Calling assumeSorted on const(std.container.Array)

[Olivier Prat via Digitalmars-d-learn]

I'm trying to call assumeSorted on a const(Array) using this code 
snippet:

    void test(T)(const ref Array!T a)
    {
        auto b = a[].assumeSorted;
        writeln(b);
    }
    void main() {
        Array!int a = [1, 5, 7, 8, 15, 100];
        test(a);
    }

Unfortunately, I keep having a compilation error:
/dlang/dmd/linux/bin64/../../src/phobos/std/range/package.d(10871): Error: 
cannot modify struct instance `result._input` of type 
`RangeT!(const(Array!int))` because it contains `const` or `immutable` members
/dlang/dmd/linux/bin64/../../src/phobos/std/range/package.d(10918): Error: 
cannot modify struct instance `result._input` of type 
`RangeT!(const(Array!int))` because it contains `const` or `immutable` members
/dlang/dmd/linux/bin64/../../src/phobos/std/range/package.d(11500): Error: template instance 
`std.range.SortedRange!(RangeT!(const(Array!int)), "a < b", 
SortedRangeOptions.assumeSorted)` error instantiating
onlineapp.d(10):        instantiated from here: `assumeSorted!("a 
< b", RangeT!(const(Array!int)))`
onlineapp.d(17):        instantiated from here: `test!int`

Would someone explain the exact nature of this error as I fail to 
understand as how just telling that an Array ConstRange is 
assumed to be sorted actually modifies anything??