Guard joined-value completion against prefix/separator overlap

[metsw24-max]

Harden the bash-completion path against invalid prefix/separator combinations that can cause std::string::substr() to throw std::out_of_range.

When a custom LongSeparator() overlaps the configured long-option prefix (for example, LongSeparator("-") with the default "--" prefix), find(longseparator) may return a position inside the prefix itself. The completion handler then constructs an arg string shorter than longprefix and subsequently calls:

arg.substr(longprefix.size())

which throws std::out_of_range.

Root Cause

The joined-value completion path assumed that any discovered separator would occur at or beyond the end of the long-option prefix. That invariant was not enforced.

For overlapping prefix/separator configurations, arg may become shorter than longprefix, making:

arg.substr(longprefix.size())

invalid.

In throwing builds, the resulting std::out_of_range escapes as a non-args exception. Under ARGS_NOEXCEPT, the throwing substr() call breaks the no-throw expectation and may result in termination when exceptions are disabled.

Fix

Only attempt joined-value completion when the separator is located at or beyond the end of the long-option prefix:

if (separator != chunk.npos && separator >= longprefix.size())

This prevents invalid substr() calls while preserving existing behavior for valid completion inputs.

Tests

Added two regression tests:

• test/completion_separator_prefix_overlap.cxx

  • Verifies that completion handling follows the normal args::Completion path instead of leaking std::out_of_range.

• test/noexcept_completion_separator_prefix_overlap.cxx

  • Verifies correct behavior under ARGS_NOEXCEPT, ensuring completion is reported through the parser error state without throwing.

Both tests fail on the unpatched code and pass with this change.

Files Changed

• args.hxx
• test/completion_separator_prefix_overlap.cxx
• test/noexcept_completion_separator_prefix_overlap.cxx

[Taywee]

Makes sense to me. Thanks for the PR.