Make sure in CI that we do not unexpectedly fall back on legacy match type reduction.

We introduce a new flag `-Yno-legacy-match-types`, which forbids
the reduction of "legacy" match types. Like `-Yno-deep-subtypes`,
it is meant to be used in our CI. With it, we check that we do not
unexpectedly fall back on legacy match types in tests for which
the specced match types are enough.

Later, we should consider integrating that behavior with the source
level so that it reaches users.

Author: sjrd

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -347,6 +347,7 @@ private sealed trait YSettings:
   val YprintPos: Setting[Boolean] = BooleanSetting("-Yprint-pos", "Show tree positions.")
   val YprintPosSyms: Setting[Boolean] = BooleanSetting("-Yprint-pos-syms", "Show symbol definitions positions.")
   val YnoDeepSubtypes: Setting[Boolean] = BooleanSetting("-Yno-deep-subtypes", "Throw an exception on deep subtyping call stacks.")
+  val YnoLegacyMatchTypes: Setting[Boolean] = BooleanSetting("-Yno-legacy-match-types", "Refuse to reduce match types with legacy/unspecified patterns")
   val YnoPatmatOpt: Setting[Boolean] = BooleanSetting("-Yno-patmat-opt", "Disable all pattern matching optimizations.")
   val YplainPrinter: Setting[Boolean] = BooleanSetting("-Yplain-printer", "Pretty-print using a plain printer.")
   val YprintSyms: Setting[Boolean] = BooleanSetting("-Yprint-syms", "When printing trees print info in symbols instead of corresponding info in trees.")

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

@@ -125,4 +125,8 @@ object MatchTypeTrace:
        |
        |    ${casesText(cases)}"""
 
+  def legacyPatternText(scrut: Type, cas: MatchTypeCaseSpec.LegacyPatMat)(using Context): String =
+    i"""Illegal match type because it contains the legacy, unspecifed case
+       |    ${caseText(cas)}"""
+
 end MatchTypeTrace

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

@@ -3428,6 +3428,9 @@ class TrackingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
       MatchResult.Stuck
 
     def recur(remaining: List[MatchTypeCaseSpec]): Type = remaining match
+      case (cas: MatchTypeCaseSpec.LegacyPatMat) :: _ if ctx.settings.YnoLegacyMatchTypes.value =>
+        val errorText = MatchTypeTrace.legacyPatternText(scrut, cas)
+        ErrorType(reporting.MatchTypeLegacyPattern(errorText))
       case cas :: remaining1 =>
         matchCase(cas) match
           case MatchResult.Disjoint =>

compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala

@@ -202,6 +202,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case ImplausiblePatternWarningID // erorNumber: 186
   case SynchronizedCallOnBoxedClassID // errorNumber: 187
   case VarArgsParamCannotBeGivenID // erorNumber: 188
+  case MatchTypeLegacyPatternID // errorNumber: 189
 
   def errorNumber = ordinal - 1
 

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -3024,6 +3024,10 @@ 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) =

compiler/test/dotty/tools/vulpix/TestConfiguration.scala

@@ -19,6 +19,7 @@ object TestConfiguration {
   val checkOptions = Array(
     // "-Yscala2-unpickler", s"${Properties.scalaLibrary}",
     "-Yno-deep-subtypes",
+    "-Yno-legacy-match-types",
     "-Yno-double-bindings",
     "-Yforce-sbt-phases",
     "-Xsemanticdb",

tests/neg/6570.scala

@@ -1,3 +1,5 @@
+// scalac: -Yno-legacy-match-types:false
+
 object Base {
   trait Trait1
   trait Trait2

tests/neg/legacy-match-types.check

@@ -0,0 +1,30 @@
+-- [E189] Type Error: tests/neg/legacy-match-types.scala:7:23 ----------------------------------------------------------
+7 |type InvNesting[X] = X match // error
+  |                     ^
+  |                     Illegal match type because it contains the legacy, unspecifed case
+  |                         case Inv[Cov[t]] => t
+8 |  case Inv[Cov[t]] => t
+-- [E189] Type Error: tests/neg/legacy-match-types.scala:10:26 ---------------------------------------------------------
+10 |type ContraNesting[X] = X match // error
+   |                        ^
+   |                        Illegal match type because it contains the legacy, unspecifed case
+   |                            case Contra[Cov[t]] => t
+11 |  case Contra[Cov[t]] => t
+-- [E189] Type Error: tests/neg/legacy-match-types.scala:15:22 ---------------------------------------------------------
+15 |type AndTypeMT[X] = X match // error
+   |                    ^
+   |                    Illegal match type because it contains the legacy, unspecifed case
+   |                        case t & Seq[Any] => t
+16 |  case t & Seq[Any] => t
+-- [E189] Type Error: tests/neg/legacy-match-types.scala:22:33 ---------------------------------------------------------
+22 |type TypeAliasWithBoundMT[X] = X match // error
+   |                               ^
+   |                               Illegal match type because it contains the legacy, unspecifed case
+   |                                   case IsSeq[t] => t
+23 |  case IsSeq[t] => t
+-- [E189] Type Error: tests/neg/legacy-match-types.scala:33:34 ---------------------------------------------------------
+33 |type TypeMemberExtractorMT[X] = X match // error
+   |                                ^
+   |                                Illegal match type because it contains the legacy, unspecifed case
+   |                                    case TypeMemberAux[t] => t
+34 |  case TypeMemberAux[t] => t

tests/neg/legacy-match-types.scala

@@ -0,0 +1,34 @@
+class Inv[T]
+class Cov[+T]
+class Contra[-T]
+
+// Nesting captures in non-covariant position
+
+type InvNesting[X] = X match // error
+  case Inv[Cov[t]] => t
+
+type ContraNesting[X] = X match // error
+  case Contra[Cov[t]] => t
+
+// Intersection type to type-test and capture at the same time
+
+type AndTypeMT[X] = X match // error
+  case t & Seq[Any] => t
+
+// Poly type alias with a bound to type-test and capture at the same time
+
+type IsSeq[X <: Seq[Any]] = X
+
+type TypeAliasWithBoundMT[X] = X match // error
+  case IsSeq[t] => t
+
+// Poly type alias with a type member refinement to extract the type member
+
+class Base {
+  type TypeMember
+}
+
+type TypeMemberAux[X] = Base { type TypeMember = X }
+
+type TypeMemberExtractorMT[X] = X match // error
+  case TypeMemberAux[t] => t

tests/pos/10747-shapeless-min-spec.scala

@@ -0,0 +1,13 @@
+trait Monoidal {
+  type to[_] <: Tuple
+}
+
+object eithers extends Monoidal {
+  class Wrap[T]
+
+  type to[t] <: Tuple = Wrap[t] match {
+    case Wrap[Nothing] => EmptyTuple
+    case Wrap[other] => other match
+      case Either[hd, tl] => hd *: to[tl]
+  }
+}

tests/pos/10747-shapeless-min.scala

@@ -1,3 +1,5 @@
+// scalac: -Yno-legacy-match-types:false
+
 trait Monoidal {
   type to[_] <: Tuple
 }

tests/pos/8647.scala

@@ -1,3 +1,5 @@
+// scalac: -Yno-legacy-match-types:false
+
 final class Two[A, B]()
 
 final class Blaaa

tests/pos/9757.scala

@@ -1,3 +1,5 @@
+// scalac: -Yno-legacy-match-types:false
+
 type RemoveFrom[R, A] = R match {
   case A & newType => newType
 }

tests/pos/i10242.scala

@@ -1,4 +1,6 @@
-// https://github.com/lampepfl/dotty/issues/10242  
+// scalac: -Yno-legacy-match-types:false
+
+// https://github.com/lampepfl/dotty/issues/10242
 type Foo[A, B <: A] = A
 
 type Bar[A] = A match {

tests/pos/i15155.scala

@@ -1,3 +1,5 @@
+// scalac: -Yno-legacy-match-types:false
+
 import scala.reflect.ClassTag
 // https://github.com/json4s/json4s/blob/355d8751391773e0d79d04402a4f9fb7bfc684ec/ext/src/main/scala-3/org/json4s/ext/package.scala#L4-L8
 type Aux[A] = { type Value = A }
@@ -8,4 +10,4 @@ type EnumValue[A <: Enumeration] = A match {
 // https://github.com/json4s/json4s/blob/355d8751391773e0d79d04402a4f9fb7bfc684ec/ext/src/main/scala/org/json4s/ext/EnumSerializer.scala#L25-L26
 class EnumSerializer[E <: Enumeration: ClassTag](enumeration: E) {
   val EnumerationClass = classOf[EnumValue[E]]
-}
+}

tests/pos/i16408.min1.scala

@@ -1,3 +1,5 @@
+// scalac: -Yno-legacy-match-types:false
+
 object Helpers:
   type NodeFun[R] = Matchable // compiles without [R] parameter
 

tests/pos/i16408.scala

@@ -1,3 +1,5 @@
+// scalac: -Yno-legacy-match-types:false
+
 import scala.util.Try
 
 trait RDF:

tests/pos/i16706.scala

@@ -1,3 +1,5 @@
+// scalac: -Yno-legacy-match-types:false
+
 import scala.deriving.Mirror
 import scala.reflect.ClassTag
 
@@ -14,4 +16,4 @@ transparent inline given derived[A](
 sealed trait Foo
 case class FooA(a: Int) extends Foo
 
-val instance = derived[Foo] // error
+val instance = derived[Foo] // error

tests/pos/i17395.scala

@@ -1,3 +1,5 @@
+// scalac: -Yno-legacy-match-types:false
+
 trait TC[T]
 
 object TC {

tests/pos/i5625b.scala

@@ -1,3 +1,5 @@
+// scalac: -Yno-legacy-match-types:false
+
 object Test {
 
   type AV[t <: AnyVal] = t
@@ -13,4 +15,4 @@ object Test {
   summon[LeafElem[Array[Int]] =:= Int]
   summon[LeafElem[Iterable[Int]] =:= Int]
   summon[LeafElem[Int] =:= Int]
-}
+}

tests/run-macros/tasty-simplified/quoted_2.scala

@@ -1,3 +1,5 @@
+// scalac: -Yno-legacy-match-types:false
+
 import Macros.simplified
 
 object Test {