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From: "de34 at live dot cn" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/114388] Behavioral change of typeid on xvalues since GCC 9
Date: Tue, 19 Mar 2024 11:43:48 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114388

--- Comment #7 from Jiang An <de34 at live dot cn> ---
(In reply to Jonathan Wakely from comment #5)
> Prior to DR 616 the expression (true ? WrapB().b : WrapD().d) was a prval=
ue
> of type B, created by copying the B (or slicing the D when the condition =
is
> false). As an rvalue, it wasn't evaluated. That's true pre-N3055 and
> post-N3055.
>=20
> DR 616 changed WrapB().b to be an xvalue, and the result of the expression
> is B&& in C++1 (and const B& in C++98 I guess).
>=20
> In C++98 the const B& is an lvalue, and in C++11 the B&& is an xvalue whi=
ch
> is a glvalue. Either way, it's correct to treat it as a glvalue of
> polymorphic type and evaluate it.
>=20
> So this change is not caused by N3055. And I think G++ is correct for both
> C++98 and C++11. So INVALID.

Pre-N3055 there were no xvalue branch in [expr.cond], and (later-called)
xvalues were handled alongwith prvalues. I don't see anything in C++98
[expr.cond] adding const& when there's no const values.

This example also shows that GCC hasn't been thinking that the result of the
expression is const B& in C++98: https://godbolt.org/z/EPv65oY1W.=