Cache AppliedType.isStable

Author: mbovel

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

@@ -165,8 +165,7 @@ object Types {
       case _: SingletonType | NoPrefix => true
       case tp: RefinedOrRecType => tp.parent.isStable
       case tp: ExprType => tp.resultType.isStable
-      case AppliedType(tycon: TypeRef, args: List[Type])
-        if defn.isCompiletimeAppliedType(tycon.symbol) && args.forall(_.isStable) => true
+      case tp: AppliedType => tp.appliedTypeIsStable
       case tp: AnnotatedType =>
         // NOTE UncheckedStableAnnot was originally meant to be put on fields,
         // not on types. Allowing it on types is a Scala 3 extension. See:
@@ -4162,11 +4161,22 @@ object Types {
     // Boolean caches: 0 = uninitialized, -1 = false, 1 = true
     private var myStableHash: Byte = 0
     private var myGround: Byte = 0
+    private var myIsStable: Boolean = false
+    private var myIsStableComputed: Boolean = false
+
 
     def isGround(acc: TypeAccumulator[Boolean])(using Context): Boolean =
       if myGround == 0 then myGround = if acc.foldOver(true, this) then 1 else -1
       myGround > 0
 
+    def appliedTypeIsStable(using Context): Boolean =
+      if !myIsStableComputed then
+        myIsStable = tycon match
+          case tycon: TypeRef => defn.isCompiletimeAppliedType(tycon.symbol) && args.forall(_.isStable)
+          case _ => false
+        myIsStableComputed = true
+      myIsStable
+
     override def underlying(using Context): Type = tycon
 
     override def superType(using Context): Type =