2 replaced http://mathematica.stackexchange.com/ with https://mathematica.stackexchange.com/
source | link

Apart from C.EC.E great answer, the specific answer to this question is something like this:

ClearAll[f];
f[x_, y_: Hold[1, 2, 3]] := {x, ReleaseHold@ y}
f[a, 2 + 3]
(*{w,5}*)
f[a]
(*{a, 1, 2, 3}*)

Apart from C.E great answer, the specific answer to this question is something like this:

ClearAll[f];
f[x_, y_: Hold[1, 2, 3]] := {x, ReleaseHold@ y}
f[a, 2 + 3]
(*{w,5}*)
f[a]
(*{a, 1, 2, 3}*)

Apart from C.E great answer, the specific answer to this question is something like this:

ClearAll[f];
f[x_, y_: Hold[1, 2, 3]] := {x, ReleaseHold@ y}
f[a, 2 + 3]
(*{w,5}*)
f[a]
(*{a, 1, 2, 3}*)
1
source | link

Apart from C.E great answer, the specific answer to this question is something like this:

ClearAll[f];
f[x_, y_: Hold[1, 2, 3]] := {x, ReleaseHold@ y}
f[a, 2 + 3]
(*{w,5}*)
f[a]
(*{a, 1, 2, 3}*)