runif with .Machine$double.xmax as boundaries
As hinted by mt1022 the reason is in runif C source:
double runif(double a, double b){ if (!R_FINITE(a) || !R_FINITE(b) || b < a) ML_ERR_return_NAN; if (a == b) return a; else { double u; /* This is true of all builtin generators, but protect against user-supplied ones */ do {u = unif_rand();} while (u <= 0 || u >= 1); return a + (b - a) * u; }}In the return argument you can see the formula a + (b - a) * u which transoform uniformly [0, 1] generated random value in the user supplied interval [a, b]. In your case it will be -M + (M + M) * u. So M + M in case it 1.79E308 + 1.79E308 generates Inf. I.e. finite + Inf * finite = Inf:
M + (M - m) * runif(1, 0, 1)# Inf