SI-5956 trigger copy generation with correct namer

the call to `addCopyMethod` passes `templateNamer`, which is supposed to be the
namer of the case class template. with two classes of the same name, `addCopyMethod`
was triggered in the wrong template.

src/compiler/scala/tools/nsc/typechecker/Namers.scala

@@ -835,13 +835,15 @@ trait Namers extends MethodSynthesis {
 
       // add the copy method to case classes; this needs to be done here, not in SyntheticMethods, because
       // the namer phase must traverse this copy method to create default getters for its parameters.
-      // here, clazz is the ClassSymbol of the case class (not the module).
-      if (clazz.isClass && !clazz.hasModuleFlag) {
+      // here, clazz is the ClassSymbol of the case class (not the module). (!clazz.hasModuleFlag) excludes
+      // the moduleClass symbol of the companion object when the companion is a "case object".
+      if (clazz.isCaseClass && !clazz.hasModuleFlag) {
         val modClass = companionSymbolOf(clazz, context).moduleClass
         modClass.attachments.get[ClassForCaseCompanionAttachment] foreach { cma =>
           val cdef = cma.caseClass
           def hasCopy(decls: Scope) = (decls lookup nme.copy) != NoSymbol
-          if (cdef.mods.isCase && !hasCopy(decls) &&
+          // SI-5956 needs (cdef.symbol == clazz): there can be multiple class symbols with the same name
+          if (cdef.symbol == clazz && !hasCopy(decls) &&
                   !parents.exists(p => hasCopy(p.typeSymbol.info.decls)) &&
                   !parents.flatMap(_.baseClasses).distinct.exists(bc => hasCopy(bc.info.decls)))
             addCopyMethod(cdef, templateNamer)

test/files/neg/t1286.check

@@ -1,9 +1,5 @@
-a.scala:1: error: Companions 'object Foo' and 'trait Foo' must be defined in same file:
-  Found in t1286/b.scala and t1286/a.scala
-trait Foo {
-      ^
 b.scala:1: error: Companions 'trait Foo' and 'object Foo' must be defined in same file:
   Found in t1286/a.scala and t1286/b.scala
 object Foo extends Foo {
        ^
-two errors found
+one error found

test/files/neg/t5956.check

@@ -0,0 +1,20 @@
+t5956.scala:1: warning: case classes without a parameter list have been deprecated;
+use either case objects or case classes with `()' as parameter list.
+object O { case class C[T]; class C }
+                          ^
+t5956.scala:2: warning: case classes without a parameter list have been deprecated;
+use either case objects or case classes with `()' as parameter list.
+object T { case class C[T]; case class C }
+                          ^
+t5956.scala:2: warning: case classes without a parameter list have been deprecated;
+use either case objects or case classes with `()' as parameter list.
+object T { case class C[T]; case class C }
+                                        ^
+t5956.scala:1: error: C is already defined as case class C
+object O { case class C[T]; class C }
+                                  ^
+t5956.scala:2: error: C is already defined as case class C
+object T { case class C[T]; case class C }
+                                       ^
+three warnings found
+two errors found

test/files/neg/t5956.scala

@@ -0,0 +1,2 @@
+object O { case class C[T]; class C }
+object T { case class C[T]; case class C }