On Monday, 26 November 2012 at 16:37:08 UTC, Manu wrote:
template isThing( alias symbol, A )
{
enum isThing = false;
}
This template works in most contexts:
int x;
struct S {}
pragma(msg, isThing!x);
pragma(msg, isThing!S);
But this fails:
pragma(msg, isThing!int);
Why does it fail on a basic type (int), but not a user defined
type (S)?
How can I fix the template declaration to not error in that
case?
I tried:
template isThing( alias symbol, A )
{
enum isThing = false;
}
template isThing( T, A )
{
enum isThing = false;
}
Hoping the T version would catch the int, but this leads to
different
errors "matches more than one template declaration".
That is annoying. There *must* be a kind of single unconstrained
template parameter accepting anything that can be an element of a
"compile time" tuple. In other words, this must work for any
arguments without hassle:
template Foo(alias Head, Tail...)
{
alias TypeTuple!(Head, Tail) Foo;
}
As others said, you can workaround the deficiency with an
if-constraint, which is unsightly. FWIW, I have all my codebase
littered with "if (A.length == 1)" rubbish.
