When using decltype around a namespace, I can write code that compiles, but the statement doesn't seem to have any effect under g++4.9.1, under clang it produces error: unexpected namespace name 'std': expected expression
For example, the following all compile under g++, but the assembly doesn't show any code generated for them.
using s = decltype(std);
auto n = typeid(decltype(std)).name();
auto sz = n.size();
std::printf("size is %zu\n", sz+1);
std::printf("this type is: %s\n\n", n.c_str());
// the only limit is your imagination
int f();
std::ostream trash = f(typeid(decltype(std)) * 10 - 6 ^ typeid(decltype(std)));
If g++ is right in allowing this? If so, what's the advantage of the code disappearing rather than causing a compile-time error?
No, it's not legal. The two decltype-specifier forms that are allowed by the grammar are (N3690, §7.1.6.2/1):
decltype-specifier:
decltype( expression )
decltype ( auto )
and a namespace name is not an expression.
The paragraph I quoted is from a standard draft post C++11, so decltype(auto) doesn't apply to C++11. The answer is the same, though.