only allow g in patterns

Author: liufengyun

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

@@ -625,7 +625,7 @@ object Parsers {
             atPos(in.offset) {
               if (in.token == IDENTIFIER)
                 termIdent()
-              else if (in.token == USCORE) {
+              else if (in.token == USCORE && inPattern) {
                 in.nextToken()
                 Ident(nme.WILDCARD)
               }

tests/neg/i1779.scala

@@ -0,0 +1,13 @@
+object Test {
+  implicit class Foo(sc: StringContext) {
+    object q {
+      def apply(arg: Any*): Int = 3
+    }
+  }
+
+  def f = {
+    val _parent = 3
+    val x = q"class $_ extends $_parent" // error
+  }
+}
+// error