Enable betterMatchTypeExtractors in >= 3.6 (#21198)

Closes #20538.

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

@@ -133,8 +133,6 @@ object Feature:
 
   def scala2ExperimentalMacroEnabled(using Context) = enabled(scala2macros)
 
-  def betterMatchTypeExtractorsEnabled(using Context) = enabled(betterMatchTypeExtractors)
-
   def quotedPatternsWithPolymorphicFunctionsEnabled(using Context) =
     enabled(quotedPatternsWithPolymorphicFunctions)
 

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

@@ -10,7 +10,7 @@ import TypeOps.refineUsingParent
 import collection.mutable
 import util.{Stats, NoSourcePosition, EqHashMap}
 import config.Config
-import config.Feature.{betterMatchTypeExtractorsEnabled, migrateTo3, sourceVersion}
+import config.Feature.{migrateTo3, sourceVersion}
 import config.Printers.{subtyping, gadts, matchTypes, capt, noPrinter}
 import config.SourceVersion
 import TypeErasure.{erasedLub, erasedGlb}
@@ -3621,10 +3621,8 @@ class MatchReducer(initctx: Context) extends TypeComparer(initctx) {
           case MatchTypeCasePattern.TypeMemberExtractor(typeMemberName, capture) =>
             /** Try to remove references to `skolem` from a type in accordance with the spec.
              *
-             *  If `betterMatchTypeExtractorsEnabled` is enabled then references
-             *  to `skolem` occuring are avoided by following aliases and
-             *  singletons, otherwise no attempt made to avoid references to
-             *  `skolem`.
+             *  References to `skolem` occuring are avoided by following aliases and
+             *  singletons.
              *
              *  If any reference to `skolem` remains in the result type,
              *  `refersToSkolem` is set to true.
@@ -3638,7 +3636,7 @@ class MatchReducer(initctx: Context) extends TypeComparer(initctx) {
                   case `skolem` =>
                     refersToSkolem = true
                     tp
-                  case tp: NamedType if betterMatchTypeExtractorsEnabled =>
+                  case tp: NamedType =>
                     val pre1 = apply(tp.prefix)
                     if refersToSkolem then
                       tp match
@@ -3656,7 +3654,7 @@ class MatchReducer(initctx: Context) extends TypeComparer(initctx) {
                             tp.derivedSelect(pre1)
                     else
                       tp.derivedSelect(pre1)
-                  case tp: LazyRef if betterMatchTypeExtractorsEnabled =>
+                  case tp: LazyRef =>
                     // By default, TypeMap maps LazyRefs lazily. We need to
                     // force it for `refersToSkolem` to be correctly set.
                     apply(tp.ref)

library/src/scala/runtime/stdLibPatches/language.scala

@@ -124,6 +124,7 @@ object language:
      *  @see [[https://github.com/scala/improvement-proposals/pull/84]]
      */
     @compileTimeOnly("`betterMatchTypeExtractors` can only be used at compile time in import statements")
+    @deprecated("The experimental.betterMatchTypeExtractors language import is no longer needed since the feature is now standard. It now has no effect, including when setting an older source version.", since = "3.6")
     object betterMatchTypeExtractors
 
     /** Experimental support for quote pattern matching with polymorphic functions

tests/neg/mt-deskolemize-2.scala

@@ -1,5 +1,3 @@
-//> using options -language:experimental.betterMatchTypeExtractors
-
 trait Expr:
   type Value
 object Expr:

tests/pos/20538.scala

@@ -0,0 +1,32 @@
+trait Column:
+  type T
+  type F[X]
+  type Q = F[T]
+
+class varchar extends Column:
+  type T = String
+
+trait notnull extends Column:
+  type F[X] = X
+
+object Error:
+
+  val firstName = new varchar with notnull
+  val lastName = new varchar with notnull
+
+  val relation = (firstName, lastName)
+
+  type RelationTypes = Tuple.InverseMap[relation.type, [X] =>> Column { type Q = X }]
+
+  summon[RelationTypes =:= (String, String)]
+
+object Works:
+
+  object firstName extends varchar with notnull
+  object lastName extends varchar with notnull
+
+  val relation = (firstName, lastName)
+
+  type RelationTypes = Tuple.InverseMap[relation.type, [X] =>> Column { type Q = X }]
+
+  summon[RelationTypes =:= (String, String)]

tests/pos/20538b.scala

@@ -0,0 +1,19 @@
+trait A:
+  type T
+  type U = T
+
+trait B extends A:
+  type T = String
+
+object C extends B
+
+
+type F[X] = A { type U = X } // works when `U` is replaced with `T`
+
+type InvF[Y] = Y match
+  case F[x] => x
+
+
+object Test:
+  summon[InvF[C.type] =:= String] // ok
+  summon[InvF[B] =:= String] // error: selector B does not uniquely determine parameter x

tests/pos/mt-deskolemize.scala

@@ -1,5 +1,3 @@
-//> using options -language:experimental.betterMatchTypeExtractors
-
 trait Expr:
   type Value