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

[Prathamesh Kulkarni <prathamesh dot kulkarni at linaro dot org>]

Hi,
For test-case containing only the following declaration:
static struct undefined_struct object;
gcc rejects it at -O0 in assemble_variable() with error "storage size
of <var> is unknown",
however no error is reported when compiled with -O2.
AFAIU that happens because at -O2, analyze_function() removes the
symbol "object" from symbol table and assemble_variable() has no
chance to process it.
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 ?

So instead of finish_decl(), I added check for incomplete struct/union
in check_global_declaration(),
which is called by analyze_function() before removing unused nodes.
The patch regresses Wcxx-compat-8.c and declspecs-1.c because the error
"storage size of <var> is unknown" shows up in these test-cases. I
modified the test-cases
to accept that error.
Does the patch look OK ?

Thank you,
Prathamesh

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