gcc/clang "call to non-constexpr function" in constant expression in CI

By Daniel Zoghalchali·Latchkey

A value used in a constant context (a constexpr variable, array bound, template argument, or static_assert) called a function that is not constexpr, so it cannot be evaluated at compile time.

What this error means

Initializing a constexpr variable or sizing a fixed array fails because the initializer calls a function not marked constexpr.

app.cpp:5:18: error: call to non-'constexpr' function 'int compute()'
    5 |   constexpr int n = compute();
      |                     ^~~~~~~

Common causes

How to fix it

Make the callee constexpr

1. Mark the called function constexpr if its body permits compile-time evaluation.
2. Otherwise use a non-constexpr variable evaluated at runtime.

constexpr int compute() { return 42; }
constexpr int n = compute();

How to prevent it

• Mark functions constexpr when they must be usable in constant expressions, and keep runtime-only logic out of constexpr initializers.

Frequently asked questions

Related guides

References

• CMake documentation
• GitHub Actions documentation