SIP-56: Better foundations for match types (#18262)

Implementation of SIP-56: Proper Specification for Match Types
https://docs.scala-lang.org/sips/match-types-spec.html

Author: sjrd

Diff for: compiler/src/dotty/tools/dotc/core/MatchTypeTrace.scala

@@ -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.*
 
@@ -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.
@@ -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
@@ -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

Diff for: compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -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.*
@@ -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}
@@ -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
@@ -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 =
+      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

Diff for: compiler/src/dotty/tools/dotc/core/Types.scala

@@ -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
 

Diff for: compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala

@@ -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) =

Diff for: compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -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"
         }

Diff for: compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala

@@ -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)
@@ -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)
-
-

Diff for: compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala

@@ -1,6 +1,7 @@
 object G {
-  final class X
-  final class Y
+  trait X
+  class Y
+  class Z
 
   trait FooSig {
     type Type
@@ -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)
   }
 }

Diff for: tests/neg/12800.scala

@@ -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
+   | )