call to immediate function is not a constant expression

[Yaraslaut]

I stumbled in weird compiler error, code in question : https://godbolt.org/z/Yox46WPMd in short

constexpr auto reflA = ^A;
form::util::printMembers<[:reflA:]>(); // <------ works
constexpr auto refl = CreateClass<CreateUniqueT<int>>();
form::util::printMembers<[:refl:]>(); // <---  error: call to immediate function 'form::util::printMembers<CreateUniqueT<int, (lambda at <source>:97:45)>>' is not a constant expression

[katzdm]

Hey @Yaraslaut - Here is what's happening.

• Your form::util::name_of function returns a std::string, which necessarily allocates some memory from dynamic storage (i.e., to hold its value).
• Dynamic memory allocated by a constant expression must be deallocated in the course of its evaluation.
• This means that form::util::name_of(^anything) can never itself be a constant expression (though it may appear in the evaluation of one).
• You call form::util::name_of(^T) in your implementation of constexpr void printMembers(). The compiler looks to see whether form::util::name_of(^T) is a subexpression of a larger constant expression, but finds that it isn't one (i.e., because std::println is not constexpr).
• The compiler therefore concludes that the "runtime path" of the constexpr function of which the call is a part is ill-formed, and "promotes" the function to effectively be consteval.
• But the "consteval path" for the function is also ill-formed, because std::println is not constexpr. So all it can do is diagnose an error.

Based on the above, I believe the issue is with your example - I'm going to close, but please re-open if I've missed something!

[Yaraslaut]

Thanks @katzdm for the detailed explanation, I indeed forgot that I changed the signature of name_of and if i keep string_view than everything is fine, thanks. I am still a bit confused why it is working for declared type, but not for defined through define_class call

[katzdm]

Ah! Good point, and the reason is actually pretty fun - libcxx implements a "short string optimization" in the std::string implementation, which stores small strings in a static buffer. In the case of the compiler running on godbolt, as many as (but no more than) 22 characters (including the null terminator) can be stored before a dynamic allocation occurs. The name rendered for CreateUniqueT<int, (lambda at /app/example.cpp:96:45)> is what overruns that 22 character limit, thus triggering a dynamic allocation and rendering the call not-constexpr.

All of that said, there is a subtle bug here: the name returned by name_of here should just be CreateUniqueT; the P2996 spec states that name_of should not include the names of template arguments.