Fix dynamic named arg format spec handling

[dinomight]

When dealing with dynamic named format args, need to account for nested named args when skipping the content of the replacement.

Fixes #4360

[vitaut]

Thanks for the PR. I think the correct fix is to disable compile-time checks if there named arguments that are not known at compile time rather than trying to approximately parse.

[dinomight]

That would disable arg ID checks also which seems to be an existing supported behavior when you mix static and dynamic named args. See how arg IDs are handled.

fmt/include/fmt/base.h

Lines 1693 to 1699 in bdbf957

	FMT_CONSTEXPR auto on_arg_id(basic_string_view<Char> id) -> int {
	for (int i = 0; i < NUM_NAMED_ARGS; ++i) {
	if (named_args_[i].name == id) return named_args_[i].id;
	}
	if (!DYNAMIC_NAMES) on_error("argument not found");
	return -1;
	}

So are you suggesting to remove that as well? What about creating some default parse function that only verifies arg IDs within the format spec that it can verify at compile time?