fix #1779: support $_ and $_id in interpolated string

Author: liufengyun

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -625,6 +625,10 @@ object Parsers {
             atPos(in.offset) {
               if (in.token == IDENTIFIER)
                 termIdent()
+              else if (in.token == USCORE) {
+                in.nextToken()
+                Ident(nme.WILDCARD)
+              }
               else if (in.token == THIS) {
                 in.nextToken()
                 This(EmptyTypeIdent)

compiler/src/dotty/tools/dotc/parsing/Scanners.scala

@@ -758,7 +758,7 @@ object Scanners {
           finishStringPart()
           nextRawChar()
           next.token = LBRACE
-        } else if (Character.isUnicodeIdentifierStart(ch)) {
+        } else if (Character.isUnicodeIdentifierStart(ch) || ch == '_') {
           finishStringPart()
           do {
             putChar(ch)

tests/run/i1779.check

@@ -0,0 +1 @@
+ extends 

tests/run/i1779.scala

@@ -0,0 +1,13 @@
+object Test {
+  implicit class Foo(sc: StringContext) {
+    object q {
+      def unapply(arg: Any): Option[(Any, Any)] =
+        Some((sc.parts(0), sc.parts(1)))
+    }
+  }
+
+  def main(args: Array[String]): Unit = {
+    val q"class $_ extends $_parent" = new Object
+    println(_parent)
+  }
+}