[Bug libstdc++/92894] "declared using local type 'test01()::X', is used but never defined" during concept satisfaction

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92894

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed on master (GCC 11) so far.

Clang's error is a bit more explicit about the problem:

error: function 'std::projected<X *, std::identity>::operator*' is used but not
defined in this translation unit, and cannot be defined in any other
translation unit because its type does not have linkage

For a type with external linkage the ranges::iter_move::operator() function
still gets instantiated, but the compiler doesn't reject it immediately because
it's possible that projected<X*, identity>::operator* is defined in another
translation unit. Since the function isn't actually called (just instantiated
to find its return type) there's no reference to it in the final object, and so
it doesn't matter that projected::operator* isn't defined anywhere. The program
is still ill-formed, but no diagnostic is required and it "works".

For a type with internal linkage (like a local class) the compiler stops
immediately, diagnosing the ill-formed call to projected::operator*. So without
this fix it's not possible to use most std::ranges:: algos with local classes.