Consider -Yexplain-lowlevel when showing goals of failed subtype assertions

Author: odersky

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

@@ -17,6 +17,8 @@ import reporting.trace
 /** Provides methods to compare types.
  */
 class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
+  import TypeComparer.show
+
   implicit val ctx = initctx
 
   val state = ctx.typerState
@@ -1573,7 +1575,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
 
   /** Show subtype goal that led to an assertion failure */
   def showGoal(tp1: Type, tp2: Type)(implicit ctx: Context) = {
-    println(i"assertion failure for $tp1 <:< $tp2, frozen = $frozenConstraint")
+    println(i"assertion failure for ${show(tp1)} <:< ${show(tp2)}, frozen = $frozenConstraint")
     def explainPoly(tp: Type) = tp match {
       case tp: TypeParamRef => ctx.echo(s"TypeParamRef ${tp.show} found in ${tp.binder.show}")
       case tp: TypeRef if tp.symbol.exists => ctx.echo(s"typeref ${tp.show} found in ${tp.symbol.owner.show}")
@@ -1605,6 +1607,11 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
 
 object TypeComparer {
 
+  private[core] def show(res: Any)(implicit ctx: Context) = res match {
+    case res: printing.Showable if !ctx.settings.YexplainLowlevel.value => res.show
+    case _ => String.valueOf(res)
+  }
+
   /** Show trace of comparison operations when performing `op` as result string */
   def explained[T](op: Context => T)(implicit ctx: Context): String = {
     val nestedCtx = ctx.fresh.setTypeComparerFn(new ExplainingTypeComparer(_))
@@ -1615,6 +1622,8 @@ object TypeComparer {
 
 /** A type comparer that can record traces of subtype operations */
 class ExplainingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
+  import TypeComparer.show
+
   private[this] var indent = 0
   private val b = new StringBuilder
 
@@ -1631,11 +1640,6 @@ class ExplainingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
       res
     }
 
-  private def show(res: Any) = res match {
-    case res: printing.Showable if !ctx.settings.YexplainLowlevel.value => res.show
-    case _ => String.valueOf(res)
-  }
-
   override def isSubType(tp1: Type, tp2: Type) =
     traceIndented(s"${show(tp1)} <:< ${show(tp2)}${if (Config.verboseExplainSubtype) s" ${tp1.getClass} ${tp2.getClass}" else ""}${if (frozenConstraint) " frozen" else ""}") {
       super.isSubType(tp1, tp2)