On Nov 5, 2020, at 6:34 AM, Pavel Černohorský via cfe-users
<cfe-users@lists.llvm.org> wrote:
>
> Hello,
>
> I would like to ask what is wrong with the following code:
> ```
>
> #include <optional>
>
> struct S
> {
> S& operator=(S&) { return *this; };
> };
>
> void thisWillNotCompileInClang()
> {
> std::optional<S> a;
> }
>
> ```
>
> Compilation command is `g++ or clang++ -std=c++17 -c source.cpp`. It compiles
> without problems with g++ 9.3.0, but not with clang++ 10.0.0 (both from
> current stable Ubuntu 20). This code is simplification of code using widely
> adopted rapidjson library which uses assignment operator with non-const
> parameter a lot. Earlier versions of clang did not complain (tried with 7.0.1
> on currently stable Debian Buster).
A note from the libstdc++ maintainer:
> I think there was a core change in C++20 to make the =default do the right
> thing (which is delete it, I think).
> Clang does compile it with -std=c++20 just not -std=c++17.
— Marshall
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