very good point on the "addresses compare == iff same object" rule.

In that case though, I think clang is right to optimize the callee (but it does introduce a problem in the caller) :

the only place you could do the equality check and observe the rule being broken is before the callee returns since the lifetime of its variable is bound to the call.

It seems that clang will not let the return pointer alias a local in the caller except when the call is the initialization of said local.

So if the caller goes :

foo x; leak(&x); x = returns_foo();

the memory will be temporary stack (and then memcpy), thus upholding the rule. (and it seems to me that this inefficiency is really required to respect the standard if we actually leak the pointer)

in the case :

foo x = returns_foo();

clang will pass the actual address of x down but that's before the object exists (and its address cannot be known yet) so the rule is still fine.

I stand corrected though, this does mean that RVO would be useful for C as well, as a way to relax the aliasing rule.

edit: nevermind that, in the first case it's perfectly legal to read/write foo through the pointer downstream so you cannot make the optimization anyway.