Fix records with type parameters (scala#19578).

This fixes a whole host of subtle issues.

 - The type parameter was not stamped correctly on the constructor
   causing the original error
 - The parsed record was not stamped with `JavaDefined`, which meant
   the duplicate constructors in the case of overrides were not
   removed.

compiler/src/dotty/tools/dotc/parsing/JavaParsers.scala

@@ -860,7 +860,7 @@ object JavaParsers {
 
       // generate the canonical constructor
       val canonicalConstructor =
-        DefDef(nme.CONSTRUCTOR, joinParams(tparams, List(header)), TypeTree(), EmptyTree)
+        DefDef(nme.CONSTRUCTOR, joinParams(Nil, List(header)), TypeTree(), EmptyTree)
           .withMods(Modifiers(Flags.JavaDefined | Flags.Synthetic, mods.privateWithin))
 
       // return the trees
@@ -872,7 +872,7 @@ object JavaParsers {
             tparams = tparams,
             needsDummyConstr = true
           )
-        ).withMods(mods)
+        ).withMods(mods.withFlags(Flags.JavaDefined | Flags.Final))
       }
       addCompanionObject(statics, recordTypeDef)
     end recordDecl

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

@@ -879,6 +879,10 @@ class Namer { typer: Typer =>
      *  with a user-defined method in the same scope with a matching type.
      */
     private def invalidateIfClashingSynthetic(denot: SymDenotation): Unit =
+
+      def isJavaRecord(owner: Symbol) =
+        owner.is(JavaDefined) && owner.derivesFrom(defn.JavaRecordClass)
+
       def isCaseClassOrCompanion(owner: Symbol) =
         owner.isClass && {
           if (owner.is(Module)) owner.linkedClass.is(CaseClass)
@@ -904,7 +908,7 @@ class Namer { typer: Typer =>
           ||
           // remove synthetic constructor of a java Record if it clashes with a non-synthetic constructor
           (denot.isConstructor
-            && denot.owner.is(JavaDefined) && denot.owner.derivesFrom(defn.JavaRecordClass)
+            && isJavaRecord(denot.owner)
             && denot.owner.unforcedDecls.lookupAll(denot.name).exists(c => c != denot.symbol && c.info.matches(denot.info))
           )
         )