SI-7715 String inpatternation s"$_" for s"${_}"

In a pattern,

```
scala> implicit class RX(val sc: StringContext) {
     | def rx = sc.parts.mkString("(.+)").r }
defined class RX

scala> "2 by 4" match { case rx"$a by $_" => a }
res0: String = 2

scala> val rx"$_ by $b" = "2 by 4"
b: String = 4
```

1 files changed, 24 insertions, 0 deletions

diff --git a/test/files/run/t7715.scala b/test/files/run/t7715.scala
new file mode 100644
index 0000000000..0ad3913016
--- /dev/null
+++ b/test/files/run/t7715.scala
@@ -0,0 +1,24 @@
+
+import PartialFunction.cond
+import util._
+
+object Test extends App {
+
+  object I { def unapply(x: String): Option[Int] = Try(x.toInt).toOption }
+  implicit class RX(val sc: StringContext) {
+    def rx = sc.parts.mkString("(.+)").r
+  }
+
+  Console println ("2 by 4" match {
+    case rx"${I(a)} by ${I(b)}" => a+b
+    case _                      => -1
+  })
+  Console println ("2 by 4" match {
+    case rx"${_} by ${I(b)}"    => b    // pattern placeholder
+    case _                      => -1
+  })
+  Console println ("2 by 4" match {
+    case rx"$_ by ${I(b)}"      => b    // is permitted this way, too
+    case _                      => -1
+  })
+}