This code:
==========
module scratch;

class C( element_t ) {

	element_t [] contents;

	element_t opIndex( int index ) {
		return contents[ index ];
	}
	
	element_t opIndexUnary( string op )( int index ) {
		
		static if( op == "++" ) {
			return ++contents[ index ];
		}
		else {
			static assert( false );
		}
	}
	
}

unittest {
	auto c = new C!int();
	
	c[0]++;
}
==========
Produces this output:
==========
$ dmd -c -unittest scratch.d
scratch.d(42): Error: c.opIndex(0) is not an lvalue
==========
But if the penultimate line: "c[0]++" is replaced by "++c[0]", there is no problem.

And finally, if the opIndex is changed to return a 'ref', there is not problem, but the opIndexUnary operator is not called - the reference is incremented as an int, I think.