Re: [C++ Patch] PR 89875 ("[7/8/9/10 Regression] invalid typeof reference to a member of an incomplete struct accepted at function scope")

[Jason Merrill <jason at redhat dot com>]

On 5/10/19 10:29 AM, Paolo Carlini wrote:
> Hi,
>
> a while ago Martin noticed that an unintended consequence of an old 
> tweak of mine - which avoided redundant error messages emitted from 
> cp_parser_init_declarator - is that, in some cases, we started accepting 
> ill-formed typeofs. Luckily, decltype isn't affected and that points to 
> the real issue: by the time that place in cp_parser_init_declarator is 
> reached, for a decltype version we already emitted a correct error 
> message. Thus I think the right way to fix the problem is simply 
> committing to tentative parse when, toward the end of 
> cp_parser_sizeof_operand we know that we must be looking at a (possibly 
> ill-formed) expression. Tested x86_64-linux.

The problem with calling cp_parser_commit_to_tentative_parse here is 
that the tentative parse you're committing to is for the enclosing 
scope, which is trying to decide whether e.g. we're parsing a 
declaration or expression.  If the operand of typeof is a well-formed 
expression, and the larger context is an expression, this will break.

Better, I think, to commit and re-parse only if you actually encounter 
an error.

Alternately, cp_parser_decltype_expr deals with this by using a 
tentative firewall and CPP_DECLTYPE; cp_parser_sizeof_operand could do 
the same, but that seems like a bigger hammer.

Jason