I agree with this, having it would be useful, and reduce the number of times you would have to evaluate something. 

On Sat, Oct 26, 2019 at 7:24 PM Justin Bassett via Std-Proposals <std-proposals@lists.isocpp.org> wrote:
This reminds me of pattern matching. I'm not sure if the pattern matching proposal allows this, though. Rust would allow something like:

if let Some(x) = some_optional {

We propose a new variant of the if statement that fuses a condition declaration with an indirection in the true branch.  We call it an 'indirect if statement'.

I find it a bit odd that this only works for unary `operator*`. I want this to be more general, but I don't know if there's a good alternative without going for full pattern matching.

I think that it does match the name, as unary `operator*` is the indirection operator. I would think the kinds of types that would benefit from this (raw and smart pointers, optionals, etc.) would all use the indirection operator, though I would certainly like to see a use case for a type that benefits for a different operator. Possibly a function pointer type.

An indirect if statement of the form:

    if (DECL : EXPR) X else Y;

is equivalent to:

        auto&& __c = EXPR;
        if (__c) {
            DECL = *__c;
        } else {
            Y;  // noteworthy: DECL is not in scope here

It would sometimes be preferable to forward __c into DECL, but sometimes not. E.g. if EXPR returned an optional<vector<...>> and DECL wanted to be a value and not a reference. I don't know what the right choice is.

I do know that I'd use this if it was in the language.

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