No longer generate a diagnostic when a small trivially copyable type is used without a reference. Before the test looked for a POD type and had no size restriction. Since the range-based for loop is only available in C++11 and POD types are trivially copyable in C++11 it's not required to test for a POD type.
Since copying a large object will be expensive its size has been restricted. 64 bytes is a common size of a cache line and if the object is aligned the copy will be cheap. No performance impact testing has been done.