Smaller testcase:

struct S {
template constexpr static T f(T k) { return g(k); }
template constexpr static T g(T k) { return k ? f(k-1) : 0; }
};
constexpr int n = S().f(1);

While instantiating f, we mark g as referenced and try to instantiate it. While instantiating g, we mark f as referenced, don't notice we're already in the middle of instantiating it, and try to instantiate it again. We should accept this code, but should reject similar code where the mutually recursive calls between f and g occur in locations where a constant expression is required.

*** Bug llvm/llvm-bugzilla-archive#13085 has been marked as a duplicate of this bug. ***

Fixed in r280190.

mentioned in issue llvm/llvm-bugzilla-archive#13085

mentioned in issue llvm/llvm-bugzilla-archive#30261

mentioned in issue llvm/llvm-bugzilla-archive#30301