[feature] check_min_cppstd should specifically report on an unknown compiler

[Minimonium]

The check_min_cppstd works, or is intended to work, with default compilers. But it should handle unknown compilers that could be added by users.

Right now it doesn't handle the None value returned by the deduced_cppstd and proceeds to compare it. Should probably throw a specialized error?
https://github.com/conan-io/conan/blob/develop/conans/client/tools/settings.py#L51

[uilianries]

Yes, indeed. When a compiler is not specified, it should throw an error.

About supporting different compiler, I would prefer adding them directly to Conan code base, so we can re-use for other cases.

[Minimonium]

I meant that we should let users an ability to handle that use case since it's not necessarily a hard error.
Right now it doesn't do it. Handle here means throwing an appropriate error which is not an InvalidConfiguration. 🙂

[uilianries]

That's why we have valid_min_cppstd, which does not raise an exception. The user is able to decide what to do.

[Minimonium]

You misunderstood. The situation when the tool itself doesn't know about a compiler is a different outcome from a situation if the cppstd is not valid.

[Minimonium]

Forgot to mention, this specified exception is required to correctly handle the standard version as described here: conan-io/conan-center-index#54 (comment)

[Minimonium]

Oh, snap. Sorry folks, but I think I've missed a very important moment:
https://github.com/uilianries/conan/blob/3fe411a8908c8c62241230061c9c439ed79b95fc/conans/client/tools/settings.py#L52
https://github.com/uilianries/conan/blob/3fe411a8908c8c62241230061c9c439ed79b95fc/conans/client/tools/settings.py#L61

These two places raise the same exception. Meaning that there is no way to distinguish these two situations. It effectively blocks all new compiler versions until they and their logic are added into the Conan repository.

The use case: https://github.com/conan-io/conan-center-index/blob/master/recipes/abseil/all/conanfile.py#L51

[Minimonium]

I've made a slight extension of cppstd operations capability in #6713 , to solve my concern in this issue and in related ones.