Compiling debug missing errors

[ag0aep6g via Digitalmars-d-learn]

On 04/09/2017 10:49 PM, Duarte wrote:
> ---------------
>
> import std.stdio;
>
> pure void foo()
> {
>     debug
>     {
>         stdout.writeln("1");
>     }
>
>     stdout.writeln("2");
> }
>
> void main(string[] args)
> {
>     foo();
> }
>
> ---------------
>
> Using either '-debug' or '-release', the second stdout will give an
> error (Error: pure function 'main.foo' cannot access mutable static data
> 'stdout') which is alright by me.
>
> The question is why the first stdout doesn't throw the same error when
> compiling with the '-debug' option? Surely the issue will be the same
> and the debug-statement should be satisfied.

You're allowed to break purity in debug code.

 From the spec: "As a concession to practicality, a pure function can 
also [...] perform impure operations in statements that are in a 
ConditionalStatement controlled by a DebugCondition"

https://dlang.org/spec/function.html#pure-functions