Fix issue #79: Incorrect parsing of complex ++ expressions

[fioriandrea]

As the issue pointed out, there was an error while parsing complex post-increment expressions involving field expressions. The fix on the parser was easy enough. However, while working on the parser, I realized that there was a bug in the compiler too. For example:

./goawk 'BEGIN { $0="3 4 5 6 7 8 9"; a=3; print ($($a++)++); print $0; print a }' prints 7\n3 4 7 6 7 8 9\n3, whereas

gawk 'BEGIN { $0="3 4 5 6 7 8 9"; a=3; print ($($a++)++); print $0; print a }' prints 7\n3 4 6 6 8 8 9\n3

./goawk 'BEGIN { n = split("12345", a, ""); i = 1; a[a[a[1]++]++]++; for (i=1;i<=n;i++) printf a[i]; print } prints 34345\n whereas

gawk 'BEGIN { n = split("12345", a, ""); i = 1; a[a[a[1]++]++]++; for (i=1;i<=n;i++) printf a[i]; print } prints 33345\n.

This pull request fixes both issues. I uncommented the TODO tests in interp/interp_test.go and added one more.

Fixes #79

[benhoyt]

This looks great at a glance, and fixes a long-standing issue. Thanks @fioriandrea! I'll take a look at the code change more closely in the next few days.

[benhoyt]

Thanks again @fioriandrea for the fix!