constexpr<\/code>\/consteval<\/code>: parsers, tables, DSLs, hashing \u2013 real code with real branches. When code fails at compile time, you can either try to guess the reason from the compiler\u2019s notes or de\u2011constexpr<\/code> it and hope the runtime reproduction matches what the compiler actually did.<\/p>\n\n\n\nThe new Constexpr Debugger available in the first CLion 2025.3 EAP build<\/a> allows you to stay in the compiler\u2019s world and see what really happens \u2013 by stepping through evaluation, inspecting values, and confirming which if constexpr <\/code>branch fired. Using it helps you understand exactly what the compiler is doing and fix issues faster.<\/p>\n\n\n\n![\"\"](\"https:\/\/blog.jetbrains.com\/wp-content\/uploads\/2025\/09\/intro_image-1.png\")
<\/figure>\n\n\n\nWith reflection coming in C++26 (P2996R13<\/a>), compile\u2011time code will increase even more. Although none of the compilers support reflection yet, the Constexpr Debugger in CLion provides a foundation that will allow for debugging this metacode in the future.<\/p>\n\n\n\n<\/i>DOWNLOAD CLION 2025.3 EAP<\/a><\/p>\n\n\n\nUpdated:<\/strong> Refer to our documentation<\/a> to learn more about working with the Constexpr Debugger in CLion.<\/p>\n\n\n\nFeatures<\/h2>\n\n\n\n
Here is what you currently can do with the Constexpr Debugger:<\/p>\n\n\n\n
\n- Start step-by-step debugging from the gutter by clicking the Debug<\/em> button next to
static_assert(...) <\/code>or a constexpr<\/code> declarator to check how it was evaluated or why it failed.<\/li>\n\n\n\n- Use the same actions as in the regular debugger: Step Into,<\/em> Step Over<\/em>, Step Out<\/em>, and Restart<\/em>. Additionally, you can use Step Backward<\/em>, the compile-time reverse stepping feature.<\/li>\n\n\n\n
- See what the compiler sees: the call stack, locals, last returns, and template arguments of the current instantiation. <\/li>\n\n\n\n
- Hover over a variable to see its values, use Evaluate Expression<\/em>, or navigate to the source code from the call stack.<\/li>\n\n\n\n
- Inspect the entire context when constant evaluation fails to determine when and why it occurred.<\/li>\n<\/ul>\n\n\n\n
How to use the Constexpr Debugger<\/h2>\n\n\n\nBasic case<\/h3>\n\n\n\n
To provide you with an overview of the basic actions that you can perform when debugging constexpr<\/code>, we\u2019ll use this compile-time Fibonacci cache implementation:<\/p>\n\n\n\n#include <array>\n\ntemplate<std::size_t N>\nstruct FibCache {\n std::array<int, N + 1> memo{};\n\n constexpr FibCache() {\n memo[0] = 0; memo[1] = 1;\n for (std::size_t i = 2; i <= N; ++i)\n memo[i] = memo[i - 1] + memo[i - 2];\n }\n\n constexpr int operator()(int n) const { return memo[n]; }\n};\n\nconstexpr int get_fibonacci(int n) {\n FibCache<8> cache;\n auto result = cache(n);\n return result;\n}\n\nconstexpr int k = get_fibonacci(6); \/\/ gutter: step through operator()\nstatic_assert(get_fibonacci(6) == 8, \"ok\"); \/\/ gutter available here too<\/pre>\n\n\n\n
The following are some basic operations you can perform when debugging:<\/p>\n\n\n\n
\n- Find the green Debug<\/em> icon next to the
constexpr<\/code> declarator or static_assert<\/code> (either will do) and click it to run a Constexpr Debugger session.<\/li>\n<\/ul>\n\n\n\n![\"\"](\"https:\/\/blog.jetbrains.com\/wp-content\/uploads\/2025\/09\/image4.png\")
<\/figure>\n\n\n\n\n- Use the regular debugger actions, or try Step Backward<\/em> to time\u2011travel one step back in your compile\u2011time evaluation.<\/li>\n<\/ul>\n\n\n\n
<\/p>\n\n\n ![\"\"](\"https:\/\/blog.jetbrains.com\/wp-content\/uploads\/2025\/09\/stepping_new_preview.jpg\")
<\/figure>\n \n\n\n\n- Inspect the state:\n
\n- Watch how the
this->memo<\/code> array fills during construction by hovering over it in the editor (see the screenshot below).<\/li>\n\n\n\n- Navigate to the Variables<\/em> pane<\/a> to see
this<\/code>, locals (like i<\/code>), and recent returns.<\/li>\n\n\n\n- Hover over any variable to see its value.<\/li>\n\n\n\n
- Navigate to the Call Stack<\/em> to see frames mapped to source.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n\n\n\n
![\"\"](\"https:\/\/blog.jetbrains.com\/wp-content\/uploads\/2025\/09\/image1.png\")
<\/figure>\n\n\n\n\n- Check the result in the Variables<\/em> pane after the constructor has precomputed all the Fibonacci numbers:<\/li>\n<\/ul>\n\n\n\n
![\"\"](\"https:\/\/blog.jetbrains.com\/wp-content\/uploads\/2025\/09\/image6.png\")
<\/figure>\n\n\n\nEdge case: non-constexpr call prevents evaluation<\/h3>\n\n\n\n
Consider the case when we have a function (fail<\/code>) that is not constexpr \u2013 it\u2019s deliberately called to indicate invalid input. This means that the original expression (parse_int(\u201chello\u201d)<\/code>) cannot be evaluated at compile time.<\/p>\n\n\n\n#include <string_view>\n\nvoid fail(); \/\/ note: non-constexpr\n\nconsteval int parse_digit(char c) {\n if (c < '0' || c > '9') {\n fail();\n }\n\n return c - '0';\n}\n\nconsteval int parse_int(std::string_view s) {\n int result = 0;\n for (std::size_t i = 0; i < s.size(); i++) {\n result = result * 10 + parse_digit(s[i]);\n }\n\n return result;\n}\n\nconstexpr auto digit = parse_int(\"12345\");\nconstexpr auto bad_digit = parse_int(\"hello\"); \/\/ start from the gutter<\/pre>\n\n\n\nStepping here lets you see c == 'x'<\/code>, the taken branch, the value of argument c, and the exact point at which the evaluation fails:<\/p>\n\n\n ![\"\"](\"https:\/\/blog.jetbrains.com\/wp-content\/uploads\/2025\/09\/edge_case_preview.jpg\")
<\/figure>\n \n\n\nCurrent limitations<\/h2>\n\n\n\n\n- Breakpoints and Run to Cursor \/ Force Run to Cursor<\/em> are not supported in
constexpr<\/code> evaluation.<\/li>\n\n\n\n- Updated:<\/strong> We’ve added support for C++ 20 modules to the 2025.3 EAP<\/a> (CPP-46489<\/a>).
C++20 modules aren\u2019t yet supported: constexpr<\/code> debugging does not navigate to entities located in imported modules.<\/s><\/li>\n\n\n\n- Updated:<\/strong> The stepping logic has been improved. This update is available in the 2025.3 EAP<\/a> (CPP-46841<\/a>, CPP-46867<\/a>).
Stepping may not work properly with some language constructs or compound statements.<\/s><\/li>\n\n\n\n- Some constructs aren\u2019t yet supported by our
constexpr<\/code> evaluator. We track these unsupported cases in YouTrack<\/a>.<\/li>\n<\/ul>\n\n\n\nWe don\u2019t have a definite timeline for when these missing bits of support might be added, but we\u2019re working on them, so stay tuned!<\/p>\n\n\n\n
Share your feedback<\/h2>\n\n\n\n
Your feedback is essential for improving this feature and making it more robust. We encourage you to try the Constexpr Debugger and share your thoughts and suggestions by submitting a ticket to our issue tracker<\/a> or commenting below.<\/p>\n\n\n\n