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From: "ldalessandro at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/97665] constexpr union array member incorrectly rejected as
 non-constexpr
Date: Mon, 02 Nov 2020 17:37:24 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97665

--- Comment #5 from Luke Dalessandro <ldalessandro at gmail dot com> ---
Ugh... replying to myself.

> You can do Foo foo =3D Foo(); and it compiles.

>> 1. I can't do `Foo foo =3D Foo();` because the purpose of the union is t=
o allocate uninitialized storage for the `Foo` during `constexpr` execution=
 when `Foo` has no default constructor. I realize now I meant to write `con=
stexpr Foo() =3D delete;`.

This means to say "trivial default constructor," not just "default
constructor". The empty default constructor was an adequate test case for t=
his,
but is misleading. The `=3D delete` should be more effective as an example.

The context for the use is to create an array that provides uninitialized
vector storage during `constexpr` evaluation in the same way a
`std::byte[T*sizeof(T)]` array combined with `reinterptret_cast<T*>` might =
be
used during normal execution.=