Using a function as argument to GRANT fails with syntax error

I can't get GRANT to take current_database() as argument. These all fail with the same error:

GRANT ALL PRIVILEGES ON DATABASE current_database() to justintestin;
GRANT ALL PRIVILEGES ON DATABASE (current_database()) to justintestin;
GRANT ALL PRIVILEGES ON DATABASE (Select current_database()) to justintestin;
GRANT ALL PRIVILEGES ON DATABASE format('%I', current_database()) to justintestin;

SQL Error [42601]: ERROR: syntax error at or near "("

Currently, I use EXECUTE:

Do 
$$ 
Begin
    Execute Format('GRANT ALL PRIVILEGES ON DATABASE %I TO %I', current_database(), 'justintestin');
End;
$$;

but the nitpicker me knows there's gotta be a simpler way. Running on the official postgres:16 Docker image:

PostgreSQL 16.2 (Debian 16.2-1.pgdg120+2) on x86_64-pc-linux-gnu, compiled by gcc (Debian 12.2.0-14) 12.2.0, 64-bit"*).