Re: ambigious ++ expression silently accepted

[Paul Eggert]

>> Some of my students wrote "{ foo++bar }" which is ambigious;
>> Either: post-increment foo and concatenate with bar
>> or: concatenate foo with pre-increment bar.
>
> It's not ambiguous. The unary operators have higher precedence
> than the binary ones, and concatenation is a binary operator.

Sorry, I don't see how mere precedence resolves the ambiguity here.
If it were only a matter of precedence, the Awk expression foo++bar
could be parsed either as ((foo++) bar), or as (foo (++bar)).  In both
parses, the ++ would be unary, and would have higher precedence than
concatenation.

I looked through the grammar defined by POSIX in
<http://www.opengroup.org/onlinepubs/000095399/utilities/awk.html>
and it appears to me that foo++bar is indeed ambiguous, since it can
be parsed in two different ways:

non_unary_expr non_unary_expr -> lvalue INCR non_unary_expr
non_unary_expr non_unary_expr -> non_unary_expr INCR lvalue

If I understand things correctly, this is a defect in the POSIX spec
for Awk, one that could be corrected by adding an extra disambiguation
rule or by complicating the grammar somewhat.