Skip widenSingletons in widenInferred if the inferred type is too large

Author: noti0na1

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

@@ -648,11 +648,13 @@ trait ConstraintHandling {
    * as those could leak the annotation to users (see run/inferred-repeated-result).
    */
   def widenInferred(inst: Type, bound: Type, widenUnions: Boolean)(using Context): Type =
+    def typeSize(tp: Type): Int = tp match
+      case tp: AndOrType => typeSize(tp.tp1) + typeSize(tp.tp2)
+      case _ => 1
+
     def widenOr(tp: Type) =
-      if widenUnions then
-        val tpw = tp.widenUnion
-        if (tpw ne tp) && !tpw.isTransparent() && (tpw <:< bound) then tpw else tp
-      else tp.hardenUnions
+      val tpw = tp.widenUnion
+      if (tpw ne tp) && !tpw.isTransparent() && (tpw <:< bound) then tpw else tp
 
     def widenSingle(tp: Type) =
       val tpw = tp.widenSingletons
@@ -665,8 +667,16 @@ trait ConstraintHandling {
     val wideInst =
       if isSingleton(bound) then inst
       else
-        val widenedFromSingle = widenSingle(inst)
-        val widenedFromUnion = widenOr(widenedFromSingle)
+        val widenedFromUnion =
+          if widenUnions && typeSize(inst) > 64 then
+            // If the inferred type `inst` is too large, the subtype check for `bound` in `widenSingle`
+            // can be expensive due to comparisons between large union types, so we avoid it by
+            // `widenUnion` directly here.
+            // See issue #19907.
+            widenOr(inst)
+          else
+            val widenedFromSingle = widenSingle(inst)
+            if widenUnions then widenOr(widenedFromSingle) else widenedFromSingle.hardenUnions
         val widened = dropTransparentTraits(widenedFromUnion, bound)
         widenIrreducible(widened)