions of an abstract type constructor,
* we can end up with patterns that do not mention all their captures anymore.
* This can happen even when the body still refers to these missing captures.
* In that case, we must always consider the case to be unmatchable, i.e., to
* become `Stuck`.
*
* See pos/i12127.scala for an example.
*/
def checkCapturesPresent(cas: HKTypeLambda, pat: Type)(using Context): collection.BitSet =
val captureCount = cas.paramNames.size
val missing = new mutable.BitSet(captureCount)
missing ++= (0 until captureCount)
new CheckCapturesPresent(cas).apply(missing, pat)

private class CheckCapturesPresent(cas: HKTypeLambda)(using Context) extends TypeAccumulator[mutable.BitSet]:
def apply(missing: mutable.BitSet, tp: Type): mutable.BitSet = tp match
case TypeParamRef(binder, num) if binder eq cas =>
missing -= num
case _ =>
foldOver(missing, tp)
end CheckCapturesPresent

/** Tries to convert a match type case pattern in HKTypeLambda form into a spec'ed `MatchTypeCasePattern`.
*
* This method recovers the structure of *legal patterns* as defined in SIP-56
* from the unstructured `HKTypeLambda` coming from the typer.
*
* It must adhere to the specification of legal patterns defined at
* https://docs.scala-lang.org/sips/match-types-spec.html#legal-patterns
*
* Returns `null` if the pattern in `caseLambda` is a not a legal pattern.
*/
private def tryConvertToSpecPattern(caseLambda: HKTypeLambda, pat: Type)(using Context): MatchTypeCasePattern | Null =
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This should have a doc comment explaining which patterns can be converted to spec patterns.

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I added a commit with comments linking to the spec on this method, on matchSpeccedPatMat and on provablyDisjoint. They are the 3 core methods that directly map to a specified algorithm of SIP-56.

var typeParamRefsAccountedFor: Int = 0

def rec(pat: Type, variance: Int): MatchTypeCasePattern | Null =
pat match
case pat @ TypeParamRef(binder, num) if binder eq caseLambda =>
typeParamRefsAccountedFor += 1
MatchTypeCasePattern.Capture(num, isWildcard = pat.paramName.is(WildcardParamName))

case pat @ AppliedType(tycon: TypeRef, args) if variance == 1 =>
val tyconSym = tycon.symbol
if tyconSym.isClass then
if tyconSym.name.startsWith("Tuple") && defn.isTupleNType(pat) then
rec(pat.toNestedPairs, variance)
else
recArgPatterns(pat) { argPatterns =>
val needsConcreteScrut = argPatterns.zip(tycon.typeParams).exists {
(argPattern, tparam) => tparam.paramVarianceSign != 0 && argPattern.needsConcreteScrutInVariantPos
}
MatchTypeCasePattern.BaseTypeTest(tycon, argPatterns, needsConcreteScrut)
}
else if defn.isCompiletime_S(tyconSym) && args.sizeIs == 1 then
val argPattern = rec(args.head, variance)
if argPattern == null then
null
else if argPattern.isTypeTest then
MatchTypeCasePattern.TypeTest(pat)
else
MatchTypeCasePattern.CompileTimeS(argPattern)
else
tycon.info match
case _: RealTypeBounds =>
recAbstractTypeConstructor(pat)
case TypeAlias(tl @ HKTypeLambda(onlyParam :: Nil, resType: RefinedType)) =>
/* Unlike for eta-expanded classes, the typer does not automatically
* dealias poly type aliases to refined types. So we have to give them
* a chance here.
* We are quite specific about the shape of type aliases that we are willing
* to dealias this way, because we must not dealias arbitrary type constructors
* that could refine the bounds of the captures; those would amount of
* type-test + capture combos, which are out of the specced match types.
*/
rec(pat.superType, variance)
case _ =>
null

case pat @ AppliedType(tycon: TypeParamRef, _) if variance == 1 =>
recAbstractTypeConstructor(pat)

case pat @ RefinedType(parent, refinedName: TypeName, TypeAlias(alias @ TypeParamRef(binder, num)))
if variance == 1 && (binder eq caseLambda) =>
parent.member(refinedName) match
case refinedMember: SingleDenotation if refinedMember.exists =>
// Check that the bounds of the capture contain the bounds of the inherited member
val refinedMemberBounds = refinedMember.info
val captureBounds = caseLambda.paramInfos(num)
if captureBounds.contains(refinedMemberBounds) then
/* In this case, we know that any member we eventually find during reduction
* will have bounds that fit in the bounds of the capture. Therefore, no
* type-test + capture combo is necessary, and we can apply the specced match types.
*/
val capture = rec(alias, variance = 0).asInstanceOf[MatchTypeCasePattern.Capture]
MatchTypeCasePattern.TypeMemberExtractor(refinedName, capture)
else
// Otherwise, a type-test + capture combo might be necessary, and we are out of spec
null
case _ =>
// If the member does not refine a member of the `parent`, we are out of spec
null

case _ =>
MatchTypeCasePattern.TypeTest(pat)
end rec

def recAbstractTypeConstructor(pat: AppliedType): MatchTypeCasePattern | Null =
recArgPatterns(pat) { argPatterns =>
MatchTypeCasePattern.AbstractTypeConstructor(pat.tycon, argPatterns)
}
end recAbstractTypeConstructor

def recArgPatterns(pat: AppliedType)(whenNotTypeTest: List[MatchTypeCasePattern] => MatchTypeCasePattern | Null): MatchTypeCasePattern | Null =
val AppliedType(tycon, args) = pat
val tparams = tycon.typeParams
val argPatterns = args.zip(tparams).map { (arg, tparam) =>
rec(arg, tparam.paramVarianceSign)
}
if argPatterns.exists(_ == null) then
null
else
val argPatterns1 = argPatterns.asInstanceOf[List[MatchTypeCasePattern]] // they are not null
if argPatterns1.forall(_.isTypeTest) then
MatchTypeCasePattern.TypeTest(pat)
else
whenNotTypeTest(argPatterns1)
end recArgPatterns

val result = rec(pat, variance = 1)
if typeParamRefsAccountedFor == caseLambda.paramNames.size then result
else null
end tryConvertToSpecPattern
end MatchTypeCaseSpec

// ------ ClassInfo, Type Bounds --------------------------------------------------

type TypeOrSymbol = Type | Symbol
Expand Down
Original file line number Diff line number Diff line change
Expand Up @@ -204,6 +204,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
case VarArgsParamCannotBeGivenID // errorNumber: 188
case ExtractorNotFoundID // errorNumber: 189
case PureUnitExpressionID // errorNumber: 190
case MatchTypeLegacyPatternID // errorNumber: 191

def errorNumber = ordinal - 1

Expand Down
4 changes: 4 additions & 0 deletions compiler/src/dotty/tools/dotc/reporting/messages.scala
Original file line number Diff line number Diff line change
Expand Up @@ -3073,6 +3073,10 @@ class MatchTypeScrutineeCannotBeHigherKinded(tp: Type)(using Context)
def msg(using Context) = i"the scrutinee of a match type cannot be higher-kinded"
def explain(using Context) = ""

class MatchTypeLegacyPattern(errorText: String)(using Context) extends TypeMsg(MatchTypeLegacyPatternID):
def msg(using Context) = errorText
def explain(using Context) = ""

class ClosureCannotHaveInternalParameterDependencies(mt: Type)(using Context)
extends TypeMsg(ClosureCannotHaveInternalParameterDependenciesID):
def msg(using Context) =
Expand Down
6 changes: 5 additions & 1 deletion compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala
Original file line number Diff line number Diff line change
Expand Up @@ -154,7 +154,11 @@ object TypeTestsCasts {

case x =>
// always false test warnings are emitted elsewhere
TypeComparer.provablyDisjoint(x, tpe.derivedAppliedType(tycon, targs.map(_ => WildcardType)))
// provablyDisjoint wants fully applied types as input; because we're in the middle of erasure, we sometimes get raw types here
val xApplied =
val tparams = x.typeParams
if tparams.isEmpty then x else x.appliedTo(tparams.map(_ => WildcardType))
TypeComparer.provablyDisjoint(xApplied, tpe.derivedAppliedType(tycon, targs.map(_ => WildcardType)))
|| typeArgsDeterminable(X, tpe)
||| i"its type arguments can't be determined from $X"
}
Expand Down
4 changes: 2 additions & 2 deletions compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala
Original file line number Diff line number Diff line change
Expand Up @@ -297,6 +297,8 @@ trait TypeAssigner {
else fntpe.resultType // fast path optimization
else
errorType(em"wrong number of arguments at ${ctx.phase.prev} for $fntpe: ${fn.tpe}, expected: ${fntpe.paramInfos.length}, found: ${args.length}", tree.srcPos)
case err: ErrorType =>
err
case t =>
if (ctx.settings.Ydebug.value) new FatalError("").printStackTrace()
errorType(err.takesNoParamsMsg(fn, ""), tree.srcPos)
Expand Down Expand Up @@ -563,5 +565,3 @@ object TypeAssigner extends TypeAssigner:
def seqLitType(tree: untpd.SeqLiteral, elemType: Type)(using Context) = tree match
case tree: untpd.JavaSeqLiteral => defn.ArrayOf(elemType)
case _ => if ctx.erasedTypes then defn.SeqType else defn.SeqType.appliedTo(elemType)


21 changes: 0 additions & 21 deletions tests/neg/12800.scala

This file was deleted.

9 changes: 5 additions & 4 deletions tests/neg/6314-1.scala
Original file line number Diff line number Diff line change
@@ -1,6 +1,7 @@
object G {
final class X
final class Y
trait X
class Y
class Z

trait FooSig {
type Type
Expand All @@ -13,14 +14,14 @@ object G {
type Foo = Foo.Type

type Bar[A] = A match {
case X & Y => String
case X & Z => String
case Y => Int
}

def main(args: Array[String]): Unit = {
val a: Bar[X & Y] = "hello" // error
val i: Bar[Y & Foo] = Foo.apply[Bar](a)
val b: Int = i // error
val b: Int = i
println(b + 1)
}
}
16 changes: 16 additions & 0 deletions tests/neg/6314-6.check
Original file line number Diff line number Diff line change
@@ -0,0 +1,16 @@
-- Error: tests/neg/6314-6.scala:26:3 ----------------------------------------------------------------------------------
26 | (new YY {}).boom // error: object creation impossible
| ^
|object creation impossible, since def apply(fa: String): Int in trait XX in object Test3 is not defined
|(Note that
| parameter String in def apply(fa: String): Int in trait XX in object Test3 does not match
| parameter Test3.Bar[X & Object with Test3.YY {...}#Foo] in def apply(fa: Test3.Bar[X & YY.this.Foo]): Test3.Bar[Y & YY.this.Foo] in trait YY in object Test3
| )
-- Error: tests/neg/6314-6.scala:52:3 ----------------------------------------------------------------------------------
52 | (new YY {}).boom // error: object creation impossible
| ^
|object creation impossible, since def apply(fa: String): Int in trait XX in object Test4 is not defined
|(Note that
| parameter String in def apply(fa: String): Int in trait XX in object Test4 does not match
| parameter Test4.Bar[X & Object with Test4.YY {...}#FooAlias] in def apply(fa: Test4.Bar[X & YY.this.FooAlias]): Test4.Bar[Y & YY.this.FooAlias] in trait YY in object Test4
| )
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