Array-only version of startsWith and skipOver

[Per Nordlöw]

Why does


/** Array-overload for `startsWith` with no explicit predicate 
predicate. */
bool startsWith(T)(scope const(T)[] haystack,
                    scope const(T)[] needle)
{
     if (haystack.length >= needle.length)
     {
         return haystack[0 .. needle.length] == needle; // range 
check is elided by LDC in release builds
     }
     return false;
}

/** Array-overload for `skipOver` with no explicit predicate 
predicate. */
bool skipOver(T)(scope ref const(T)[] haystack,
                  scope const(T)[] needle)
{
     if (startsWith(haystack, needle))
     {
         haystack = haystack[needle.length .. $];
         return true;
     }
     return false;
}

///
@safe pure nothrow @nogc unittest
{
     string x = "beta version";
     assert(x.skipOver("beta"));
}


fail to compile as


array_algorithm.d(59,22): Error: template 
`array_algorithm.skipOver` cannot deduce function from argument 
types `!()(string, string)`, candidates are:
     assert(x.skipOver("beta"));
                      ^
array_algorithm.d(44,6):        `skipOver(T)(ref scope const(T)[] 
haystack, scope const(T)[] needle)`
bool skipOver(T)(scope ref const(T)[] haystack,