disallow begin inside indexing

part of #23354, #25458

NEWS.md

@@ -184,6 +184,9 @@ Language changes
   * `=>` now has its own precedence level, giving it strictly higher precedence than
     `=` and `,` ([#25391]).
 
+  * `begin` is disallowed inside indexing expressions, in order to enable the syntax
+    `a[begin]` (for selecting the first element) in the future ([#23354]).
+
   * Underscores for `_italics_` and `__bold__` are now supported by the Base Markdown
     parser. ([#25564])
 

src/julia-parser.scm

@@ -1318,6 +1318,8 @@
 
 ;; parse expressions or blocks introduced by syntactic reserved words
 (define (parse-resword s word)
+  (if (and (eq? word 'begin) end-symbol)
+      (parser-depwarn s "\"begin\" inside indexing expression" ""))
   (with-bindings
    ((expect-end-current-line (input-port-line (ts:port s))))
    (with-normal-context