Bug #20439: Invalid string format with n$ flag is ignored in some case

• Status: Open
• Assignee: -
• Target version: -
• ruby -v: ruby 3.3.0 (2023-12-25 revision 5124f9ac75) +YJIT +MN [arm64-darwin22]
• Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN

[tompng (tomoya ishida)]

String format including n$ flag and no type specifier raises ArgumentError

"%1$ " % 1 # invalid format character - % (ArgumentError)
"%1$," % 1 # malformed format string - %, (ArgumentError)

But when it ends with n$ flag, or the next char to n$ flag is "\n" or "\0", there is no error. Returns string with n$ flag removed.

"%1$\n" % 1 #=> "%\n"
"%1$\0" % 1 #=> "%\u0000"
"%1$" % 1 #=> "%"

[nobu (Nobuyoshi Nakada)]

• Is duplicate of Bug #20438: String format "%\n" and "%\0" does not raise format error added

[nobu (Nobuyoshi Nakada)]

• Related to Bug #13315: Single "%" at the end of `printf` format string appears in the result added