[cfe-dev] return lvalue move instead of copy?

[Stephan Bergmann via cfe-dev]

Given code like

   struct S1 { S1(S1 &&); };
   struct S2: S1 {};
   S1 f(S2 s) { return s; }

and the rules for when s in the return statement is treated like an 
rvalue when picking the ctor to use ([class.copy.elision]/3 in C++17, 
[class.copy]/32 in C++14, and [class.copy]/32 plus CWG1579 in C++11), my 
interpretation is that the code is correct and should pick the S1 move ctor.

However, any version of Clang up to recent trunk rejects the code, 
trying to call the implicitly deleted S1 copy ctor.  GCC >= 8.1 accepts 
the code, and I wonder whether Clang or GCC (and my interpretation) is 
wrong.