__traits(compiles) is true with warnings as errors

[Tejas]

On Wednesday, 13 October 2021 at 17:58:23 UTC, Jonathan Levi 
wrote:
> When dmd is passed the "-w" tag, it "treats warnings as errors" 
> but not with the "__traits(compiles)" expression.  Is this the 
> intended action?
>
> This code should compile even with "-w", but it does not.
>
>     int i;
>     static if (__traits(compiles,i += 5.2))
>     	i += 5.2;
>
>
> `pragma(msg, __traits(compiles,i += 5.2))` is `true`, even 
> though it will stop the compiler.
>
> My specific use case, where I am hindered by this, is with a 
> guarded template function akin to this:
>
>     void fun(T,U)(ref T a, const U b)
>     if (__traits(compiles, a += b))
>     {
>         a += b;
>     }
>
> I think this is a bug.  The Docs for __traits compiles state 
> "Returns a bool true if all of the arguments compile . . ." 
> which is not holding true.

But it _is_ compiling, just with a warning.

If you wish to force the code to compile, you can manually cast 
it to prevent the compiler from erroring out:
```d

void main(){
	int i;
	static if (__traits(compiles,i += 5.2))
		i += cast(int)5.2;
}
```

Or in general:

```d
void fun(T,U)(ref T a, const U b)
      if (__traits(compiles, a += b))
      {
          a += cast(T)b;
      }
```

It seems the `__traits(compiles, ..)` feature was designed to 
operate independent of the command line flags passed to the D 
compiler.

I don't know if that's a bug or not though; best wait for someone 
more experienced to chime in.