NIntegrate can't integrate function which returns list, but can integrate list directly

Question

I have the following dummy Code

f[arg : {__?NumericQ}] := 2*arg;
g := f[{x, y}]
NIntegrate[g, {x, 0, 1}, {y, 0, 1}]
NIntegrate[f[{x, y}], {x, 0, 1}, {y, 0, 1}]

Both tries yield

NIntegrate::inum: Integrand f[{x,y}] is not numerical at {x,y} = {0.5,0.5}.

Despite

f[{x, y}] /. x -> 0.5 /. y -> 0.5
{1.,1.}
f[{0.5, 0.5}]
{1.,1.}

However, if I enter the function directly, I get the expected result

NIntegrate[2*{x, y}, {x, 0, 1}, {y, 0, 1}]
{1., 1.}

Why does the first try fail and the last try evaluate?

Answer 1

The issue here is that even with using Evaluate, neither g nor f[{x,y}] will evaluate without satisfying the NumericQ that has been imposed on arg. There are a number of solutions mentioned in the related links that @MichaelE2 has linked to in the comments, and other solutions offered by @flinty & @Bob Hanlon. However, if control of the function being input into NIntegrate is possible, you may find inspiration in the following simple solution:

h[arg:{__}]:=2*arg;
NIntegrate[h[{x,y}],{x,0,1},{y,0,1}]

{1.,1.}