Long code complain about 'writeln is not defined' even though it is not there

[ryuukk]

Try the following code:

https://gist.github.com/ryuukk/e6e02960496c491b3370ddbd8841979f

And it'll give the error:

onlineapp.d(680): Error: `writeln` is not defined, perhaps `import std.stdio;

[Luís Ferreira]

Somewhat related to #744