Print failures in boundsViolations, via TypeComparer.explaining

TypeComparer.explaining is like TypeComparer.explained, but instead of
just returning the trace, returns the result, still allowing the trace
to be accessed via .lastTrace, as exemplified by implementing
TypeComparer.explained in terms of TypeComparer.explaining.

Also add, but leave commented out the call of, a trace.dumpStack, which
is like Thread.dumpStack(), but outputing to System.out, like all our
tracing does - so the two don't interact when unbuffering onto the
terminal.  Also, we can do customisations like filtering out stack
elements, limiting the stack.

Author: dwijnand

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

@@ -3268,9 +3268,10 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
 
   /** The trace of comparison operations when performing `op` */
   def explained[T](op: ExplainingTypeComparer => T, header: String = "Subtype trace:", short: Boolean)(using Context): String =
-    val cmp = explainingTypeComparer(short)
-    inSubComparer(cmp)(op)
-    cmp.lastTrace(header)
+    explaining(cmp => { op(cmp); cmp.lastTrace(header) }, short)
+
+  def explaining[T](op: ExplainingTypeComparer => T, short: Boolean)(using Context): T =
+    inSubComparer(explainingTypeComparer(short))(op)
 
   def reduceMatchWith[T](op: MatchReducer => T)(using Context): T =
     inSubComparer(matchReducer)(op)
@@ -3440,6 +3441,9 @@ object TypeComparer {
   def explained[T](op: ExplainingTypeComparer => T, header: String = "Subtype trace:", short: Boolean = false)(using Context): String =
     comparing(_.explained(op, header, short))
 
+  def explaining[T](op: ExplainingTypeComparer => T, short: Boolean = false)(using Context): T =
+    comparing(_.explaining(op, short))
+
   def reduceMatchWith[T](op: MatchReducer => T)(using Context): T =
     comparing(_.reduceMatchWith(op))
 
@@ -3873,7 +3877,7 @@ class ExplainingTypeComparer(initctx: Context, short: Boolean) extends TypeCompa
   override def recur(tp1: Type, tp2: Type): Boolean =
     def moreInfo =
       if Config.verboseExplainSubtype || ctx.settings.verbose.value
-      then s" ${tp1.getClass} ${tp2.getClass}"
+      then s" ${tp1.className} ${tp2.className}"
       else ""
     val approx = approxState
     def approxStr = if short then "" else approx.show

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

@@ -691,11 +691,22 @@ object TypeOps:
       val hiBound = instantiate(bounds.hi, skolemizedArgTypes)
       val loBound = instantiate(bounds.lo, skolemizedArgTypes)
 
-      def check(using Context) = {
-        if (!(lo <:< hiBound)) violations += ((arg, "upper", hiBound))
-        if (!(loBound <:< hi)) violations += ((arg, "lower", loBound))
+      def check(tp1: Type, tp2: Type, which: String, bound: Type)(using Context) = {
+        val isSub = TypeComparer.explaining { cmp =>
+          val isSub = cmp.isSubType(tp1, tp2)
+          if !isSub then
+            if !ctx.typerState.constraint.domainLambdas.isEmpty then
+              typr.println(i"${ctx.typerState.constraint}")
+            if !ctx.gadt.symbols.isEmpty then
+              typr.println(i"${ctx.gadt}")
+            typr.println(cmp.lastTrace(i"checkOverlapsBounds($lo, $hi, $arg, $bounds)($which)"))
+            //trace.dumpStack()
+          isSub
+        }//(using ctx.fresh.setSetting(ctx.settings.verbose, true)) // uncomment to enable moreInfo in ExplainingTypeComparer recur
+        if !isSub then violations += ((arg, which, bound))
       }
-      check(using checkCtx)
+      check(lo, hiBound, "upper", hiBound)(using checkCtx)
+      check(loBound, hi, "lower", loBound)(using checkCtx)
     }
 
     def loop(args: List[Tree], boundss: List[TypeBounds]): Unit = args match

compiler/src/dotty/tools/dotc/reporting/trace.scala

@@ -27,6 +27,18 @@ object trace extends TraceSyntax:
   object log extends TraceSyntax:
     inline def isEnabled: true = true
     protected val isForced = false
+
+  def dumpStack(limit: Int = -1): Unit = {
+    val out = Console.out
+    val exc = new Exception("Dump Stack")
+    var stack = exc.getStackTrace
+      .filter(e => !e.getClassName.startsWith("dotty.tools.dotc.reporting.TraceSyntax"))
+      .filter(e => !e.getClassName.startsWith("dotty.tools.dotc.reporting.trace"))
+    if limit >= 0 then
+      stack = stack.take(limit)
+    exc.setStackTrace(stack)
+    exc.printStackTrace(out)
+  }
 end trace
 
 /** This module is carefully optimized to give zero overhead if Config.tracingEnabled