https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106604
Bug ID: 106604
Summary: Fully-specified deduction guide in anonymous namespace
warns as-if a function? Unsuppressably?
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: nabijaczleweli at nabijaczleweli dot xyz
Target Milestone: ---
Reproduced this separately on 10.2.1-6 (bullseye), 12.1.0-7 (sid from 2 weeks
ago), trunk (according to godbolt).
TL;DR: https://gcc.godbolt.org/z/869TdbvEe
Given:
namespace {
template <class T>
struct test {
test(T) noexcept {}
};
UNUSED test(bool) -> test<bool>;
}
If UNUSED is blank, GCC produces:
<source>:7:12: warning: '{anonymous}::test(int) -> test<int>' declared
'static' but never defined [-Wunused-function]
7 | UNUSED test(int) -> test<int>;
| ^~~~
Compiler returned: 0
If UNUSED is [[maybe_unused]] (which was my solution originally since it works
on Clang):
<source>:0:16: error: 'decl-specifier' in declaration of deduction guide
0 | #define UNUSED [[maybe_unused]]
| ^
<source>:7:5: note: in expansion of macro 'UNUSED'
7 | UNUSED test(int) -> test<int>;
| ^~~~~~
<source>:7:12: warning: '{anonymous}::test(int) -> test<int>' declared
'static' but never defined [-Wunused-function]
7 | UNUSED test(int) -> test<int>;
| ^~~~
Compiler returned: 1
So:
* the deduction guide is decidedly not "declared 'static'" (anon namespace
means it has static linkage, sure, but it's not 'static')
* of course it's not defined?? can you even define a deduction guide?
* how do you suppress this? (besides template<class = void>)?
