From 5aaea2f6e847be0881f2116cc206e2057d64b524 Mon Sep 17 00:00:00 2001
From: Dale Wijnand <dale.wijnand@gmail.com>
Date: Sat, 17 Aug 2024 12:01:47 +0100
Subject: [PATCH] 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.
---
 .../dotty/tools/dotc/core/TypeComparer.scala  | 12 ++++++++----
 .../src/dotty/tools/dotc/core/TypeOps.scala   | 19 +++++++++++++++----
 .../dotty/tools/dotc/reporting/trace.scala    | 12 ++++++++++++
 3 files changed, 35 insertions(+), 8 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
index 0f74ca40843b..5aab2e277693 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/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
diff --git a/compiler/src/dotty/tools/dotc/core/TypeOps.scala b/compiler/src/dotty/tools/dotc/core/TypeOps.scala
index 8089735bdb0f..0d8801b646ee 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeOps.scala
+++ b/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
diff --git a/compiler/src/dotty/tools/dotc/reporting/trace.scala b/compiler/src/dotty/tools/dotc/reporting/trace.scala
index fbbc3d990969..732e779e9bf7 100644
--- a/compiler/src/dotty/tools/dotc/reporting/trace.scala
+++ b/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