Accept and ignore using in method calls (to aid cross-building with Scala 3)

spec/06-expressions.md

@@ -240,6 +240,7 @@ depending on whether `B` is mixed in with class `Root` or `A`.
 ```ebnf
 SimpleExpr    ::=  SimpleExpr1 ArgumentExprs
 ArgumentExprs ::=  ‘(’ [Exprs] ‘)’
+                |  ‘(’ ‘using’ Exprs ‘)’
                 |  ‘(’ [Exprs ‘,’] PostfixExpr ‘:’ ‘_’ ‘*’ ‘)’
                 |  [nl] BlockExpr
 Exprs         ::=  Expr {‘,’ Expr}
@@ -328,6 +329,9 @@ sum(List(1, 2, 3, 4))
 
 would not typecheck.
 
+An argument list may begin with the soft keyword `using` to facilitate cross-compilation with Scala 3.
+The keyword is ignored.
+
 ### Named and Default Arguments
 
 If an application is to use named arguments ´p = e´ or default

src/compiler/scala/tools/nsc/ast/parser/Parsers.scala

@@ -1916,6 +1916,7 @@ self =>
 
     /** {{{
      *  ArgumentExprs ::= `(` [Exprs] `)`
+     *                  | `(` `using` Exprs `)`
      *                  | [nl] BlockExpr
      *  }}}
      */
@@ -1924,9 +1925,15 @@ self =>
         if (isIdent) treeInfo.assignmentToMaybeNamedArg(expr()) else expr()
       )
       in.token match {
-        case LBRACE   => List(blockExpr())
-        case LPAREN   => inParens(if (in.token == RPAREN) Nil else args())
-        case _        => Nil
+        case LBRACE => List(blockExpr())
+        case LPAREN => inParens {
+          in.token match {
+            case RPAREN => Nil
+            case IDENTIFIER if in.name == nme.using && lookingAhead(isExprIntro) => in.nextToken() ; args()
+            case _ => args()
+          }
+        }
+        case _      => Nil
       }
     }
     /** A succession of argument lists. */

src/reflect/scala/reflect/internal/StdNames.scala

@@ -684,6 +684,7 @@ trait StdNames {
     // Scala 3 soft keywords
     val infix: NameType           = nameType("infix")
     val open: NameType            = nameType("open")
+    val using: NameType           = nameType("using")
 
     // Compiler utilized names
 

test/files/neg/i15552.check

@@ -0,0 +1,7 @@
+i15552.scala:4: error: not found: value g
+  def f() = g(42)(using ctx)  // error: no g
+            ^
+i15552.scala:22: error: not found: value using
+  def f() = g(using)          // error: no using, does not actually suggest Using
+              ^
+2 errors

test/files/neg/i15552.scala

@@ -0,0 +1,40 @@
+// scalac: -Werror
+class C {
+  val ctx: C = new C()
+  def f() = g(42)(using ctx)  // error: no g
+}
+
+class Innocent {
+  val using = 42
+  def g(i: Int) = i
+  def f() = g(using)
+}
+
+class Malicious {
+  val using = 42
+  def g(i: Int) = i
+  def f() = g(using using)
+}
+
+class Suggest {
+  import scala.util._
+  def g(i: Int) = i
+  def f() = g(using)          // error: no using, does not actually suggest Using
+}
+
+class Fancy {
+  def g(i: Int) = i
+  def f() = g(using if (true) 42 else 17)
+}
+
+/*
+was:
+
+i15552.scala:4: error: not found: value g
+  def f() = g(42)(using ctx)
+            ^
+i15552.scala:4: error: not found: value using
+  def f() = g(42)(using ctx)
+                  ^
+i15552.scala:4: error: postfix operator ctx needs to be enabled
+ */