Re: Best way to make a template function conditionally @trusted

[Paul Backus via Digitalmars-d-learn]

On Thursday, 1 April 2021 at 22:35:01 UTC, tsbockman wrote:
Suppose I have a templated struct member function for which I 
can compute at compile-time when the function is memory safe, 
and when it is not. But, the compiler cannot correctly 
determine this automatically.

What is the best way to express this in code? Other than 
straight-up duplicating the implementation, the only answer 
I've come up with so far is to create a `private @system` 
implementation function, and then separate `public @system` and 
`public @trusted` wrappers with appropriate template 
constraints.

Is there a better way?

Here's a technique I've used:

    // infer function attributes
    auto func(...)
    {
        // do your compile-time memory-safety check here
        enum bool shouldBeSystem = ...;

        static if (shouldBeSystem) {
            // force inference of @system
            cast(void) () @system {}();
        }

        () @trusted {
            // rest of code goes here
        }();
    }

As long as the compile-time check is correct, this is sound: the 
@trusted lambda can be called from @safe code if and only if 
`shouldBeSystem == false`.