__traits(compiles, ...) with syntax errors

[Steven Schveighoffer]

On 1/14/19 4:47 PM, Steven Schveighoffer wrote:
> How many times have you written something like:
> 
> void foo(T)(T t) if (__traits(compiles, t.bar())) // or is(typeof(t.bar()))
> {
>     t.bar();
> }
> 
> And somehow, somewhere, this isn't called? Then you remove the 
> constraint, and find that there's a syntax error in the bar template 
> function (like a missing semicolon).
> 
> I often times, to avoid this, simply just use __traits(hasMember, T, 
> "bar"), and live with the consequences.
> 
> I'd love to have something like __traits(compiles), but have it be an 
> error if the syntax is incorrect.
> 
> In other words, the constraint above only fails if there is a missing 
> symbol, or incorrect type conversion, not that there is a stupid syntax 
> error that would NEVER compile.
> 
> Ironically, it would be something like compilesOrSyntaxError.

Another option is to have __traits(compiles) spit out an error type that 
has truthiness if it's not an error (for backwards compatibility), then 
you could something like:

if(__traits(compiles, ...) != COMPILER_ERROR.SYNTAX)

-Steve