Improve debugging for explained

Author: liufengyun

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

@@ -2076,15 +2076,14 @@ object TypeComparer {
   /** Show trace of comparison operations when performing `op` as result string */
   def explaining[T](say: String => Unit)(op: Context => T)(implicit ctx: Context): T = {
     val nestedCtx = ctx.fresh.setTypeComparerFn(new ExplainingTypeComparer(_))
-    val res = op(nestedCtx)
-    say(nestedCtx.typeComparer.lastTrace())
+    val res = try { op(nestedCtx) } finally { say(nestedCtx.typeComparer.lastTrace()) }
     res
   }
 
   /** Like [[explaining]], but returns the trace instead */
   def explained[T](op: Context => T)(implicit ctx: Context): String = {
     var trace: String = null
-    explaining(trace = _)(op)
+    try { explaining(trace = _)(op) } catch { case ex: Throwable => ex.printStackTrace }
     trace
   }
 }

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -405,8 +405,8 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
 
   /** Is `tp1` a subtype of `tp2`?  */
   def isSubType(tp1: Type, tp2: Type): Boolean = {
+    debug.println(TypeComparer.explained(implicit ctx => tp1 <:< tp2))
     val res = (tp1 != nullType || tp2 == nullType) && tp1 <:< tp2
-    debug.println(s"${tp1} <:< ${tp2} = $res")
     res
   }