How do you call hashOf() on a string?

[Enjoys Math via Digitalmars-d-learn]

module smallest_grammar;
import std.conv;
import std.algorithm;
struct Symbol(T) {
public:
        this(T sym, bool isVar) {
                this.sym = sym;
                this.is_var = isVar;
        }

        @property T symbol() { return sym; }
        @property bool isVar() { return is_var; }


private:
        T sym;
        bool is_var = false;
}


struct String(T) {
public:
        this(const T[] s) {
                this.s = s.dup;
        }
        
        alias s this;

        string toString() const { return to!string(s); }
	string flattened() const { string t = "";  foreach (c; s)	t ~= 
to!string(c);  return t; }

        size_t toHash() const @safe pure nothrow
    {
                return flattened().hashOf();
    }

    bool opEquals(ref const String t) const @safe pure nothrow
    {
                if (t.length != this.length)
                        return false;

                for(size_t k=0; k < t.length; k++)
                        if (t.s[k] != this.s[k]) return false;

                return true;
    }


private
        T[] s;
}

-----


My hash function:

        size_t toHash() const @safe pure nothrow
    {
                return flattened().hashOf();
    }

-----

Is yielding:
smallest_grammar.d(35): Error: @safe function 
'smallest_grammar.String!string.String.toHash' cannot call 
@system function 'object.hashOf!string.hashOf'