reserve default field value syntax

ref JuliaLang#10146, JuliaLang#11536

src/julia-syntax.scm

@@ -1169,6 +1169,15 @@
  (let ((mut (cadr e))
  (sig (caddr e))
  (fields (cdr (cadddr e))))
+ (let loop ((f fields))
+ (if (null? f)
+ '()
+ (let ((x (car f)))
+ (cond ((or (symbol? x) (decl? x) (and (pair? x) (eq? (car x) 'line)))
+ (loop (cdr f)))
+ ((and (assignment? x) (or (symbol? (cadr x)) (decl? (cadr x))))
+ (error (string "\"" (deparse x) "\" inside type definition is reserved")))
+ (else '())))))
  (expand-binding-forms
  (receive (name params super) (analyze-type-sig sig)
  (struct-def-expr name params super fields mut)))))