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From: "rs2740 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/109232] New: Using deduced return type in an unevaluated
 context leads to codegen
Date: Tue, 21 Mar 2023 13:15:52 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 13.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109232

            Bug ID: 109232
           Summary: Using deduced return type in an unevaluated context
                    leads to codegen
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rs2740 at gmail dot com
  Target Milestone: ---

auto begin(auto&& r) {
    return r.begin();
}

namespace {
    struct R {
        int* begin();
    };
}

static_assert(__is_same(decltype(begin(R())), int*));
int main() {}

Even though ::begin is only used in an unevaluated context, GCC at -O0
generates code for begin<R>, leading to a warning and a failure to link
(https://gcc.godbolt.org/z/cdajoP7f3):

<source>:10:14: warning: 'int* {anonymous}::R::begin()' used but never defi=
ned
   10 |         int* begin();
      |              ^~~~~
/opt/compiler-explorer/gcc-trunk-20230320/bin/../lib/gcc/x86_64-linux-gnu/1=
3.0.1/../../../../x86_64-linux-gnu/bin/ld:
/tmp/cc7gqEhh.o: in function `auto begin<(anonymous namespace)::R&>((anonym=
ous
namespace)::R&)':
<source>:2: undefined reference to `(anonymous namespace)::R::begin()'
collect2: error: ld returned 1 exit status
Execution build compiler returned: 1=