Lambda implicit capture fails with variable declared from structured binding

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Core issue 2313 changed the standard so that structured bindings are never names of variables, making them never capturable.

P0588R1‘s reformulation of lambda capture wording makes this prohibition explicit:

If a lambda-expression […] captures a structured binding (explicitly
or implicitly), the program is ill-formed.

Note that this wording is supposedly a placeholder while the committee figures out exactly how such captures should work.

Previous answer kept for historical reasons:

This technically should compile, but there’s a bug in the standard here.

The standard says that lambdas can only capture variables. And it says that a non-tuple-like structured binding declaration doesn’t introduce variables. It introduces names, but those names aren’t names of variables.

A tuple-like structured binding declaration, on the other hand, does introduce variables. a and b in auto [a, b] = std::make_tuple(1, 2); are actual
reference-typed variables. So they can be captured by a lambda.

Obviously this is not a sane state of affairs, and the committee knows this, so a fix should be forthcoming (though there appears be some disagreement over exactly how capturing a structured binding should work).

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