On 9/16/20 9:04 AM, Steven Schveighoffer wrote:
> static assert(__traits(compiles, fun!foo(args)));

ugh.

static assert(__traits(compiles, doppleganger!foo(args)));

On Tuesday, 8 September 2020 at 18:59:00 UTC, Steven Schveighoffer wrote:
>    void toString(Output)(Output output) if (isOutputRange!(Output, dchar))
>    {
>       put(output, "hello, this is an S!"); // no import of std.range.put
>    }
...
> if writeln checked using the proposed __traits(instantiates) instead of __traits(compiles) on the call to S.toString(lockedTextOutput) (or whatever it does), then instead of the wrong path, I get a compilation error, telling me that my toString doesn't compile.

In theory, `is(typeof(S.init.toString(suitableOutputRange)))` could be true even if there is a semantic error in the body of your S.toString method. The typeof expression only needs to resolve the method then produce the type of that method. You specified the return type as `void` (not `auto`), so it is overkill to check the semantics of the method body at this point in compilation (it might not even get called).

If typeof was changed to do less work, perhaps `is(typeof(...))` could do what you want.

On 9/16/20 12:59 PM, Nick Treleaven wrote:
> On Tuesday, 8 September 2020 at 18:59:00 UTC, Steven Schveighoffer wrote:
>>    void toString(Output)(Output output) if (isOutputRange!(Output, dchar))
>>    {
>>       put(output, "hello, this is an S!"); // no import of std.range.put
>>    }
> ....
>> if writeln checked using the proposed __traits(instantiates) instead of __traits(compiles) on the call to S.toString(lockedTextOutput) (or whatever it does), then instead of the wrong path, I get a compilation error, telling me that my toString doesn't compile.
> 
> In theory, `is(typeof(S.init.toString(suitableOutputRange)))` could be true even if there is a semantic error in the body of your S.toString method. The typeof expression only needs to resolve the method then produce the type of that method. You specified the return type as `void` (not `auto`), so it is overkill to check the semantics of the method body at this point in compilation (it might not even get called).
> 
> If typeof was changed to do less work, perhaps `is(typeof(...))` could do what you want.

Ehh... I don't care what it returns. I care that it can be called. The fact that I wrote void in my toy example is not important.

But you bring up a good point. Something like "see if this lambda compiles" might included figuring out the return type of a function. Which in turn means to analyze the body (if it's an auto return).

So the formulation of a constraint may have to avoid using "see if this lambda works" techniques to avoid having to compile everything.

-Steve