[cfe-users] Code which compiles with g++ but not with clang++

[Pavel Černohorský via cfe-users]

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).

Thank you very much for your time and explanation. Kind regards,

Pavel Cernohorsky

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