Fix precondition violation in parse_key when parsing '[[[' table headers

[gistrec]

Description of the bug

parse_table_header() can call parse_key() with an invalid starting character after consuming a table header prefix.

For example, with input like [[[1]]], the parser consumes the first [ and the second [ as an array-of-tables header, then enters parse_key() while *cp still points at the third [. That character is neither a bare-key character nor a string delimiter, so it violates this precondition in parse_key():

TOML_ASSERT_ASSUME(is_bare_key_character(*cp) || is_string_delimiter(*cp));

In debug builds this hits the assertion and aborts the process instead of returning a normal parse_error.

Reproduction

#include "toml++/toml.hpp"
int main()
{
    toml::parse("[[[1]]]");
}

With a debug clang build this currently fails with:

Assertion failed: (is_bare_key_character(*cp) || is_string_delimiter(*cp)),
function parse_key, file parser.inl

Similar small cases are [[[a]]], [[[]], and [[=.

Fix

Before calling parse_key(), parse_table_header() now checks that the next character is a valid key starter.
If it is not, the parser reports a normal parse_error instead of entering parse_key() with a broken precondition.

Tests

Added regression tests for malformed array-of-table headers such as:

• [[[1]]]
• [[[a]]]
• [[[]]
• [[=

Valid table headers and array-of-table headers still parse as before.

[gistrec]

Write me if I should remove the comments from the code

[marzer]

Write me if I should remove the comments from the code

Yes, please. These look like classic LLM comment vomit. Otherwise the fix is good 👍🏼

[marzer]

Thanks!