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
}

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)

On Thursday, 7 April 2016 at 21:36:37 UTC, Alex Parrill wrote:
> 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.
>> [...]
> 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)

is(typeof()) gives the expected results:

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) { }

unittest
{
	static assert(is(typeof(foo!(int, int))));	//I get this
	static assert(!is(typeof(foo!(bool, bool))));	//I get this
	static assert(!is(typeof(foo!(int))));		//I think I get this
	static assert(is(typeof(bar!(int, int[]))));	//I get this
	static assert(!is(typeof(bar!(int, int))));	//I get this
	static assert(!is(typeof(bar!(int))));		//I don't get this
}

(note well I have changed the assertion 3 and 6).

There must be a subtle difference between __traits(compile,...) and is(typeof()).
Does "compiles" mean that you've get something but that this thing is not always of a valid type ?

It looks like the bug has already been reported. There are a few associated with __traits(compiles), but this one seems most relevant:
https://issues.dlang.org/show_bug.cgi?id=3448

It also suggests that this is relevant:
https://issues.dlang.org/show_bug.cgi?id=965