Fix #10285: Fix combinations of trait setter names and other derived names

Adding "_=" to the last part of a name does not work if that name is a unique name
or other derived name, since the resulting name would print like `x_=$1` instead
of `x$1_=`. Use a new semantic name for synthetic setter suffix instead.

Author: odersky

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

@@ -367,6 +367,7 @@ object NameKinds {
   val ModuleClassName: SuffixNameKind = new SuffixNameKind(OBJECTCLASS, "$", optInfoString = "ModuleClass")
   val ImplMethName: SuffixNameKind = new SuffixNameKind(IMPLMETH, "$")
   val AdaptedClosureName: SuffixNameKind = new SuffixNameKind(ADAPTEDCLOSURE, "$adapted") { override def definesNewName = true }
+  val SyntheticSetterName: SuffixNameKind = new SuffixNameKind(SETTER, "_=")
 
   /** A name together with a signature. Used in Tasty trees. */
   object SignedName extends NameKind(SIGNED) {

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

@@ -1,4 +1,5 @@
-package dotty.tools.dotc
+package dotty.tools
+package dotc
 package core
 
 import java.security.MessageDigest
@@ -9,7 +10,7 @@ import Names._, StdNames._, Contexts._, Symbols._, Flags._, NameKinds._, Types._
 import util.Chars.{isOperatorPart, digit2int}
 import Definitions._
 import nme._
-import Decorators.concat
+import Decorators._
 
 object NameOps {
 
@@ -69,7 +70,7 @@ object NameOps {
     def isLocalDummyName: Boolean = name startsWith str.LOCALDUMMY_PREFIX
     def isReplWrapperName: Boolean = name.toString contains str.REPL_SESSION_LINE
     def isReplAssignName: Boolean = name.toString contains str.REPL_ASSIGN_SUFFIX
-    def isSetterName: Boolean = name endsWith str.SETTER_SUFFIX
+    def isSetterName: Boolean = name.endsWith(str.SETTER_SUFFIX) || name.is(SyntheticSetterName)
     def isScala2LocalSuffix: Boolean = testSimple(_.endsWith(" "))
     def isSelectorName: Boolean = testSimple(n => n.startsWith("_") && n.drop(1).forall(_.isDigit))
     def isAnonymousClassName: Boolean = name.startsWith(str.ANON_CLASS)
@@ -347,17 +348,21 @@ object NameOps {
 
     def setterName: TermName = name.exclude(FieldName) ++ str.SETTER_SUFFIX
 
+    def syntheticSetterName = SyntheticSetterName(name.exclude(FieldName))
+
     def getterName: TermName =
-      name.exclude(FieldName).mapLast(n =>
+      val name1 = name.exclude(FieldName)
+      if name1.is(SyntheticSetterName) then name1.exclude(SyntheticSetterName)
+      else name1.mapLast(n =>
         if (n.endsWith(str.SETTER_SUFFIX)) n.take(n.length - str.SETTER_SUFFIX.length).asSimpleName
         else n)
 
     def fieldName: TermName =
       if (name.isSetterName)
-        if (name.is(TraitSetterName)) {
-          val TraitSetterName(_, original) = name
-          original.fieldName
-        }
+        if name.is(SyntheticSetterName) then
+          name.exclude(SyntheticSetterName)
+            .replace { case TraitSetterName(_, original) => original }
+            .fieldName
         else getterName.fieldName
       else FieldName(name.toSimpleName)
 

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

@@ -29,6 +29,8 @@ object NameTags extends TastyFormat.NameTags {
 
   final val PARAMACC = 33       // Used for a private parameter alias
 
+  final val SETTER = 34         // A synthesized += suffix.
+
   def nameTagToString(tag: Int): String = tag match {
     case UTF8 => "UTF8"
     case QUALIFIED => "QUALIFIED"

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

@@ -25,7 +25,7 @@ object Mixin {
   def traitSetterName(getter: TermSymbol)(using Context): TermName =
     getter.ensureNotPrivate.name
       .expandedName(getter.owner, TraitSetterName)
-      .asTermName.setterName
+      .asTermName.syntheticSetterName
 }
 
 /** This phase performs the following transformations:

tests/run/i10285.scala

@@ -0,0 +1,25 @@
+trait Companion {
+
+  trait AbstractFoo {
+    val a0 = 1
+    private val b0 = 2
+    def b1 = b0
+    val (a, b) = {
+      val av  = 1
+      val bv  = 2
+      (av, bv)
+    }
+  }
+}
+
+object Bar extends Companion {
+  class FooImpl extends AbstractFoo
+  val foo = new FooImpl()
+}
+
+object Test {
+  val foo = new Bar.FooImpl
+  def main(args: Array[String]): Unit =
+    assert(Bar.foo.a0 + Bar.foo.b1 == 3)
+    assert(Bar.foo.a + Bar.foo.b == 3)
+}