c-lang-constants-under-evaluation is a variable defined in `cc-defs.el'.
Its value is

Alist of constants in the process of being evaluated.
The `cdr' of each entry indicates how far we've looked in the list
of definitions, so that the def for var FOO in c-mode can be defined in
terms of the def for that same var FOO (which will then rely on the
fallback definition for all modes, to break the cycle).