Re: reject decl with incomplete struct/union type in check_global_declaration()

[Nathan Sidwell <nathan at acm dot org>]

On 01/14/16 16:57, Joseph Myers wrote:
> On Thu, 14 Jan 2016, Prathamesh Kulkarni wrote:

> > g++ rejects it during parsing. I tried similarly in C FE by adding a
> > check for decl with incomplete struct/union type in finish_decl(),
> > however that fails to compile the following case:
> > typedef struct foo foo_t;
> > foo_t x;
> > struct foo { int i; };
> > g++ rejects the above case as well but gcc accepts it.
> > Do C and C++ standards differ in this regard ?
>
> I don't know about C++, but this sort of thing is valid C if the type is
> complete at the end of the translation unit or is an incomplete array type
> (but not in the case where the variable is static - such a case with
> static, if supported, is an extension).

It's ill-formed C++.

7.1.1/8 allows you to use a declared but undefined struct with 'extern', but not 
 without it.

'The name of a declared but undefined class can be used in an extern 
declaration. Such a declaration can only be used in ways that do not require a 
complete class type'

(Although it doesn't seem to explicitly say that such a name can be used without 
an 'extern', the implication is that it cannot).

nathan