DefinitionsApi.{IntClass, ...} ought to be vals, not defs, to facilitate use as a stable identifier pattern

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See #6815 and https://groups.google.com/d/topic/scala-internals/COqh09lG0P0/discussion for previous discussion/work. But is still broken, at least in terms of the usability of reflection.

The fix version of #6815 says 2.10.2-RC1, but in 2.10.4 there's no progress.

scala> u.emptyValDef match { case u.emptyValDef => }
<console>:9: error: stable identifier required, but u.emptyValDef found.
 Note that value emptyValDef is not stable because its type, => u.ValDef, is volatile.
              u.emptyValDef match { case u.emptyValDef => }
                                           ^

In 2.11.0-RC3 that example works, but more relevant ones don't.

Welcome to Scala version 2.11.0-RC3 (Java HotSpot(TM) 64-Bit Server VM, Java 1.7.0_51).
Type in expressions to have them evaluated.
Type :help for more information.

scala> val u = scala.reflect.runtime.universe
u: scala.reflect.api.JavaUniverse = scala.reflect.runtime.JavaUniverse@4ce1748a

scala> import u._
import u._

scala> (null: Any) match { case TypeRef(_, definitions.IntClass, _) => }
<console>:12: error: stable identifier required, but u.definitions.IntClass found.
 Note that method IntClass is not stable because its type, => u.ClassSymbol, is volatile.
              (null: Any) match { case TypeRef(_, definitions.IntClass, _) => }
                                                              ^

It seems the behavior is a consequence of IntClass and friends appearing as defs via the reflection API, whereas in reality they are lazy vals. If I cast the universe to scala.reflect.internal.SymbolTable then I can pattern match on IntClass.

Given that pattern matching is essentially the only way to get at the structure of a typeref, if pattern matching on the core types doesn't work at all without reassigning to dummy vals it seems like a problem. It isn't so bad if one knows and can apply the secret incantation to expose the internals - maybe it is being taken as a given that everyone does that anyway.

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Imported From: https://issues.scala-lang.org/browse/SI-8483?orig=1
Reporter: @paulp
Affected Versions: 2.11.1
See #6815

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@paulp said:
I withdraw the suggestion that there is a workaround. One simply trades in one set of bugs for another.

scala> val u = scala.reflect.runtime.universe
u: scala.reflect.api.JavaUniverse = scala.reflect.runtime.JavaUniverse@7e73b262

scala> import u._
import u._

scala> def f[T: WeakTypeTag] = weakTypeOf[T]
f: [T](implicit evidence$1: u.WeakTypeTag[T])u.Type

scala> f[List[_]]
res0: u.Type = scala.List[_]

// result of f[List[_]] if u is asInstanceOf[scala.reflect.internal.SymbolTable]
scala> f[List[_]]
scala.ScalaReflectionException: object $iw not found.
  at scala.reflect.internal.Mirrors$RootsBase.staticModule(Mirrors.scala:160)
  at scala.reflect.internal.Mirrors$RootsBase.staticModule(Mirrors.scala:22)
  at $typecreator1$1.apply(<console>:13)

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@xeno-by said:
With the recent pull request that exposes c.internal, casting is actually discouraged, even for those who know their way around internals. This thing is a bug that I overlooked. Too bad that it didn't end up being on the aforementioned pull request, because now we'll have to wait for 2.12 :(

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@retronym said:
Changing the defs to vals might end up being a two-way binary compatible change.

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@retronym said:
Retitling this to make it sound less like a bug in the pattern matcher.

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@paulp said:
Those with long memories may recall they first appeared as vals and I argued to change them into defs. Those reasons still stand: the problem is that there are no stable defs, so you are always left to choose between subsets of desirable qualities. See comments in #4605.

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@szeiger said:
Scheduling for 2.12.0-M5. This would be the right time to get this into 2.12 before we try to freeze binary compatibility.