mapcar*: fix broken.

* eval.c (lazy_mapcar_func): We must capture the return value
of iter_step, since we refer to it in the next statement,
expecting it to have stepped. This bug causes a behavior as if
the original list had an extra nil.

* tests/012/lazy.tl: Tests. Poor test coverage is why this
sort of thing comes up and bites us.

2 files changed, 7 insertions, 1 deletions

diff --git a/eval.c b/eval.c
index b2e0605a..814f99c5 100644
--- a/eval.c
+++ b/eval.c
@@ -5428,7 +5428,7 @@ static val lazy_mapcar_func(val env, val lcons)
   us_cons_bind (fun, iter, env);
 
   us_rplaca(lcons, funcall1(fun, iter_item(iter)));
-  us_rplacd(env, iter_step(iter));
+  us_rplacd(env, iter = iter_step(iter));
 
   if (iter_more(iter))
     us_rplacd(lcons, make_lazy_cons(us_lcons_fun(lcons)));
diff --git a/tests/012/lazy.tl b/tests/012/lazy.tl
new file mode 100644
index 00000000..be04412a
--- /dev/null
+++ b/tests/012/lazy.tl
@@ -0,0 +1,6 @@
+(load "../common")
+
+(test [mapcar* list nil] nil)
+(test [mapcar* list '(1)] ((1)))
+(test [mapcar* list '(1 2 3)] ((1) (2) (3)))
+(test [mapcar* list '(1 2 3) '(a b c)] ((1 a) (2 b) (3 c)))