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, 18 insertions, 0 deletions

diff --git a/test/files/neg/t7715.scala b/test/files/neg/t7715.scala
new file mode 100644
index 0000000000..637ab8df6d
--- /dev/null
+++ b/test/files/neg/t7715.scala
@@ -0,0 +1,18 @@
+
+import PartialFunction.cond
+import util._
+
+object Test extends App {
+  val days = (1 to 12).toList
+
+  days map s"On the $_th day of Christmas" foreach println
+
+  val rf = (n: Int) => s"\\*{$_}"(n).r
+  def stars(n: Int)(s: String) = {
+    val r = rf(n)
+    cond(s) { case r(_*) => true }
+  }
+  Console println stars(5)("*****")
+
+  days zip days map s"${_: Int} by ${_: Int}".tupled foreach println
+}