https://issues.dlang.org/show_bug.cgi?id=16034
Paul Backus <[email protected]> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |[email protected] --- Comment #3 from Paul Backus <[email protected]> --- The problem is not the lambda's parameter, but its return value. Essentially, we would like the lambda to return by `ref` when `r.front` returns by `ref`, so that it does not create an unnecessary copy. For a normal function, the solution would be to use `auto ref`, but the lambda syntax does not support this (issue 21243), so we must either fix that issue first or find a workaround. One workaround is to use a string mixin, like this: private enum getFrontLambda = q{(return ref R r) => r.front}; enum bool isInputRange(R) = /* ... */ && ( is(typeof(mixin("ref ", getFrontLambda))) || is(typeof(mixin(getFrontLambda))) ) /* ... */ This essentially replicates the logic of inferring `ref` in user code: attempt to compile with `ref`, then fall back to non-`ref` if it fails. --
