int[5] foo(int[2] a, int[3] b) {
    typeof(return) result = a ~ b; // OK
    return result;
}
int[5] bar(int[2] a, int[3] b) {
    return a ~ b;                  // Error
}
void main() {}


DMD 2.066alpha gives:

test2.d(6,12): Error: cannot implicitly convert expression (cast(int[])a ~ cast(int[])b) of type int[] to int[5]


I suggest to support the code in bar() too. (A small optimization can even remove any heap allocations from the bar() function).

I am not sure, but perhaps the solution is similar to a value range propagation: to propagate the statically known length of an array. (Expecially for immutable arrays, as in Issue 10594 ).