std.range.hasLength does not work if the .length member is defined inside a static if. It's a common case for ranges wrapping other ranges and trying to expose their input's properties:
---
static if (hasLength!Input)
    int length() { return _input.length;}
----
for example.


Here is some code demonstrating the problem:

----
import std.range;

struct Lengthy(bool i) {
    int front() { return 1;}
    void popFront();
    static if (i)
        int length() { return 1;}
}

void main() {
    Lengthy!true i;
    Lengthy!false ni;
    assert(hasLength!(typeof(i))); // Error: AssertError. Considers i has no .length defined.
    assert(!hasLength!(typeof(ni)));
}
----

And a possible solution that seems to work quite well in practice:
----
template hasLength(R) {
    enum bool hasLength = __traits(compiles, R.length);
}
----