aaron.ballman added a comment.

In D90042#2390203 <https://reviews.llvm.org/D90042#2390203>, @flx wrote:

> Thanks for the suggestion, I had never hear of creduce!

Glad to have introduced you to it -- it's a great tool!

> After a bit of trial an error I seem to have found a more minimal example:
>
>   namespace std {                                                             
>                                                  
>   template <typename> class function;                                         
>                                                  
>   template <typename a, typename... b> class function<a(b...)> {              
>                                                  
>   public:                                                                     
>                                                  
>     void operator()(b...);                                                    
>                                                  
>   };                                                                          
>                                                  
>   } // namespace std                                                          
>                                                  
>   struct c {                                                                  
>                                                  
>     c();                                                                      
>                                                  
>     c(const c &);                                                             
>                                                  
>   };                                                                          
>                                                  
>   std::function<void(c &)> f;                                                 
>                                                  
>   void d() {                                                                  
>                                                  
>     c Orig;                                                                   
>                                                  
>     c Copy = Orig;                                                            
>                                                  
>     f(Copy);                                                                  
>                                                  
>   }  
>
>
> To be frank I can't spot a meaningful difference to the std::function copy we 
> already have.

Aha, I may have spotted it. The call operators have subtly different signatures 
and the signature we have in our test file is wrong. Note the `&&` in our test 
file compared to what the standard defines: 
http://eel.is/c++draft/func.wrap.func#inv which is what's causing the 
difference here: https://godbolt.org/z/hxfM7P


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D90042/new/

https://reviews.llvm.org/D90042

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