unpack.c: Cast negative signed offset to signedHEADmaster

As N1570§6.5.16.2p3 states, E1 += E2 is equivalent to E1 = E1 + (E2).
This means that E1 + (E2) will be subject to (6.5.6p4) "the usual
arithmetic conversions". Since both types have equal integer conversion
rank (6.3.1.1p1), E1 which was intmax_t will first be converted to
uintmax_t before being added to (E2). This is not the intended result as
it relies on integer wrapping behaviour again. Casting E2 to intmax_t
first prevents the conversion of E1 and produces the correct result.

1 files changed, 1 insertions, 1 deletions

diff --git a/unpack.c b/unpack.c
index 7786ca6..45ba8fd 100644
--- a/unpack.c
+++ b/unpack.c
@@ -79,7 +79,7 @@ static void read_fields(struct dest dest, const void *src_, enum pack_endian end
 				vals = val.unsigned_;
 			} else {
 				vals = minval(s);
-				vals += val.unsigned_ ^ (UINTMAX_C(1) << (s * 8 - 1));
+				vals += (intmax_t)(val.unsigned_ ^ (UINTMAX_C(1) << (s * 8 - 1)));
 			}
 			val.signed_ = vals;
 			tr_debug("val.s: %" PRIdMAX, val.signed_);