mruby-strftime: prevent crash on msvc with unsupported %- flag

add validation to detect gnu extension %- flag and raise argumenterror
on msvc instead of crashing. update test to use portable %m format.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-strftime/src/strftime.c

@@ -73,6 +73,18 @@ mrb_time_strftime(mrb_state *mrb, mrb_value self)
       memcpy(segment, fmt_ptr, (size_t)segment_len);
       segment[segment_len] = '\0';
 
+#ifdef _MSC_VER
+      /* Check for GNU extension %-flag which crashes on MSVC */
+      /* Scan for %- patterns in the format string */
+      for (const char *p = segment; *p != '\0'; p++) {
+        if (p[0] == '%' && p[1] == '-') {
+          mrb_free(mrb, segment);
+          mrb_raisef(mrb, E_ARGUMENT_ERROR,
+                     "strftime format flag '%-' not supported on this platform (use '%%#' on Windows)");
+        }
+      }
+#endif
+
       /* Allocate buffer for formatted output */
       buf_size = INITIAL_BUFFER_SIZE;
       buf = (char *)mrb_malloc(mrb, buf_size);

mrbgems/mruby-strftime/test/strftime.rb

@@ -129,7 +129,8 @@ end
 assert('Time#strftime with various time components') do
   t = Time.gm(2023, 6, 15, 14, 23, 7)
 
-  assert_equal '6', t.strftime('%-m')   if t.strftime('%-m') != ''  # Skip if platform doesn't support %-
+  # Use portable format specifiers that work on all platforms
+  assert_equal '06', t.strftime('%m')   # Month with leading zero (portable)
   assert_equal '15', t.strftime('%d')
   assert_equal '14', t.strftime('%H')
   assert_equal '23', t.strftime('%M')