deprecate using _ as an rvalue

Author: JeffBezanson

NEWS.md

@@ -41,6 +41,9 @@ Language changes
   * `isa` is now parsed as an infix operator with the same precedence as `in`
     ([#19677]).
 
+  * The identifier `_` can be assigned, but accessing its value is deprecated,
+    allowing this syntax to be used in the future for discarding values ([#9343], [#18251]).
+
 Breaking changes
 ----------------
 

src/jlfrontend.scm

@@ -99,6 +99,8 @@
                   (or (memq (car e) '(toplevel line module import importall using export
                                                error incomplete))
                       (and (eq? (car e) 'global) (every symbol? (cdr e))))))
+         (if (eq? e '_)
+             (syntax-deprecation #f "_ as an rvalue" ""))
          e)
         (else
          (let ((last *in-expand*))

src/julia-syntax.scm

@@ -3293,15 +3293,20 @@ f(x) = yt(x)
     (define (compile e break-labels value tail)
       (if (or (not (pair? e)) (memq (car e) '(null ssavalue quote inert top core copyast the_exception $
                                                    globalref outerref cdecl stdcall fastcall thiscall llvmcall)))
-          (let ((e (if (and arg-map (symbol? e))
-                       (get arg-map e e)
-                       e)))
-            (cond (tail  (emit-return e))
-                  (value e)
-                  ((or (eq? e 'true) (eq? e 'false)) #f)
-                  ((symbol? e) (emit e) #f)  ;; keep symbols for undefined-var checking
-                  ((and (pair? e) (eq? (car e) 'outerref)) (emit e) #f)  ;; keep globals for undefined-var checking
-                  ((and (pair? e) (eq? (car e) 'globalref)) (emit e) #f) ;; keep globals for undefined-var checking
+          (let ((e1 (if (and arg-map (symbol? e))
+                        (get arg-map e e)
+                        e)))
+            (if (and value (or (eq? e '_)
+                               (and (pair? e) (or (eq? (car e) 'outerref)
+                                                  (eq? (car e) 'globalref))
+                                    (eq? (cadr e) '_))))
+                (syntax-deprecation #f "_ as an rvalue" ""))
+            (cond (tail  (emit-return e1))
+                  (value e1)
+                  ((or (eq? e1 'true) (eq? e1 'false)) #f)
+                  ((symbol? e1) (emit e1) #f)  ;; keep symbols for undefined-var checking
+                  ((and (pair? e1) (eq? (car e1) 'outerref)) (emit e1) #f)  ;; keep globals for undefined-var checking
+                  ((and (pair? e1) (eq? (car e1) 'globalref)) (emit e1) #f) ;; keep globals for undefined-var checking
                   (else #f)))
           (case (car e)
             ((call new foreigncall)