On Friday, January 24, 2025 3:50:15 AM MST Dom DiSc via Digitalmars-d-learn wrote:
> Is it possible in D to find out if the parameter given to a function is a literal or a compile time constant?

Not when calling the function, no. It should be possible to test whether a particular argument is a known at compile time by writing an is expression that uses it in a context where it must be known at compile time, since if the result compiles and results in a type other than void, then the value is known at compile time, and if you get void (and thus the is expression is false), then it isn't known at compile time. However, that would have to be tested separately from the function call. The function itself doesn't care about its arguments at all once it's been compiled. It just takes them and runs whether that's part of CTFE or at runtime. The function itself is the same regardless of what you pass to it.

> Of course, if it has _only_ parameters known at compile time, the function will be executed during CTFE, so I can check for the execution time. But if it has also runtime-parameters, it will only be executed during runtime. Is it still possible to find out, which values are known at compile-time?

No. For a function to be called with CTFE, it must be used in a context where the result must be known at compile time - e.g. providing the value of an enum or directly initializing a member variable. In such a case, the arguments to the function must also be known at compile time, or you'll get a compilation error, but if you call a function entirely with arguments that are known at compile time, but it's not called in a context where the result must be known at compile time, then it will be called at runtime.

> They should be immutable, but do they have some other, more specific property?

Immutability is not a requirement for CTFE.

I suggest that you read this article:

https://wiki.dlang.org/Compile-time_vs._compile-time

- Jonathan M Davis