Created attachment 8681 [details]
Simple file to demonstrate the problem

If a class is defined with a single operator[] method,
then gdb can invoke this method with the normal x[i] syntax.

If, however,  there are separate const "rhs" and non-const "lhs" 
methods defined,  then trying to print or assign using x[i] syntax
produces the message "Could not find operator[].".

See example t.c.  When compiled as is (SEPARATE_SUB_METHODS defined)
there are methods 
	const T& operator[](const int index) const 
and
	T& operator[](const int index) {

Running gdb shows:

(gdb) b 53
Breakpoint 1 at 0x40076a: file t.cc, line 53.
(gdb) r
(gdb) p x
$1 = {storage = {10, 20, 30, 40, 50, 60, 70, 80, 90, 100}}
(gdb) p x[3]
Could not find operator[].
(gdb) show version
GNU gdb (GDB) 7.10
Copyright (C) 2015 Free Software Foundation, Inc.


When SEPARATE_SUB_METHODS is undefined there is only one method
	T& operator[](const int index)  {
and gdb works fine with this:

(gdb) b 53
Breakpoint 1 at 0x40076a: file t2.cc, line 53.
(gdb) r
Starting program: /localhome/sharman/tmp/c++test/t2 

Breakpoint 1, main (NameError: Installation error: gdb.execute_unwinders function is missing

argc=1, argv=0x7fffffffdbc8) at t2.cc:53
53		printf("%2d: %d\n", i, x[i]);
(gdb) p x
$1 = {storage = {10, 20, 30, 40, 50, 60, 70, 80, 90, 100}}
(gdb) p x[3]
NameError: Installation error: gdb.execute_unwinders function is missing
$2 = (int &) @0x7fffffffdab8: 30
(gdb) set var x[3] = 33
(gdb) p x[3]
$3 = (int &) @0x7fffffffdab8: 33
(gdb) p x
$4 = {storage = {10, 20, 33, 40, 50, 60, 70, 80, 90, 100}}
(gdb)

Works since version gdb-8.0.