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From: "jason at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/102229] [11/12 Regression] 'decltype(auto)' cannot be
 cv-qualified
Date: Tue, 07 Sep 2021 19:43:33 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102229

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org

--- Comment #5 from Jason Merrill <jason at gcc dot gnu.org> ---
The standard isn't very clear on this point; it seems like a core issue.

9.2.6/10: A constexpr specifier used in an object declaration declares the
object as const.

9.2.9.6.2/2.2: for a variable declared with a type that contains a placehol=
der
type, T is the declared type of the variable and E is the initializer.

/5: If the placeholder-type-specifier is of the form type-constraint opt
decltype(auto), T shall be the placeholder alone.

The question is whether constexpr adds const before deduction (so it's part=
 of
"the declared type of the variable") or after.  The latter interpretation s=
eems
more useful, so let's go with that.=