The new Constexpr Debugger available in the first CLion 2025.3 EAP build allows you to stay in the compiler’s world and see what really happens – by stepping through evaluation, inspecting values, and confirming which if constexpr branch fired. Using it helps you understand exactly what the compiler is doing and fix issues faster.

Introducing the Constexpr Debugger

by Alexander Karaev

From the article:

“Not a constant expression.” You’ve seen the diagnostics and the note trail, but never the actual state. Until now.

Modern C++ pushes more logic into constexpr/consteval: parsers, tables, DSLs, hashing – real code with real branches. When code fails at compile time, you can either try to guess the reason from the compiler’s notes or de‑constexpr it and hope the runtime reproduction matches what the compiler actually did.

The new Constexpr Debugger available in the first CLion 2025.3 EAP build allows you to stay in the compiler’s world and see what really happens – by stepping through evaluation, inspecting values, and confirming which if constexpr branch fired. Using it helps you understand exactly what the compiler is doing and fix issues faster.