Deprecate the use of ? as an identifier

Ref #6286

base/docs/helpdb/Base.jl

@@ -250,7 +250,7 @@ the byte representation is different.
 
 This would create a 25-by-30000 `BitArray`, linked to the file associated with stream `s`.
 """
-Mmap.mmap(io, ::BitArray, dims = ?, offset = ?)
+Mmap.mmap(io, ::BitArray, dims, offset)
 
 """
     sizeof(T)
@@ -2151,7 +2151,7 @@ julia> pop!(d, "e", 4)
 4
 ```
 """
-pop!(collection,key,?)
+pop!(collection,key,default)
 
 """
     pop!(collection) -> item

src/julia-parser.scm

@@ -151,6 +151,8 @@
 (define whitespace-newline #f)
 ; enable parsing `where` with high precedence
 (define where-enabled #t)
+; the question mark is only allowed as a token in ternary expressions
+(define allow-question-mark #f)
 
 (define current-filename 'none)
 
@@ -187,6 +189,10 @@
   `(with-bindings ((whitespace-newline #f))
                   ,@body))
 
+(define-macro (with-allow-question-mark . body)
+  `(with-bindings ((allow-question-mark #t))
+                  ,@body))
+
 ;; --- lexer ---
 
 (define (newline? c) (eqv? c #\newline))
@@ -541,11 +547,14 @@
                    t)))))
 
 (define (take-token s)
-  (or
-   (begin0 (ts:pbtok s)
-           (aset! s 3 #f))
-   (begin0 (ts:last-tok s)
-           (ts:set-tok! s #f))))
+  (let ((t (or
+             (begin0 (ts:pbtok s)
+                     (aset! s 3 #f))
+             (begin0 (ts:last-tok s)
+                     (ts:set-tok! s #f)))))
+   (if (and (not allow-question-mark) (eq? t '?))
+       (syntax-deprecation s "`?` used as an identifier" ""))
+   t))
 
 ;; --- misc ---
 
@@ -736,7 +745,7 @@
     (cond ((eq? (peek-token s) '?)
            (begin (if (not (ts:space? s))
                       (syntax-deprecation s (string (deparse ex) "?") (string (deparse ex) " ?")))
-                  (take-token s) ; take the ?
+                  (with-allow-question-mark (take-token s)) ; take the ?
                   (let ((t (with-whitespace-newline (without-range-colon (peek-token s)))))
                     (if (not (ts:space? s))
                         (syntax-deprecation s (string (deparse ex) " ?" (deparse t)) (string (deparse ex) " ? " (deparse t)))))

test/enums.jl

@@ -78,7 +78,7 @@ end
 @test Integer(_zerobi) == 1
 
 # can't use non-identifiers as enum members
-@test_throws ArgumentError eval(:(@enum(Test2, ?)))
+@test_throws ArgumentError eval(:(@enum Test2  x ? 1 : 2))
 @test_throws ArgumentError eval(:(@enum Test22 1=2))
 
 # other Integer types of enum members