SI-7296 Remove arity limit for case classes

When venturing above the pre-ordained limit of twenty
two, `Companion extends FunctionN` and `Companion.unapply`
are sacrificed. But oh-so-many other case class features
work perfectly: equality/hashing/stringification, the apply
method, and even pattern matching (which already bypasses
unapply.)

There was some understandable fear of the piecemeal when
I tabled this idea on scala-internals [1]. But I'd like
to persist as this limit is a needless source of pain for
anyone using case classes to bind to database, XML or JSON
schemata.

[1] https://groups.google.com/forum/#!topic/scala-internals/RRu5bppi16Y

1 files changed, 10 insertions, 5 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Unapplies.scala b/src/compiler/scala/tools/nsc/typechecker/Unapplies.scala
index 589e5ce6fd..d55dce70c7 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Unapplies.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Unapplies.scala
@@ -115,11 +115,16 @@ trait Unapplies extends ast.TreeDSL
   /** The module corresponding to a case class; overrides toString to show the module's name
    */
   def caseModuleDef(cdef: ClassDef): ModuleDef = {
-    // > MaxFunctionArity is caught in Namers, but for nice error reporting instead of
-    // an abrupt crash we trim the list here.
-    def primaries      = constrParamss(cdef).head take MaxFunctionArity map (_.tpt)
-    def inheritFromFun = !cdef.mods.hasAbstractFlag && cdef.tparams.isEmpty && constrParamss(cdef).length == 1
-    def createFun      = gen.scalaFunctionConstr(primaries, toIdent(cdef), abstractFun = true)
+    val params = constrParamss(cdef)
+    def inheritFromFun = !cdef.mods.hasAbstractFlag && cdef.tparams.isEmpty && (params match {
+      case List(ps) if ps.length <= MaxFunctionArity => true
+      case _ => false
+    })
+    def createFun = {
+      def primaries = params.head map (_.tpt)
+      gen.scalaFunctionConstr(primaries, toIdent(cdef), abstractFun = true)
+    }
+
     def parents        = if (inheritFromFun) List(createFun) else Nil
     def toString       = DefDef(
       Modifiers(OVERRIDE | FINAL | SYNTHETIC),