std::vector<T> is a fairly complicated class that involves dynamic allocation. While clang++ is sometimes able to elide heap allocations, it is a fairly tricky optimization and you should not rely on it. Example:
int foo() {
int* p = new int{5};
return *p;
}
foo(): # @foo() mov eax, 5 ret
As an example, using std::array<T> (which does not dynamically allocate) produces fully-inlined code:
#include <array>
int foo() {
std::array v{1, 2, 3, 4, 5};
return v[4];
}
foo(): # @foo() mov eax, 5 ret
As Marc Glisse noted in the other answer’s comments, this is what the Standard says in [expr.new] #10:
An implementation is allowed to omit a call to a replaceable global allocation function ([new.delete.single], [new.delete.array]). When it does so, the storage is instead provided by the implementation or provided by extending the allocation of another new-expression. The implementation may extend the allocation of a new-expression e1 to provide storage for a new-expression e2 if the following would be true were the allocation not extended: […]