[fhgffy]

An explicit empty short-option value (-d "") or positional argument is currently accepted as the double value 0. strtod returns zero without consuming any characters, but arg_dbl_scanfn only checks whether the end pointer points to the string terminator.

Require the conversion to consume characters before storing the value. Empty input now follows the existing invalid-double error path without changing the stored default or argument count. Omitted optional values retain their current behavior. Long options already report a missing-argument error for an explicit empty value; the double scanner now also rejects that value, consistent with the integer scanner.

Six tests cover short and positional empty values, explicit long-option values, omitted versus empty optional values, repeated arguments, valid zero/negative/exponent values, and neighboring invalid inputs. Five of these tests fail on the original code; the conversion controls pass.

Validation on Linux with GCC 14.2.0 and CMake 3.31.6:

• Debug and Release: static library, individual sources, and generated amalgamation tests pass
• Shared Debug, system getopt, and long-only configurations: all three CTest targets pass in each configuration
• ASan/UBSan: all three CTest targets pass with leak detection disabled
• Installed CMake package consumer passes the empty-value regression
• A separate 29-assertion parser/callback test passes with bundled getopt, system getopt, and LONG_ONLY; the original code fails eight empty-input assertions in each configuration

The full static/source/amalgamation suite contains 176 tests; the shared-library suite contains 169. The amalgamation was regenerated before compiling. Its line-ending normalization command (dos2unix) is unavailable in this environment; generation, compilation, and tests completed with the existing line endings. Clang, Windows/MSVC, Valgrind, and LeakSanitizer were not tested.

[tomghuang]

@fhgffy: Really appreciate your patch to correct the behavior. All the pipeline checks passed, so I'll merge it. Thanks!