On Wed, Mar 11, 2020 at 11:05 AM Nathan Sidwell <nat...@acm.org> wrote:
> 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. > Looks like the condition on the call to start_lambda_scope in cp_parser_init_declarator just needs to be adjusted to handle non-template inline variables. > 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++-boun...@gcc.gnu.org> on behalf of Jonathan > Wakely via Libstdc++ <libstd...@gcc.gnu.org> > > Sent: Wednesday, March 11, 2020 10:26 > > To: Nathan Sidwell > > Cc: libstd...@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 > >
