[PATCH] Fix PR c++/70121 (premature folding of const var that was implicitly captured)

[Patrick Palka]

On Thu, Mar 10, 2016 at 6:06 PM, Patrick Palka <patrick@parcs.ath.cx> wrote:
> On Thu, Mar 10, 2016 at 5:58 PM, Patrick Palka <patrick@parcs.ath.cx> wrote:
>> Within a lambda we should implicitly capture an outer const variable
>> only if it's odr-used in the body of the lambda.  But we are currently
>> making the decision of whether to capture such a variable, or else to
>> fold it to a constant, too early -- before we can know whether it's
>> being odr-used or not.  So we currently always fold a const variable to
>> a constant if possible instead of otherwise capturing it, but of course
>> doing this is wrong if e.g. the address of this variable is taken inside
>> the lambda's body.
>>
>> This patch reverses the behavior of process_outer_var_ref, so that we
>> always implicitly capture a const variable if it's capturable, instead
>> of always trying to first fold it to a constant.  This behavior however
>> is wrong too, and introduces a different but perhaps less important
>> regression: if we implicitly capture by value a const object that is not
>> actually odr-used within the body of the lambda, we may introduce a
>> redundant call to its copy/move constructor, see pr70121-2.C.
>>
>> Ideally we should be capturing a variable only if it's not odr-used
>
> Er, this sentence should read
>
>   Ideally we should be _implicitly_ capturing a variable only if it
> _is_ odr-used ...

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