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From: "hewillk at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/100894] New: The std::common_reference implementation
 seems to be wrong
Date: Thu, 03 Jun 2021 16:35:12 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100894

            Bug ID: 100894
           Summary: The std::common_reference implementation seems to be
                    wrong
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

In [meta#trans.other-6.3.1], the standard specifies "If T1 and T2 are refer=
ence
types and COMMON-REF(T1, T2) is well-formed, then the member typedef type
denotes that type", where COMMON-REF is defined in [meta#trans.other-3.5]: =
"If
A and B are both lvalue reference types, COMMON-REF(A, B) is COND-RES(COPYC=
V(X,
Y) &, COPYCV(=E2=80=8B Y, X) &) if that type exists and is a reference type=
."

libstdc++ does not check that COMMON-REF(A, B) must be a reference type, wh=
ich
will lead to incorrect determination of the common reference type according=
 to
bullet 1 in the following cases.

https://godbolt.org/z/7Mc7jjesK

#include <concepts>

struct A {};
struct B { B(A); };
static_assert(
  std::same_as<
  std::common_reference_t<A&, B&&>, B>);=