Merge pull request #15500 from dotty-staging/backport-15423

Backport #15423: Refine Matchtype checking

Author: odersky

compiler/src/dotty/tools/dotc/core/MatchTypeTrace.scala

@@ -4,6 +4,7 @@ package core
 
 import Types._, Contexts._, Symbols._, Decorators._
 import util.Property
+import Names.Name
 
 /** A utility module to produce match type reduction traces in error messages.
  */
@@ -13,6 +14,7 @@ object MatchTypeTrace:
     case TryReduce(scrut: Type)
     case NoMatches(scrut: Type, cases: List[Type])
     case Stuck(scrut: Type, stuckCase: Type, otherCases: List[Type])
+    case NoInstance(scrut: Type, stuckCase: Type, fails: List[(Name, TypeBounds)])
     case EmptyScrutinee(scrut: Type)
   import TraceEntry._
 
@@ -62,6 +64,9 @@ object MatchTypeTrace:
   def stuck(scrut: Type, stuckCase: Type, otherCases: List[Type])(using Context) =
     matchTypeFail(Stuck(scrut, stuckCase, otherCases))
 
+  def noInstance(scrut: Type, stuckCase: Type, fails: List[(Name, TypeBounds)])(using Context) =
+    matchTypeFail(NoInstance(scrut, stuckCase, fails))
+
   /** Record a failure that scrutinee `scrut` is provably empty.
    *  Only the first failure is recorded.
    */
@@ -82,7 +87,7 @@ object MatchTypeTrace:
       case _ =>
         op
 
-  private def caseText(tp: Type)(using Context): String = tp match
+  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 => $body"
@@ -114,6 +119,13 @@ object MatchTypeTrace:
            |  Therefore, reduction cannot advance to the remaining case$s
            |
            |    ${casesText(otherCases)}"""
+    case NoInstance(scrut, stuckCase, fails) =>
+      def params = if fails.length == 1 then "parameter" else "parameters"
+      i"""  failed since selector  $scrut
+         |  does not uniquely determine $params ${fails.map(_._1)}%, % in
+         |    ${caseText(stuckCase)}
+         |  The computed bounds for the $params are:
+         |    ${fails.map((name, bounds) => i"$name$bounds")}%\n    %"""
 
   def noMatchesText(scrut: Type, cases: List[Type])(using Context): String =
     i"""failed since selector  $scrut

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -60,6 +60,8 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
   /** Indicates whether the subtype check used GADT bounds */
   private var GADTused: Boolean = false
 
+  protected var canWidenAbstract: Boolean = true
+
   private var myInstance: TypeComparer = this
   def currentInstance: TypeComparer = myInstance
 
@@ -757,9 +759,11 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
 
     def tryBaseType(cls2: Symbol) = {
       val base = nonExprBaseType(tp1, cls2)
-      if (base.exists && (base `ne` tp1))
-        isSubType(base, tp2, if (tp1.isRef(cls2)) approx else approx.addLow) ||
-        base.isInstanceOf[OrType] && fourthTry
+      if base.exists && (base ne tp1)
+        && (!caseLambda.exists || canWidenAbstract || tp1.widen.underlyingClassRef(refinementOK = true).exists)
+      then
+        isSubType(base, tp2, if (tp1.isRef(cls2)) approx else approx.addLow)
+        || base.isInstanceOf[OrType] && fourthTry
           // if base is a disjunction, this might have come from a tp1 type that
           // expands to a match type. In this case, we should try to reduce the type
           // and compare the redux. This is done in fourthTry
@@ -776,7 +780,9 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
                 || narrowGADTBounds(tp1, tp2, approx, isUpper = true))
               && (tp2.isAny || GADTusage(tp1.symbol))
 
-            isSubType(hi1, tp2, approx.addLow) || compareGADT || tryLiftedToThis1
+            (!caseLambda.exists || canWidenAbstract) && isSubType(hi1, tp2, approx.addLow)
+            || compareGADT
+            || tryLiftedToThis1
           case _ =>
             // `Mode.RelaxedOverriding` is only enabled when checking Java overriding
             // in explicit nulls, and `Null` becomes a bottom type, which allows
@@ -2849,7 +2855,16 @@ object TypeComparer {
     comparing(_.tracked(op))
 }
 
+object TrackingTypeComparer:
+  enum MatchResult:
+    case Reduced(tp: Type)
+    case Disjoint
+    case Stuck
+    case NoInstance(fails: List[(Name, TypeBounds)])
+
 class TrackingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
+  import TrackingTypeComparer.*
+
   init(initctx)
 
   override def trackingTypeComparer = this
@@ -2887,31 +2902,36 @@ class TrackingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
   }
 
   def matchCases(scrut: Type, cases: List[Type])(using Context): Type = {
-    def paramInstances = new TypeAccumulator[Array[Type]] {
-      def apply(inst: Array[Type], t: Type) = t match {
-        case t @ TypeParamRef(b, n) if b `eq` caseLambda =>
-          inst(n) = approximation(t, fromBelow = variance >= 0).simplified
-          inst
+
+    def paramInstances(canApprox: Boolean) = new TypeAccumulator[Array[Type]]:
+      def apply(insts: Array[Type], t: Type) = t match
+        case param @ TypeParamRef(b, n) if b eq caseLambda =>
+          insts(n) =
+            if canApprox then
+              approximation(param, fromBelow = variance >= 0).simplified
+            else constraint.entry(param) match
+              case entry: TypeBounds =>
+                val lo = fullLowerBound(param)
+                val hi = fullUpperBound(param)
+                if isSubType(hi, lo) then lo.simplified else Range(lo, hi)
+              case inst =>
+                assert(inst.exists, i"param = $param\nconstraint = $constraint")
+                inst.simplified
+          insts
         case _ =>
-          foldOver(inst, t)
-      }
-    }
+          foldOver(insts, t)
 
-    def instantiateParams(inst: Array[Type]) = new TypeMap {
+    def instantiateParams(insts: Array[Type]) = new ApproximatingTypeMap {
+      variance = 0
       def apply(t: Type) = t match {
-        case t @ TypeParamRef(b, n) if b `eq` caseLambda => inst(n)
+        case t @ TypeParamRef(b, n) if b `eq` caseLambda => insts(n)
         case t: LazyRef => apply(t.ref)
         case _ => mapOver(t)
       }
     }
 
-    /** Match a single case.
-     *  @return  Some(tp)     if the match succeeds with type `tp`
-     *           Some(NoType) if the match fails, and there is an overlap between pattern and scrutinee
-     *           None         if the match fails and we should consider the following cases
-     *                        because scrutinee and pattern do not overlap
-     */
-    def matchCase(cas: Type): Option[Type] = trace(i"match case $cas vs $scrut", matchTypes) {
+    /** Match a single case. */
+    def matchCase(cas: Type): MatchResult = trace(i"match case $cas vs $scrut", matchTypes) {
       val cas1 = cas match {
         case cas: HKTypeLambda =>
           caseLambda = constrained(cas)
@@ -2922,34 +2942,52 @@ class TrackingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
 
       val defn.MatchCase(pat, body) = cas1: @unchecked
 
-      if (isSubType(scrut, pat))
-        // `scrut` is a subtype of `pat`: *It's a Match!*
-        Some {
-          caseLambda match {
-            case caseLambda: HKTypeLambda =>
-              val instances = paramInstances(new Array(caseLambda.paramNames.length), pat)
-              instantiateParams(instances)(body).simplified
-            case _ =>
-              body
-          }
-        }
+      def matches(canWidenAbstract: Boolean): Boolean =
+        val saved = this.canWidenAbstract
+        this.canWidenAbstract = canWidenAbstract
+        try necessarySubType(scrut, pat)
+        finally this.canWidenAbstract = saved
+
+      def redux(canApprox: Boolean): MatchResult =
+        caseLambda match
+          case caseLambda: HKTypeLambda =>
+            val instances = paramInstances(canApprox)(new Array(caseLambda.paramNames.length), pat)
+            instantiateParams(instances)(body) match
+              case Range(lo, hi) =>
+                MatchResult.NoInstance {
+                  caseLambda.paramNames.zip(instances).collect {
+                    case (name, Range(lo, hi)) => (name, TypeBounds(lo, hi))
+                  }
+                }
+              case redux =>
+                MatchResult.Reduced(redux.simplified)
+          case _ =>
+            MatchResult.Reduced(body)
+
+      if caseLambda.exists && matches(canWidenAbstract = false) then
+        redux(canApprox = true)
+      else if matches(canWidenAbstract = true) then
+        redux(canApprox = false)
       else if (provablyDisjoint(scrut, pat))
         // We found a proof that `scrut` and `pat` are incompatible.
         // The search continues.
-        None
+        MatchResult.Disjoint
       else
-        Some(NoType)
+        MatchResult.Stuck
     }
 
     def recur(remaining: List[Type]): Type = remaining match
       case cas :: remaining1 =>
         matchCase(cas) match
-          case None =>
+          case MatchResult.Disjoint =>
             recur(remaining1)
-          case Some(NoType) =>
+          case MatchResult.Stuck =>
             MatchTypeTrace.stuck(scrut, cas, remaining1)
             NoType
-          case Some(tp) =>
+          case MatchResult.NoInstance(fails) =>
+            MatchTypeTrace.noInstance(scrut, cas, fails)
+            NoType
+          case MatchResult.Reduced(tp) =>
             tp
       case Nil =>
         val casesText = MatchTypeTrace.noMatchesText(scrut, cases)

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -5977,7 +5977,13 @@ object Types {
         case _ =>
           scrutinee match
             case Range(lo, hi) => range(bound.bounds.lo, bound.bounds.hi)
-            case _ => tp.derivedMatchType(bound, scrutinee, cases)
+            case _ =>
+              if cases.exists(isRange) then
+                Range(
+                  tp.derivedMatchType(bound, scrutinee, cases.map(lower)),
+                  tp.derivedMatchType(bound, scrutinee, cases.map(upper)))
+              else
+                tp.derivedMatchType(bound, scrutinee, cases)
 
     override protected def derivedSkolemType(tp: SkolemType, info: Type): Type =
       if info eq tp.info then tp
@@ -6013,7 +6019,7 @@ object Types {
   /** A range of possible types between lower bound `lo` and upper bound `hi`.
    *  Only used internally in `ApproximatingTypeMap`.
    */
-  private case class Range(lo: Type, hi: Type) extends UncachedGroundType {
+  case class Range(lo: Type, hi: Type) extends UncachedGroundType {
     assert(!lo.isInstanceOf[Range])
     assert(!hi.isInstanceOf[Range])
 

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -200,6 +200,7 @@ class CompilationTests {
   @Test def runAll: Unit = {
     implicit val testGroup: TestGroup = TestGroup("runAll")
     aggregateTests(
+      compileFile("tests/run-custom-args/typeclass-derivation1.scala", defaultOptions.without(yCheckOptions*)),
       compileFile("tests/run-custom-args/tuple-cons.scala", allowDeepSubtypes),
       compileFile("tests/run-custom-args/i5256.scala", allowDeepSubtypes),
       compileFile("tests/run-custom-args/no-useless-forwarders.scala", defaultOptions and "-Xmixin-force-forwarders:false"),

tests/neg/6570-1.check

@@ -0,0 +1,32 @@
+-- [E007] Type Mismatch Error: tests/neg/6570-1.scala:23:27 ------------------------------------------------------------
+23 |  def thing = new Trait1 {} // error
+   |                           ^
+   |                           Found:    Object with Trait1 {...}
+   |                           Required: N[Box[Int & String]]
+   |
+   |                           Note: a match type could not be fully reduced:
+   |
+   |                             trying to reduce  N[Box[Int & String]]
+   |                             failed since selector  Box[Int & String]
+   |                             is uninhabited (there are no values of that type).
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg/6570-1.scala:36:54 ------------------------------------------------------------
+36 |  def foo[T <: Cov[Box[Int]]](c: Root[T]): Trait2 = c.thing  // error
+   |                                                    ^^^^^^^
+   |                                                Found:    M[T]
+   |                                                Required: Trait2
+   |
+   |                                                where:    T is a type in method foo with bounds <: Cov[Box[Int]]
+   |
+   |
+   |                                                Note: a match type could not be fully reduced:
+   |
+   |                                                  trying to reduce  M[T]
+   |                                                  failed since selector  T
+   |                                                  does not uniquely determine parameter x in
+   |                                                    case Cov[x] => N[x]
+   |                                                  The computed bounds for the parameter are:
+   |                                                    x >: Box[Int]
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg/6570-1.scala

@@ -33,7 +33,7 @@ class Asploder extends Root[Cov[Box[Int & String]]] {
 }
 
 object Main {
-  def foo[T <: Cov[Box[Int]]](c: Root[T]): Trait2 = c.thing
+  def foo[T <: Cov[Box[Int]]](c: Root[T]): Trait2 = c.thing  // error
   def explode = foo(new Asploder)
 
   def main(args: Array[String]): Unit =

tests/neg/6570.scala

@@ -21,7 +21,7 @@ object UpperBoundParametricVariant {
   trait Child[A <: Cov[Int]] extends Root[A]
 
   // we reduce `M[T]` to `Trait2`, even though we cannot be certain of that
-  def foo[T <: Cov[Int]](c: Child[T]): Trait2 = c.thing
+  def foo[T <: Cov[Int]](c: Child[T]): Trait2 = c.thing  // error
 
   class Asploder extends Child[Cov[String & Int]] {
     def thing = new Trait1 {} // error
@@ -42,7 +42,7 @@ object InheritanceVariant {
 
   trait Child extends Root { type B <: { type A <: Int } }
 
-  def foo(c: Child): Trait2 = c.thing
+  def foo(c: Child): Trait2 = c.thing  // error
 
   class Asploder extends Child {
     type B = { type A = String & Int }
@@ -98,7 +98,7 @@ object UpperBoundVariant {
 
   trait Child extends Root { type A <: Cov[Int] }
 
-  def foo(c: Child): Trait2 = c.thing
+  def foo(c: Child): Trait2 = c.thing  // error
 
   class Asploder extends Child {
     type A = Cov[String & Int]

tests/neg/i11982.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/i11982.scala:22:38 ---------------------------------------------------------------------------------
+22 |  val p1: ("msg", 42) = unpair[Tshape]  // error: no singleton value for Any
+   |                                      ^
+   |                                      No singleton value available for Any.

tests/neg/i11982.scala

@@ -0,0 +1,27 @@
+package tuplefun
+object Unpair {
+
+  def pair[A, B](using a: ValueOf[A], b: ValueOf[B]): Tuple2[A, B] =
+    (a.value, b.value)
+
+  def unpair[X <: Tuple2[?, ?]](
+      using a: ValueOf[Tuple.Head[X]],
+            b: ValueOf[Tuple.Head[Tuple.Tail[X]]]
+  ): Tuple2[Tuple.Head[X], Tuple.Head[Tuple.Tail[X]]] =
+    type AA = Tuple.Head[X]
+    type BB = Tuple.Head[Tuple.Tail[X]]
+    pair[AA, BB](using a, b)
+}
+
+object UnpairApp {
+  import Unpair._
+
+  type Tshape = ("msg", 42)
+
+  // the following won't compile when in the same file as Unpair
+  val p1: ("msg", 42) = unpair[Tshape]  // error: no singleton value for Any
+
+  @main def pairHello: Unit =
+    assert(p1 == ("msg", 42))
+    println(p1)
+}