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From: "matthurd at acm dot org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/99926] New: Parameter packs and variadic arguments: Clang,
 gcc, and msvc differ on this one
Date: Tue, 06 Apr 2021 07:47:41 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99926

            Bug ID: 99926
           Summary: Parameter packs and variadic arguments: Clang, gcc,
                    and msvc differ on this one
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: matthurd at acm dot org
  Target Milestone: ---

I found three compiler differences from three compilers:
https://godbolt.org/z/cEoYrn4T8 - two are wrong.

g++ trunk and 10.2 affected.

I thought gcc may be correct and clang may be incorrect in this compiler
difference so I filed a bug with llvm. Richard Smith surmised gcc was incor=
rect
and clang is correct, so I have closed the clang bug and I'm opening a gcc =
bug
here if you'll indulge me.

[Note: clang issue: https://bugs.llvm.org/show_bug.cgi?id=3D49852]

________________


gcc, clang, and msvc all compile this fun one:

auto foo(auto......) { return 42; }
int add_three() { return foo<int,int>(3,4,5); }

____________________

But they argue about this curious one:

[[nodiscard]] constexpr auto foo(auto...t...) noexcept {return (... + t);}
int add_three() { return foo<int,int>(3,4,5);} //gcc(7),  clang(12), msvc(e=
rr)
int add_more()  { return foo(3,4,5,6);       } //gcc(18), clang(18), msvc(e=
rr)

https://godbolt.org/z/cEoYrn4T8
____________________

It looks like gcc may be failing to extend the deduction as clang does. Like
clang, EDG extends it as well, Richard reported. msvc will give the same an=
swer
as clang if auto is not used and it is a normal template expansion. This le=
aves
gcc as the outlier I guess.


That gcc may be wrong makes sense, though it makes a C-style variadic after=
 a
parameter pack unreachable which is a wee semantic whole in the grammar I
guess.

--Matt.=