Weird error C2131 with constexpr declared variable

Question

I'm trying to compile the following snippet with VS2017 but it is throwing an error C2131: expression did not evaluate to a constant without any further detail in the output log.

class Example { public: constexpr Example() : m_int() { for (int i = 0; i < 256; ++i) { for (int j = 0; j < 256; ++j) { m_int[i] = i; } } } private: int m_int[256]; }; int main() { constexpr Example vv; // <-- error C2131 here return 0; }

However for some reason unknown to me, if I stop the j counter at < 255 (instead of 256), the code compile fine. What's the explanation behind this and how can I fix it?

Don't see anything obviously wrong, so this looks like a compiler bug. gcc compiles the above without any issues. — Sam Varshavchik
– Sam Varshavchik, Commented Dec 7, 2019 at 4:48
I have just found the solution, I'll post the answer right away, thanks! — Robotex
– Robotex, Commented Dec 7, 2019 at 5:02
What does the inner loop even do? It’s just assigning to the same element over and over again. — L. F.
– L. F., Commented Dec 7, 2019 at 5:28

Robotex · Accepted Answer · 2019-12-07 05:19:24Z

Further searching landed me on the following link and after some trial and error with the parameters values I found the solution.

Adding /constexpr:steps10000000 to the project additional options in C++ compiler settings has fixed the error and now it compiles fine. I wish VS2017 would throw the error C4593 in this case instead of C2131 as it would have saved me a headache!

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