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From: "daniel.kruegler at googlemail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/51155] Incorrect sizeof and sizeof... behavior in template partial specialization
Date: Wed, 16 Nov 2011 09:13:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D51155

Daniel Kr=C3=BCgler <daniel.kruegler at googlemail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |daniel.kruegler at
                   |                            |googlemail dot com

--- Comment #1 from Daniel Kr=C3=BCgler <daniel.kruegler at googlemail dot =
com> 2011-11-16 08:36:52 UTC ---
(In reply to comment #0)
> 14.5.5.8 of n324 states:
>=20
> "A partially specialized non-type argument expression shall not involve a
> template parameter of the
> partial specialization except when the argument expression is a simple
> identi=EF=AC=81er."
>=20
> From what I understand, this seems to be broken when using sizeof:
[..]
> template <int a>
> struct foo<sizeof(a), a> { }; // not correct and compiles

The core language is a bit fuzzy here, but this example should be no proble=
m at
all, because sizeof(a) does not actually depend on the value of a, it just
corresponds to the equivalent of sizeof(int), which is fine. I don't think =
that
gcc should change the compiler behaviour here unless the core language is
better. IMO the corresponding CWG issue is this one:

http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1315

I observe that Comeau online accepts this as well.

> Additional bugs when using sizeof.. with a variadic template
>=20
> template <int n, int... xs>
> struct foo { };
>=20
> // partial specialization:
> template <int... xs>
> struct foo<sizeof...(xs), xs...> { }; // not correct and doesn't compile
>                                       // although it should if sizeof is
> allowed
>=20
> template <int... xs>
> struct foo<sizeof(xs), xs...> { };    // not correct and compiles
>                                       // sizeof(xs) evaluates to sizeof(i=
nt)
>                                       // which is incorrect

I agree that this should be ill-formed, it looks as if gcc incorrectly perf=
orms
an implicit unpacking of the parameter pack xs here. It does so correctly
within the body of a class template.