__traits(compiles) and template instantiation

[Alex Parrill via Digitalmars-d-learn]

On Thursday, 7 April 2016 at 20:31:12 UTC, jmh530 wrote:
> I've been playing around with __traits and I find myself 
> confused on one aspect. In the code below, I was testing 
> whether some templates would compile given types. For the most 
> part it works as I would expect.
>
> I think I get why the third one works with foo!(int). My guess 
> is that it assumed that U is the same as T and both are int. 
> However, that wouldn't make sense with the last one where I use 
> bar!(int). In that one it's basically ignoring the second 
> template constraint. So I don't understand what's going on for 
> that last line to compile. To confirm I wasn't crazy, I get an 
> error with
> alias bar_ = bar!(int);
>
>
>
> import std.traits : isNumeric;
> import std.range : isInputRange;
>
> void foo(T, U)(T x, U y) if (isNumeric!T && isNumeric!U) { }
>
> void bar(T, U)(T x, U y) if (isNumeric!T && isInputRange!U) { }
>
> void main()
> {
> 	assert(__traits(compiles, foo!(int, int)));	//I get this
> 	assert(!__traits(compiles, foo!(bool, bool)));	//I get this
> 	assert(__traits(compiles, foo!(int)));		//I think I get this
> 	assert(__traits(compiles, bar!(int, int[])));	//I get this
> 	assert(!__traits(compiles, bar!(int, int)));	//I get this
> 	assert(__traits(compiles, bar!(int)));		//I don't get this
> }

Neither the third nor sixth lines should be true.

     alias wrongfoo = foo!int; /* Error: template instance foo!int 
does not match template declaration foo(T, U)(T x, U y) if 
(isNumeric!T && isNumeric!U) */
     alias rightfoo = foo!(int, int); /* ok */

File a DMD bug.

(Also, you can use static assert here to check the assertions at 
build-time instead of run-time)