Avoid field/field clashes

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

@@ -369,6 +369,7 @@ object NameKinds {
   val FieldName: SuffixNameKind = new SuffixNameKind(FIELD, "$$local") {
       override def mkString(underlying: TermName, info: ThisInfo) = underlying.toString
   }
+  val ExplicitFieldName: SuffixNameKind = new SuffixNameKind(EXPLICITFIELD, "$field")
   val ExtMethName: SuffixNameKind = new SuffixNameKind(EXTMETH, "$extension")
   val ParamAccessorName: SuffixNameKind = new SuffixNameKind(PARAMACC, "$accessor")
   val ModuleClassName: SuffixNameKind = new SuffixNameKind(OBJECTCLASS, "$", optInfoString = "ModuleClass")

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

@@ -37,6 +37,9 @@ object NameTags extends TastyFormat.NameTags {
   final val AVOIDLOWER = 36
   final val AVOIDBOTH = 37
 
+  inline val EXPLICITFIELD = 38  // An explicitly named field, introduce to avoid a clash
+                                 // with a regular field of the underlying name
+
   def nameTagToString(tag: Int): String = tag match {
     case UTF8 => "UTF8"
     case QUALIFIED => "QUALIFIED"

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

@@ -9,7 +9,9 @@ import dotty.tools.dotc.core.StdNames._
 import ast._
 import Trees._
 import Flags._
+import Names.Name
 import NameOps._
+import NameKinds.{FieldName, ExplicitFieldName}
 import SymUtils._
 import Symbols._
 import Decorators._
@@ -249,7 +251,28 @@ class Constructors extends MiniPhase with IdentityDenotTransformer { thisPhase =
         splitStats(stats1)
       case Nil =>
     }
+
+    /** Check that we do not have both a private field with name `x` and a private field
+     *  with name `FieldName(x)`. These will map to the same JVM name and therefore cause
+     *  a duplicate field error. If that case arises (as in i13862.scala), use an explicit
+     *  name `x$field` instead of `FieldName(x).
+     */
+    def checkNoFieldClashes() =
+      val fieldNames = mutable.HashSet[Name]()
+      for case field: ValDef <- clsStats do
+        field.symbol.name match
+          case FieldName(_) =>
+          case name => fieldNames += name
+      for case field: ValDef <- clsStats do
+        field.symbol.name match
+          case fldName @ FieldName(name) if fieldNames.contains(name) =>
+            val newName = ExplicitFieldName(name)
+            report.log(i"avoid field/field conflict by renaming $fldName to $newName")
+            field.symbol.copySymDenotation(name = newName).installAfter(thisPhase)
+          case _ =>
+
     splitStats(tree.body)
+    checkNoFieldClashes()
 
     // The initializers for the retained accessors */
     val copyParams = accessors flatMap { acc =>

tests/run/i13862.scala

@@ -0,0 +1,11 @@
+trait Foo(val num: Int)                 // a trait with a parameter stored in a val
+
+class Bar(num: Int) extends Foo(num):   // an extending class with a parameter of the same name
+  def bar = this.num                    // implicitly creates another num in Bar
+
+@main def Test = Bar(123)
+
+class Bar2(n: Int) extends Foo(n):   // an extending class with a parameter of the same name
+  private val num = n
+  def bar = this.num                    // implicitly creates another num in Bar
+