I was way too terse with my answer.

You could also try specifying the branch name directly:

FetchContent_Declare(
    MicroTeX
    GIT_REPOSITORY https://github.com/NanoMichael/MicroTeX.git
    GIT_TAG openmath
    GIT_SHALLOW FALSE
)

If you've already configured the project once, delete the _deps directory (or at least the microtex-src folder) and rerun CMake so it performs a fresh clone. I've run into cases where FetchContent kept using the cached repository and ignored changes to GIT_TAG.

If it still fails, does the error say it can't find the openmath ref, or is it failing during the clone itself? That detail would help narrow down whether it's a branch name issue or something else.

I've tried to change the GIT_TAG to origin/openmath or "openmath". However, doing so blocks all builds, as CMake says it could not clone the repository.

Can you show us the error message?

I used GIT_TAG origin/openmath and it fetched correctly, however broke the configure step because that branch's code is full of compiler warnings, treated as error. (If I use origin/master, I get an unmet tinyxml2 dependency instead, which I haven't bothered installing).

I think you should pass the commit hash matching that tag.

This is correct, you should, for security (not technical) reasons:

it is advisable to use a hash for GIT_TAG rather than a branch or tag name. A commit hash is more secure and helps to confirm that the downloaded contents are what you expected.^1

But branch and tag names work too.

Since the openmath branch has not changed in 2 years, you could always try:

  GIT_TAG 086f4eb740270b28bd0c61a0a359aea9300d61ae # Current openmath HEAD.

For which I get the same result as GIT_TAG origin/openmath - ie it fetches, just has compilation errors compiling the MicroTex source.

Cheers.

From a quick look at the documentation of FetchContent_Declare, I think you should pass the commit hash matching that tag.