Incorrect cast in polymorphique returns of builtins

[Thierry Delisle]

Given the function

forall(dtype T)
static inline T & identity(T & i) {
	return i;
}

The following code compiles if identity is a regular function but doesn't if it is a CFA builtin.

struct A { int a; };
void foo( A & a ) {
	printf("%d %d\n", a.a, identity(a).a);
}