Some code that compiles but shouldn't

[Heromyth]

On Monday, 30 December 2019 at 18:18:49 UTC, uranuz wrote:
> I have created library/ framework to handle JSON-RPC requests 
> using D methods. I use some *template magic* to translate 
> JSON-RPC parameters and return values from/ and to JSON. And I 
> have encountered funny bug that at first was hard to find. My 
> programme just segfaulted when call to this method occured:
>
> 	void getCompiledTemplate(HTTPContext ctx)
> 	{
> 		import std.exception: enforce;
> 		enforce(ctx, `ctx is null`);
> 		enforce(ctx.request, `ctx.request is null`);
> 		enforce(ctx.request.form, `ctx.request is null`);
> 		enforce(ivyEngine !is null, `ivyEngine is null`);
> 		string moduleName = ctx.request.form[`moduleName`];
> 		auto mod = ivyEngine.getByModuleName(moduleName);
> 		return ctx.response.write(mod.toStdJSON().toString());
> 	}
>
>
> So as you see I have added a lot of enforce to test if all 
> variables are not null. But nothing was null and the reason of 
> segfault were unclear.
> Today I just went home. Opened a bottle of beer. And have 
> noticed that function is marked as returning `void`, but in 
> fact it doesn't. When I fixed this segfault have gone. But why 
> this even compiled?! Interesting...

We also have a piece of code, see here 
https://github.com/huntlabs/hunt-examples/blob/master/website-basic/source/app/controller/IndexController.d#L133.

It's a wrong function declaration.

There is no error message when compiling it. However, the 
showString() can't be called via the browser, because the router 
mapping fails.

What happened for this is that the IndexController is not used 
directly by others. It's used in `mixin MakeController;`, see 
https://github.com/huntlabs/hunt-framework/blob/master/source/hunt/framework/application/Controller.d#L153.

So the conclusion is the mixin needs to do more works to check 
the validation in mixined code.