Inline GADT state restoring in TypeComparer

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

@@ -26,6 +26,11 @@ sealed trait GadtConstraint (
 
   import dotty.tools.dotc.config.Printers.{gadts, gadtsConstr}
 
+  private[core] def getConstraint: Constraint = constraint
+  private[core] def getMapping: SimpleIdentityMap[Symbol, TypeVar] = mapping
+  private[core] def getReverseMapping: SimpleIdentityMap[TypeParamRef, Symbol] = reverseMapping
+  private[core] def getWasConstrained: Boolean = wasConstrained
+
   /** Exposes ConstraintHandling.subsumes */
   def subsumes(left: GadtConstraint, right: GadtConstraint, pre: GadtConstraint)(using Context): Boolean = {
     def extractConstraint(g: GadtConstraint) = g.constraint
@@ -198,6 +203,25 @@ sealed trait GadtConstraint (
     this.reverseMapping = other.reverseMapping
     this.wasConstrained = other.wasConstrained
 
+  def restore(constr: Constraint, mapping: SimpleIdentityMap[Symbol, TypeVar], revMapping: SimpleIdentityMap[TypeParamRef, Symbol], wasConstrained: Boolean): Unit =
+    this.myConstraint = constr
+    this.mapping = mapping
+    this.reverseMapping = revMapping
+    this.wasConstrained = wasConstrained
+
+  def rollbackGadtUnless(op: => Boolean): Boolean =
+    val savedConstr = myConstraint
+    val savedMapping = mapping
+    val savedReverseMapping = reverseMapping
+    val savedWasConstrained = wasConstrained
+    var result = false
+    try
+      result = op
+    finally
+      if !result then
+        restore(savedConstr, savedMapping, savedReverseMapping, savedWasConstrained)
+    result
+
   // ---- Protected/internal -----------------------------------------------
 
   override protected def constraint = myConstraint

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

@@ -265,26 +265,26 @@ trait PatternTypeConstrainer { self: TypeComparer =>
       (tp, pt) match {
         case (AppliedType(tyconS, argsS), AppliedType(tyconP, argsP)) =>
           val saved = state.nn.constraint
-          val savedGadt = ctx.gadt.fresh
           val result =
-            tyconS.typeParams.lazyZip(argsS).lazyZip(argsP).forall { (param, argS, argP) =>
-              val variance = param.paramVarianceSign
-              if variance == 0 || assumeInvariantRefinement ||
-                // As a special case, when pattern and scrutinee types have the same type constructor,
-                // we infer better bounds for pattern-bound abstract types.
-                argP.typeSymbol.isPatternBound && patternTp.classSymbol == scrutineeTp.classSymbol
-              then
-                val TypeBounds(loS, hiS) = argS.bounds
-                val TypeBounds(loP, hiP) = argP.bounds
-                var res = true
-                if variance <  1 then res &&= isSubType(loS, hiP)
-                if variance > -1 then res &&= isSubType(loP, hiS)
-                res
-              else true
+            ctx.gadt.rollbackGadtUnless {
+              tyconS.typeParams.lazyZip(argsS).lazyZip(argsP).forall { (param, argS, argP) =>
+                val variance = param.paramVarianceSign
+                if variance == 0 || assumeInvariantRefinement ||
+                  // As a special case, when pattern and scrutinee types have the same type constructor,
+                  // we infer better bounds for pattern-bound abstract types.
+                  argP.typeSymbol.isPatternBound && patternTp.classSymbol == scrutineeTp.classSymbol
+                then
+                  val TypeBounds(loS, hiS) = argS.bounds
+                  val TypeBounds(loP, hiP) = argP.bounds
+                  var res = true
+                  if variance <  1 then res &&= isSubType(loS, hiP)
+                  if variance > -1 then res &&= isSubType(loP, hiS)
+                  res
+                else true
+              }
             }
           if !result then
             constraint = saved
-            ctx.gadt.restore(savedGadt)
           result
         case _ =>
           // Give up if we don't get AppliedType, e.g. if we upcasted to Any.

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

@@ -1443,10 +1443,13 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
     else if tp1 eq tp2 then true
     else
       val saved = constraint
-      val savedGadt = ctx.gadt.fresh
+      val savedGadtConstr = ctx.gadt.getConstraint
+      val savedMapping = ctx.gadt.getMapping
+      val savedReverseMapping = ctx.gadt.getReverseMapping
+      val savedWasConstrained = ctx.gadt.getWasConstrained
       inline def restore() =
         state.constraint = saved
-        ctx.gadt.restore(savedGadt)
+        ctx.gadt.restore(savedGadtConstr, savedMapping, savedReverseMapping, savedWasConstrained)
       val savedSuccessCount = successCount
       try
         recCount += 1
@@ -2050,10 +2053,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
       gadts.println(i"narrow gadt bound of $tparam: ${tparam.info} from ${if (isUpper) "above" else "below"} to $bound ${bound.toString} ${bound.isRef(tparam)}")
       if (bound.isRef(tparam)) false
       else
-        val savedGadt = ctx.gadt.fresh
-        val success = gadtAddBound(tparam, bound, isUpper)
-        if !success then ctx.gadt.restore(savedGadt)
-        success
+        ctx.gadt.rollbackGadtUnless(gadtAddBound(tparam, bound, isUpper))
     }
   }