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From: "ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/59716] [10/11 Regression] variadic template multiple
 parameter pack expansion fails
Date: Thu, 07 Apr 2022 13:28:11 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D59716

--- Comment #9 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Hmm, this PR is essentially a dup of PR88580 but I don't think it's a 10/11
regression: between r8-2715 and r10-3735 it seems we just went from incorre=
ctly
rejecting the testcase (the behavior before r8-2715) to miscompiling it.  A=
fter
r10-3735 we went back to incorrectly rejecting it (arguably an improvement)=
.=20
So AFAICT we never correctly handled this testcase before r12-174.


Output when compiled with GCC trunk:

  Construction of: DebugPrinter<Head, T>::DebugPrinter(Y ...) [with Y =3D {=
int,
char, void*, int}; Head =3D int; T =3D {char, int, float}] Values: 1, c, 0x=
ddddd,
1,=20
  Construction of: DebugPrinter<Head, T>::DebugPrinter(Y ...) [with Y =3D {=
float,
char, void*, float}; Head =3D float; T =3D {void*, int, float}] Values: 2.2=
, c,
0xddddd, 2.2,=20

GCC 10/11 reject the testcase.

Output when compiled with GCC 7.4/8/9 (miscompile):

  Construction of: DebugPrinter<Head, T>::DebugPrinter(Y ...) [with Y =3D {=
int,
bool, int}; Head =3D int; T =3D {char, int, float}] Values: 1, 0, 1,=20
  Construction of: DebugPrinter<Head, T>::DebugPrinter(Y ...) [with Y =3D {=
float,
bool, float}; Head =3D float; T =3D {void*, int, float}] Values: 2.2, 0, 2.=
2,=20

GCC < 7.4 reject the testcase.

Output when compiled with Clang (equivalent to GCC trunk output):

  Construction of: DebugPrinter<int, char, int, float>::DebugPrinter(Y...)
[Head =3D int, T =3D <char, int, float>, Y =3D <int, char, void *, int>] Va=
lues: 1,
c, 0xddddd, 1,=20
  Construction of: DebugPrinter<float, void *, int, float>::DebugPrinter(Y.=
..)
[Head =3D float, T =3D <void *, int, float>, Y =3D <float, char, void *, fl=
oat>]
Values: 2.2, c, 0xddddd, 2.2,=