[PATCH] lib/base64: fix -Wconstant-conversion warning in INIT_1()

[Rosalie Fang <rosaliefang@google.com>]

When expanding BASE64_REV_INIT() for v in 0x7c..0x7f (124..127), the
unselected ternary branch (v) - '0' + 52 evaluates to 128..131, which
exceeds SCHAR_MAX (127). Recent Clang versions diagnose implicit integer
constant conversions in unevaluated ternary arms when initializing the
s8 lookup tables:

  lib/base64.c:58:18: error: implicit conversion from 'int' to 's8'
  (aka 'signed char') changes value from 131 to -125
  [-Werror,-Wconstant-conversion]
     58 |         [BASE64_IMAP] = BASE64_REV_INIT('+', ',')
        |                         ^~~~~~~~~~~~~~~~~~~~~~~~~
  lib/base64.c:34:42: note: expanded from macro 'INIT_1'
     34 |                 : (v) >= '0' && (v) <= '9' ? (v) - '0' + 52 \
        |                                              ~~~~~~~~~~^~~~

Explicitly cast the digit-branch expression to s8 to silence the
warning.

Fixes: c4eb7ad32eab ("lib/base64: optimize base64_decode() with reverse lookup tables")
Assisted-by: LLM
Signed-off-by: Rosalie Fang <rosaliefang@google.com>
---
 lib/base64.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/base64.c b/lib/base64.c
index 325c7332b049..1489c048b1fd 100644
--- a/lib/base64.c
+++ b/lib/base64.c
@@ -31,7 +31,7 @@ static const char base64_tables[][65] = {
 #define INIT_1(v, ch_62, ch_63) \
 	[v] = (v) >= 'A' && (v) <= 'Z' ? (v) - 'A' \
 		: (v) >= 'a' && (v) <= 'z' ? (v) - 'a' + 26 \
-		: (v) >= '0' && (v) <= '9' ? (v) - '0' + 52 \
+		: (v) >= '0' && (v) <= '9' ? (s8)((v) - '0' + 52) \
 		: (v) == (ch_62) ? 62 : (v) == (ch_63) ? 63 : -1
 
 /*
-- 
2.56.0.385.gac671c819a-goog