Question

How std.range.ElementType should be used in template constraints?

I assumed this way, but I was wrong

import std.range;
auto f(T)(T x)
    if (ElementType!(T) is uint) // adding this line causes lot of error messages 
                                // first of which is:  found ')' when expecting '.' following uint
{
    return x;
}
f(map!"a"([1,2,3,4]));

Answer 1

The is expression is not the same as the is operator. Documentation on the is expression is here: http://dlang.org/expression.html#IsExpression. The is operator does a bitwise comparison of values whereas the is expression compares types (and does crazy pattern matching). Your constraint should be written like this:

auto f(T)(T x)
     if (is(ElementType!(T) == uint))

Or, if you want to match anything implicitly convertible to a uint as well:

auto f(T)(T x)
     if (is(ElementType!(T) : uint))

Further, I’ve fixed your invocations of ElementType, adding the “!” where appropriate; ElementType is a template, not a function.