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From: Bart <bc@freeuk.com>
Newsgroups: comp.lang.c
Subject: Re: Top 10 most common hard skills listed on resumes...
Date: Sun, 1 Sep 2024 00:37:49 +0100
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On 31/08/2024 23:31, Keith Thompson wrote:
> Bart <bc@freeuk.com> writes:
> [...]
>> Given this:
>>
>>     x + y = z;
>>     (x + y) = z;
>>
>> My compiler produces the same AST for the LHS of '=' in both cases.
> [...]
> 
> If that's the case (and I don't doubt that it is), then your compiler is
> not following the grammar specified by the ISO C standard.  Since
> `x + y` is not a unary-expression, `x + y = z` is not a syntactically
> valid assignment expression.

Yet no compiler out of the dozen I tried reported a syntax error (except 
SDCC).

So what AST is produced by gcc, if any?

Most compilers, including mine, complain that an lvalue is expected.

> 
> A parser that strictly follows the ISO C grammar would reject
> (diagnose, flag, whatever) `x + y = z;` just as it would reject `x = y +;`.

Which one does that (apart from SDCC)?


> This is an observation, not a complaint.  It doesn't imply that your
> compiler is non-conforming or buggy.  A parser that doesn't strictly
> follow the ISO C grammar could still be part of a conforming compiler.
> 

I can also say that the C grammar is buggy:

     assignment-expression:
         conditional-expression
         unary-expression asssignment-operator assignment-expression

When attempting to parse an assignment-expression, do you go for a 
conditional-expression or unary-expression?

The latter is a subset of the former. If you go for a 
conditional-expression and find that an assignment-operator follows, now 
you have to perform some analysis on the LHS to see if that 
conditional-expression contains only a unary-expression.

However, if it's not a unary-expression, it will fail for other reasons
anyway, because all those other things that a conditional-expression 
will be, can't be lvalues.

That also applies to many unary-expressions, such as 42, or a++; those 
can't be lvalues either, even though the syntax is valid.

So it makes sense to do only an lvalue test.