GCC Bugzilla Bug 12391

Consider:

typedef struct { int a; } b_t;
int foo() { b_t d; struct b_t * c = &d; c->a; }

gcc 3.2 says :

a.c: In function `foo':
a.c:2: warning: initialization from incompatible pointer type
a.c:2: dereferencing pointer to incomplete type

GNU C version 3.4 20030914 (experimental) (i686-pc-linux-gnu) says :

a.c: In function `foo':
a.c:2: error: dereferencing pointer to incomplete type

Note the missing warning on initialisation. This can be moderately confusing
because the latter looks exactly like a "missing header" problem, but in fact
the problem is prepending the "struct" before the typedef name, which is
actually an incomplete type of "struct b_t".

The missing warning makes it obvious that it's not as simple as a missing
header.

Right, a mailine regression. What a vicious piece of code. The g++
front end gives this, btw, which I think is a good reason to change
your programming language :-)

g/x> /home/bangerth/bin/gcc-3.4-pre/bin/c++ -c x.c -W -Wall
x.c: In function `int foo()':
x.c:2: error: using typedef-name `b_t' after `struct'
x.c:2: warning: statement has no effect

The regresion happened between 20030830 and 20030902.
I want to say it was caused by: <http://gcc.gnu.org/ml/gcc-patches/2003-09/msg00003.html>
Or one of these two:
2003-08-30  Zack Weinberg  <zack@codesourcery.com>

        * c-tree.h: Delete COMPARE_DIFFERENT_TU from enumeration.
        * c-typeck.c (same_translation_unit_p): New function.
        (comptypes): Use it instead of flags parameter to identify
        structure types from different translation units.
        * c-decl.c (duplicate_decls): Always call comptypes with
        COMPTYPE_STRICT flags argument.
        (c_reset_state): Set BLOCK_SUPERCONTEXT of the block formed
        to file_scope_decl.

2003-08-30  Zack Weinberg  <zack@codesourcery.com>

        * c-tree.h (C_TYPE_INCOMPLETE_VARS): New macro.
        * c-decl.c (struct c_scope): Remove "incomplete" field.
        (pushdecl): Attach variables with incomplete types to
        the TYPE_MAIN_VARIANT of the incomplete type in question.
        (finish_struct): Look at C_TYPE_INCOMPLETE_VARS for variables
        to complete, not at current_scope->incomplete.  All such
        variables do need completion.

These are the only patches which touched the c front-end which could cause this problem.

This is a very minor issue: postponed until GCC 3.4.1.

Fixed by the c-decl.c rewrite:
http://gcc.gnu.org/ml/gcc-patches/2004-03/msg01280.html

fixed in 3.5

Suspending bugs which could be fixed by backporting the c-decl.c rewrite to
3.4.

Fixed in 3.5, won't be fixed in 3.4.x.