Disable legacy match types under source version >= 3.4.

Instead of through the flag -Yno-legacy-match-types.

Codebases that require legacy match types will need to use
`-source:3.3` to compile from now on.

Author: sjrd

Diff for: compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -347,7 +347,6 @@ 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.")

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

@@ -10,8 +10,9 @@ import TypeOps.refineUsingParent
 import collection.mutable
 import util.{Stats, NoSourcePosition, EqHashMap}
 import config.Config
-import config.Feature.migrateTo3
+import config.Feature.{migrateTo3, sourceVersion}
 import config.Printers.{subtyping, gadts, matchTypes, noPrinter}
+import config.SourceVersion
 import TypeErasure.{erasedLub, erasedGlb}
 import TypeApplications._
 import Variances.{Variance, variancesConform}
@@ -3455,7 +3456,7 @@ 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 =>
+      case (cas: MatchTypeCaseSpec.LegacyPatMat) :: _ if sourceVersion.isAtLeast(SourceVersion.`3.4`) =>
         val errorText = MatchTypeTrace.legacyPatternText(scrut, cas)
         ErrorType(reporting.MatchTypeLegacyPattern(errorText))
       case cas :: remaining1 =>

Diff for: compiler/test/dotty/tools/vulpix/TestConfiguration.scala

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

Diff for: tests/neg/6570.scala

@@ -1,4 +1,4 @@
-// scalac: -Yno-legacy-match-types:false
+// scalac: -source:3.3
 
 object Base {
   trait Trait1

Diff for: tests/pos/10747-shapeless-min.scala

@@ -1,4 +1,4 @@
-// scalac: -Yno-legacy-match-types:false
+// scalac: -source:3.3
 
 trait Monoidal {
   type to[_] <: Tuple

Diff for: tests/pos/8647.scala

@@ -1,4 +1,4 @@
-// scalac: -Yno-legacy-match-types:false
+// scalac: -source:3.3
 
 final class Two[A, B]()
 

Diff for: tests/pos/9757.scala

@@ -1,4 +1,4 @@
-// scalac: -Yno-legacy-match-types:false
+// scalac: -source:3.3
 
 type RemoveFrom[R, A] = R match {
   case A & newType => newType

Diff for: tests/pos/i10242.scala

@@ -1,4 +1,4 @@
-// scalac: -Yno-legacy-match-types:false
+// scalac: -source:3.3
 
 // https://github.com/lampepfl/dotty/issues/10242
 type Foo[A, B <: A] = A

Diff for: tests/pos/i15155.scala

@@ -1,4 +1,4 @@
-// scalac: -Yno-legacy-match-types:false
+// scalac: -source:3.3
 
 import scala.reflect.ClassTag
 // https://github.com/json4s/json4s/blob/355d8751391773e0d79d04402a4f9fb7bfc684ec/ext/src/main/scala-3/org/json4s/ext/package.scala#L4-L8

Diff for: tests/pos/i16706.scala

@@ -1,4 +1,4 @@
-// scalac: -Yno-legacy-match-types:false
+// scalac: -source:3.3
 
 import scala.deriving.Mirror
 import scala.reflect.ClassTag

Diff for: tests/pos/i17395.scala

@@ -1,4 +1,4 @@
-// scalac: -Yno-legacy-match-types:false
+// scalac: -source:3.3
 
 trait TC[T]
 

Diff for: tests/pos/i5625b.scala

@@ -1,4 +1,4 @@
-// scalac: -Yno-legacy-match-types:false
+// scalac: -source:3.3
 
 object Test {
 

Diff for: tests/run-macros/tasty-simplified/quoted_2.scala

@@ -1,4 +1,4 @@
-// scalac: -Yno-legacy-match-types:false
+// scalac: -source:3.3
 
 import Macros.simplified