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From: "arthur.j.odwyer at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/98249] New: Improper ADL on the `arg` in `new (arg) T`
Date: Fri, 11 Dec 2020 21:56:09 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98249

            Bug ID: 98249
           Summary: Improper ADL on the `arg` in `new (arg) T`
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: arthur.j.odwyer at gmail dot com
  Target Milestone: ---

// https://godbolt.org/z/xavfej

#include <new>
struct Incomplete;
template<class T> struct Holder { T t; };
Holder<Incomplete> *p;
void test() {
    ::new (p) int;
    new (p) int;
}

In an "ADL call," the compiler needs to compute the associated types of
`Holder<Incomplete>`, which means it needs to complete that type in order to
check for friend declarations.  In a "non-ADL call" (such as a qualified ca=
ll),
the compiler does NOT need to complete `Holder<Incomplete>`.  Since complet=
ing
`Holder<Incomplete>` produces a hard error, we can detect the difference
between an ADL and a non-ADL call.

All other compilers use "non-ADL calls" for both `::new (p) int` and `new (=
p)
int`: they don't attempt to complete the incomplete type. GCC uses "ADL cal=
ls"
for both, which means it hard-errors on both lines.

I'm not sure what the Standard has to say about this, but GCC disagrees with
all of Clang/MSVC/ICC, so I think GCC ought to change.

(Detected while testing libc++ patch https://reviews.llvm.org/D92884)=