I do not really expect setting a watch point on 'this' to be very useful, but the the error message is strange:

Breakpoint 2, Foo::Foo (this=0xbfffee54, i=0) at ../simple/simple_test_app.cpp:335
335             b = 2 + s + t;
(gdb) watch this
Hardware watchpoint 3: this
(gdb) watch -l this
A syntax error in expression, near `const *) 0xbfffee40'.
(gdb) watch -l this->a
Hardware watchpoint 4: -location this->a
(gdb) watch -l b
Hardware watchpoint 5: -location b

(the second item)