[PATCH] D123576: Support constructing empty function_ref from other callables that can be "empty"

[Duncan P. N. Exon Smith via Phabricator via llvm-commits]

dexonsmith added a comment.

In D123576#3467855 <https://reviews.llvm.org/D123576#3467855>, @dblaikie wrote:

> I worry this is overly general (it's not obvious that any functor with boolean conversion has the semantics this patch relies on) & inconsistent with the standard library (`std::function` for instance doesn't provide this (or a narrower version of it) functionality - so we'll still have the bugs for any API that takes a `function_ref` and passes it to `std::function`)

Seems nice to make it safer than the STL. Would you be more comfortable if it were special-cased to known functions that behave this way (std::function and llvm::unique_function)? For your other concern, I'm wondering if we could also make it symmetric (add `operator std::function()` that adds this check somehow) or if that would lead to ambiguities.

> I don't know that there's /no/ useful direction here, but I have my reservations.
>
> (@rsmith as the original author of `function_ref`)




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https://reviews.llvm.org/D123576