ODR violation in ranges

[Nathan Sidwell]

On 3/11/20 6:56 AM, Tam S. B. wrote:
> IIUC using lambda in inline variable initializer is not ODR violation. This is covered in CWG 2300 ( http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1510r0.html#2300 ).

ah, thanks for the pointer.  For lambdas in function scope we do achieve 
this (the mangling of the lambda uses the function as scope and a 
function-specific index).

We do not do so for lambdas in the initializer of namespace-scope inline 
variables, which is I think the missing bit.  That's an ABI thing, I'll 
ask over there.

The ranges header we'll die horribly if we ever emit non-inline code to 
evaluate the lambda at runtime.  We'll give it a name that depends on 
the order of (namespace-scope?) lambdas. :(

inline constexpr auto foo = [] (int i) {return i * 2;};
int bob (int i)
{ return foo (i); }

compile with -fno-inline and observe '_ZNKUliE_clEi'

sigh,

nathan
> 
> ________________________________________
> From: Libstdc++ <libstdc++-bounces@gcc.gnu.org> on behalf of Jonathan Wakely via Libstdc++ <libstdc++@gcc.gnu.org>
> Sent: Wednesday, March 11, 2020 10:26
> To: Nathan Sidwell
> Cc: libstdc++@gcc.gnu.org; GCC Patches
> Subject: Re: ODR violation in ranges
> 
> On 11/03/20 06:08 -0400, Nathan Sidwell wrote:
>> Jonathan,
>> the ranges header contains code like:
>>     inline constexpr __adaptor::_RangeAdaptorClosure all
>>       = [] <viewable_range _Range> (_Range&& __r)
>>       {
>> if constexpr (view<decay_t<_Range>>)
>>    return std::forward<_Range>(__r);
>> else if constexpr (requires { ref_view{std::forward<_Range>(__r)}; })
>>    return ref_view{std::forward<_Range>(__r)};
>> else
>>    return subrange{std::forward<_Range>(__r)};
>>       };
>>
>> (line 1236)
>>
>> When you strip away all the templateyness, you have:
>>
>>
>> inline constexpr auto all = [] () {};
>>
>>
>> That's an ODR violation -- the initializers in different TUs are not
>> the same!
>>
>> As you can guess, I can't turn this into a header unit (well, I can,
>> but merging duplicates complains at you)
> 
> CC libstdc++@ and Patrick.
> 
> I did wonder if using lambdas for those global variables would be OK.
> 
> I think we'll need a new class template for each view adaptor, rather
> than using the _RangeAdaptorClosure to hold a closure.
> 
> Patrick, can you look into that please?
> 


-- 
Nathan Sidwell