https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109367
Nathaniel Shead <nshead at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |nshead at gcc dot gnu.org
--- Comment #5 from Nathaniel Shead <nshead at gcc dot gnu.org> ---
I think the warning is correct (i.e. this is not a bug). The namespace-scope
declaration of the lambda is not keyed to any decl and so will be distinct in
each TU. Consider the example given here:
https://eel.is/c++draft/basic.def.odr#18
More formally, a lambda type has no linkage in general. But for a definable
item D with definitions in multiple TUs, any entities declared within D
(including lambda closure types) are treated as if only one was declared.
But a type alias is not a definable item
(https://eel.is/c++draft/basic.def.odr#1) and so that doesn't apply in this
case, so we just follow the normal linkage rules and so there can be no
possible matching corresponding declaration of `S::f(T)` in this example since
the type referred to by T has no linkage and thus will not correspond to any
declaration in other TUs.
The original code could be "fixed" by instead doing:
struct T_impl { using type = decltype([]{}); };
using T = T_impl::type;
or perhaps even
struct T : decltype([]{}) {};
since this would attach the lambda to be within a definable item that can be
declared across different TUs.
