Adapt mirror synthesis for old classes

Author: EugeneFlesselle

compiler/src/dotty/tools/dotc/transform/SymUtils.scala

@@ -19,6 +19,7 @@ import Phases._
 import ast.tpd.Literal
 
 import dotty.tools.dotc.transform.sjs.JSSymUtils.sjsNeedsField
+import dotty.tools.tasty.TastyFormat
 
 import scala.annotation.tailrec
 
@@ -107,6 +108,13 @@ object SymUtils:
 
     def isGenericProduct(using Context): Boolean = whyNotGenericProduct.isEmpty
 
+    /** Is a case class for which mirrors support access to default arguments.
+     */
+    def mirrorSupportsDefaultArguments(using Context): Boolean = self.isClass && {
+      val d = self.asClass.classDenot
+      TastyFormat.isVersionCompatible(28, 4, 1, d.tastyMajorVersion, d.tastyMinorVersion, d.tastyExperimentalVersion)
+    }
+
     /** Is this an old style implicit conversion?
      *  @param directOnly            only consider explicitly written methods
      *  @param forImplicitClassOnly  only consider methods generated from implicit classes

compiler/src/dotty/tools/dotc/transform/SyntheticMembers.scala

@@ -646,7 +646,7 @@ class SyntheticMembers(thisPhase: DenotTransformer) {
           synthesizeDef(meth, vrefss => body(cls, vrefss.head.head))
       }
     }
-    def overrideMethod(name: TermName, info: Type, cls: Symbol, body: (Symbol, Tree) => Context ?=> Tree): Unit = { // TODO online override if changed
+    def overrideMethod(name: TermName, info: Type, cls: Symbol, body: (Symbol, Tree) => Context ?=> Tree): Unit = {
       val meth = newSymbol(clazz, name, Synthetic | Method | Override, info, coord = clazz.coord)
       meth.enteredAfter(thisPhase)
       newBody = newBody :+ synthesizeExperimentalDef(meth, vrefss => body(cls, vrefss.head.head))
@@ -668,8 +668,8 @@ class SyntheticMembers(thisPhase: DenotTransformer) {
       addParent(defn.Mirror_ProductClass.typeRef)
       addMethod(nme.fromProduct, MethodType(defn.ProductClass.typeRef :: Nil, monoType.typeRef), cls,
         fromProductBody(_, _, optInfo).ensureConforms(monoType.typeRef))  // t4758.scala or i3381.scala are examples where a cast is needed
-      if cls.primaryConstructor.hasDefaultParams then overrideMethod(nme.defaultArgument,
-        MethodType(defn.IntType :: Nil, defn.AnyType), cls, defaultArgumentBody(_, _, optInfo))
+      if cls.mirrorSupportsDefaultArguments && cls.primaryConstructor.hasDefaultParams then overrideMethod(
+        nme.defaultArgument, MethodType(defn.IntType :: Nil, defn.AnyType), cls, defaultArgumentBody(_, _, optInfo))
     }
     def makeSumMirror(cls: Symbol, optInfo: Option[MirrorImpl.OfSum]) = {
       addParent(defn.Mirror_SumClass.typeRef)

compiler/src/dotty/tools/dotc/typer/Synthesizer.scala

@@ -414,7 +414,8 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
       val elemLabels = accessors.map(acc => ConstantType(Constant(acc.name.toString)))
       val elemsLabels = TypeOps.nestedPairs(elemLabels)
 
-      val elemHasDefaults = accessors.map(acc => ConstantType(Constant(acc.is(HasDefault))))
+      val supportsDefaults = cls.mirrorSupportsDefaultArguments
+      val elemHasDefaults = accessors.map(acc => ConstantType(Constant(supportsDefaults && acc.is(HasDefault))))
       val elemsHasDefaults = TypeOps.nestedPairs(elemHasDefaults)
 
       val typeElems = tps.getOrElse(accessors.map(mirroredType.resultType.memberInfo(_).widenExpr))