Resolve # ## #

1 files changed, 11 insertions, 3 deletions

diff --git a/module/c/preprocessor2.scm b/module/c/preprocessor2.scm
index 3cb5913f..4678ded7 100644
--- a/module/c/preprocessor2.scm
+++ b/module/c/preprocessor2.scm
@@ -26,7 +26,8 @@
 
 (define parameter-map? (of-type? (alist-of string? (list-of lexeme?))))
 
-(define (concat-token? token)            (equal? "##"  (punctuator-token? token)))
+(define (concat-token? token)            (and (equal? "##"  (punctuator-token? token))
+                                              (not (member "##" (lexeme-noexpand token)))))
 (define (stringify-token? token)         (equal? "#"   (punctuator-token? token)))
 (define (left-parenthesis-token? token)  (equal? "("   (punctuator-token? token)))
 (define (right-parenthesis-token? token) (equal? ")"   (punctuator-token? token)))
@@ -90,8 +91,15 @@
                     (loop (cdr l) r))
                    ((placemaker-token? (car r))
                     (loop (cdr l) (cons (car l) (cdr r))))
-                   (else (loop (cdr l) (cons (concatenate-tokens (car l) (car r))
-                                             (cdr r)))))))
+                   (else
+                    ;; 6.10.3.3 p. 3
+                    ;; I believe that ## is the only special case where the
+                    ;; result of concatenation is differente from the token directly.
+                    (let ((token (concatenate-tokens (car l) (car r))))
+                      (let ((token (if (concat-token? token)
+                                       (modify token lexeme-noexpand xcons "##")
+                                       token)))
+                        (loop (cdr l) (cons token (cdr r)))))))))
           (else
            (let ((pre post (break concat-token? right)))
              (loop (append left (reverse pre)) post))))))