https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88759
Bug ID: 88759
Summary: `decltype(auto)` as return type of abbreviated
function template strips cv-qualifications and
referenceness
Product: gcc
Version: unknown
Status: UNCONFIRMED
Keywords: rejects-valid
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ensadc at mailnesia dot com
Target Milestone: ---
https://wandbox.org/permlink/hPwFtdVsAd4kIu44
(-fconcepts is required to silent the warning "use of 'auto' in parameter
declaration only available with -fconcepts", but otherwise the default setting
is enough to trigger the error.)
====
decltype(auto) f(auto x) {
return x[0];
}
const int x[] = {0};
extern decltype(f(x)) a;
extern const int& a;
====
prog.cc:8:19: error: conflicting declaration 'const int& a'
8 | extern const int& a;
| ^
prog.cc:7:23: note: previous declaration as 'int a'
7 | extern decltype(f(x)) a;
| ^
====
Originally discovered in
https://stackoverflow.com/questions/54062172/understanding-decltypeauto-difference-in-case-of-template-vs-auto-functions.
I rewrote the code to better illustrate the issue.
`decltype(f(x))` should be `const int&`, but GCC thinks it's `int`.
It seems that `decltype(auto)` is treated the same as `auto` when used as
return type of abbreviated function template.
Changing the abbreviated function template to a regular function template (i.e.
changing `decltype(auto) f(auto x)` to `template<class T> decltype(auto) f(T
x)`) removes the error.
Probably related to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69448.
