////////////////////////////// test.d /////////////////////////////
struct S
{
    @property size_t count() const
    {
        return 1;
    }
}

int[S.init.count()] x; // OK
int[S.init.count  ] y; // error
// Error: need `this` for `count` of type `const @property ulong()`
///////////////////////////////////////////////////////////////////

Additionally, the error message is misleading. Older versions of compilers produced a better error:

2.079.0 to 2.087.1: Failure with output: test.d(9): Error: integer constant expression expected instead of `S().count`