Avoid using ExplainingTypeComparer in regular code

The operations of an ExplainingTypeComparer are expensive. So we should only
run it when producing an error message.

Author: odersky

Diff for: compiler/src/dotty/tools/dotc/core/TypeOps.scala

@@ -691,20 +691,18 @@ object TypeOps:
       val hiBound = instantiate(bounds.hi, skolemizedArgTypes)
       val loBound = instantiate(bounds.lo, skolemizedArgTypes)
 
-      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))
-      }
+      def check(tp1: Type, tp2: Type, which: String, bound: Type)(using Context) =
+        val isSub = TypeComparer.isSubType(tp1, tp2)
+        if !isSub then
+          // inContext(ctx.fresh.setSetting(ctx.settings.verbose, true)):  // uncomment to enable moreInfo in ExplainingTypeComparer
+            TypeComparer.explaining: cmp =>
+              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)"))
+            violations += ((arg, which, bound))
+
       check(lo, hiBound, "upper", hiBound)(using checkCtx)
       check(loBound, hi, "lower", loBound)(using checkCtx)
     }