Seq/String/Array pattern matches: proposed solution

I have this suspicion that we've been overthinking this. This was
almost too easy, so I'm afraid I'm overlooking something, but it looks
good to me. Here are three bits of code, all of which match in 2.7, two
of which fail in 2.8 and throw runtime errors, or would fail silently
were there a default case. After my proposed patch, instead we get
this:

scala> "bob".reverse match { case Seq('b', 'o', 'b') => true }
:5: error: pattern type is incompatible with scrutinee ;
found : Seq[A]
required: String
"bob".reverse match { case Seq('b', 'o', 'b') => true }
^

scala> "bob".toArray match { case Seq('b', 'o', 'b') => true }
:5: error: pattern type is incompatible with scrutinee ;
found : Seq[A]
required: Array[Char]
"bob".toArray match { case Seq('b', 'o', 'b') => true }
^

scala> "bob".toList match { case Seq('b', 'o', 'b') => true }
res2: Boolean = true

Failing seems like outright desirable behavior to me. I built the
compiler with my change and 4 tests fail. Let's see what they are.

1) files/pos/t0999.scala no longer compiles. Feature. It will never
match. Test case only exists looking for a crasher (or so I infer based
on the fact that it crashes scala 2.7.0.)

object A {
val d: Double = Math.sqrt(5 match {
case x: Double => x
})
}

2) files/pos/bug0646.scala no longer compiles. Feature. It will never
match.

new NodeSeq { val theSeq = books.child } match {
case t @ Blabla => t
}

3) files/neg/bug1878.scala has an extra error. Feature. It will never
match, and the test only exists looking for a crasher.

val err1 = "" match { case Seq(f @ _*, ',') => f }

4) files/run/withIndex.scala does not compile. This is the only
legitimate error. It has something to do with Arrays and underscores,
because the same construct with Lists matches fine. Basically, it is:

scala> Array(("a",0)) match { case _: Array[(_, _)] => true }
:5: error: pattern type is incompatible with scrutinee ;
found : Array[(_, _)]
required: Array[(java.lang.String, Int)]
Array(("a",0)) match { case _: Array[(_, _)] => true }
^

Oh yes, the patch. All of two lines. In inferTypedPattern, right after
widening pt0, add:

if (!(pt <:< pattp) && pt.typeSymbol.isEffectivelyFinal)
error(pos, "pattern type is incompatible with scrutinee "+foundReqMsg(pattp, pt0))