Add specialized printer for GADT constraint

Author: Aleksander Boruch-Gruszecki

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

@@ -20,6 +20,7 @@ object Printers {
   val dottydoc: Printer = noPrinter
   val exhaustivity: Printer = noPrinter
   val gadts: Printer = noPrinter
+  val gadtsConstr: Printer = noPrinter
   val hk: Printer = noPrinter
   val implicits: Printer = noPrinter
   val implicitsDetailed: Printer = noPrinter

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

@@ -724,7 +724,7 @@ object Contexts {
     private[this] var boundCache: SimpleIdentityMap[Symbol, TypeBounds],
     private[this] var dirtyFlag: Boolean
   ) extends GADTMap with ConstraintHandling {
-    import dotty.tools.dotc.config.Printers.gadts
+    import dotty.tools.dotc.config.Printers.{gadts, gadtsConstr}
 
     def this() = this(
       myConstraint = new OrderingConstraint(SimpleIdentityMap.Empty, SimpleIdentityMap.Empty, SimpleIdentityMap.Empty),
@@ -734,6 +734,8 @@ object Contexts {
       dirtyFlag = false
     )
 
+    override def constr_println(msg: => String): Unit = gadtsConstr.println(msg)
+
     override def debugBoundsDescription(implicit ctx: Context): String = {
       val sb = new mutable.StringBuilder
       sb ++= constraint.show