Avoid sharing a mutable empty GadtConstraint!

Author: dwijnand

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

@@ -814,7 +814,7 @@ object Contexts {
       .updated(notNullInfosLoc, Nil)
       .updated(compilationUnitLoc, NoCompilationUnit)
     searchHistory = new SearchRoot
-    gadt = GadtConstraint.empty
+    gadt = GadtConstraint()
   }
 
   @sharable object NoContext extends Context((null: ContextBase | Null).uncheckedNN) {

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

@@ -10,10 +10,8 @@ import util.{SimpleIdentitySet, SimpleIdentityMap}
 import collection.mutable
 import printing._
 
-import scala.annotation.internal.sharable
-
 object GadtConstraint:
-  @sharable val empty: GadtConstraint =
+  def apply(): GadtConstraint =
     new ProperGadtConstraint(OrderingConstraint.empty, SimpleIdentityMap.empty, SimpleIdentityMap.empty, false)
 
 /** Represents GADT constraints currently in scope */

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -3774,7 +3774,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                 adaptToSubType(wtp)
           case CompareResult.OKwithGADTUsed
           if pt.isValueType
-             && !inContext(ctx.fresh.setGadt(GadtConstraint.empty)) {
+             && !inContext(ctx.fresh.setGadt(GadtConstraint())) {
                val res = (tree.tpe.widenExpr frozen_<:< pt)
                if res then
                  // we overshot; a cast is not needed, after all.