Adapt mirror synthesis for old classes

Author: EugeneFlesselle

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

@@ -16,6 +16,7 @@ import Constants.Constant
 import Annotations.Annotation
 import Phases.*
 import ast.tpd.Literal
+import dotty.tools.tasty.TastyHeaderUnpickler.TastyVersion
 import transform.Mixin
 
 import dotty.tools.dotc.transform.sjs.JSSymUtils.sjsNeedsField
@@ -115,6 +116,15 @@ class 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.is(JavaDefined) && !self.is(Scala2x) && self.isClass && {
+//        val tastyVersion: TastyVersion = ???
+//        tastyVersion.major == 28 && tastyVersion.minor >= 4
+        true // TODO
+      }
+
     /** 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

@@ -666,8 +666,8 @@ 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

@@ -411,7 +411,8 @@ 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))