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)
