Under -source:3.3 and below, always use the legacy match type algorithm.

This should improve consistency with the actual earlier compilers,
since it means the matching algorithm will be intact.

Note that the new behavior of `provablyDisjoint` is always applied,
even under `-source:3.3`. This includes using `provablyDisjoint`
instead of `provablyEmpty`. So it is still possible that something
behaves differently than the actual earlier compilers.

Author: sjrd

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

@@ -32,6 +32,8 @@ 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}
@@ -5120,6 +5122,9 @@ object Types {
   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)

tests/neg/6570.check

@@ -14,7 +14,7 @@
    |                           does not uniquely determine parameter x in
    |                             case UpperBoundParametricVariant.Cov[x] => Base.N[x]
    |                           The computed bounds for the parameter are:
-   |                             x <: Int
+   |                             x >: Int
    |
    | longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg/6570.scala:29:29 --------------------------------------------------------------
@@ -98,7 +98,7 @@
     |                                does not uniquely determine parameter t in
     |                                  case UpperBoundVariant.Cov[t] => Base.N[t]
     |                                The computed bounds for the parameter are:
-    |                                  t <: Int
+    |                                  t >: Int
     |
     | longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg/6570.scala:107:29 -------------------------------------------------------------