The property pure, depending on how defined, may be undecidable, as in the halting problem.

However, it might be useful to be able to tell the optimizer that a function is to be considered semantically pure, even though it may not be in a strict computational sense. This would be similar to the use of the mutable keyword.


> On 4 Mar 2025, at 08:07, Tymi via Std-Proposals <std-proposals_at_[hidden]> wrote:
>
> Some functions have no side effects, and while they can be often optimised, the compiler is not aware of that.
> Consider this example:
> ```cpp
> template <std::arithmetic T> constexpr T(const T a, const T b) { return a + b; }
> ```
> That function could be declared as pure, or in other words, produces no side effects.
>
> I think this is a great way to optimise frequent calls, such as a pure function's result can be cached for the same arguments provided.
>
> Diagnostics: if a pure function has side effects, the compiler shall issue a warning and calling that function is undefined.
>
> pure member function shall be declared as const, otherwise, the program is ill-formed.
>
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