g++ build failure (Was: C++ PATCH: PR 12735)

[Joern Rennecke <joern dot rennecke at superh dot com>]

> 2003-11-12  Mark Mitchell  <mark@codesourcery.com>
> 
> 	PR c++/12735
> 	* cp-tree.h (duplicate_decls): Return a tree.
> 	* decl.c (duplicate_decls): Clarify documentation.  Return
> 	error_mark_node to indicate a failed redeclaration.
> 	* friend.c (do_friend): Handle that case.
> 	* name-lookup.c (pushdecl): Likewise.

This breaks the build on hosts with older versions of gcc
(independent of the target):

bash-2.05$ gcc -v
Reading specs from /usr/lib/gcc-lib/i386-redhat-linux/2.96/specs
gcc version 2.96 20000731 (Red Hat Linux 7.1 2.96-98)

gcc -c   -g -O2 -DIN_GCC -DCROSS_COMPILE  -W -Wall -Wwrite-strings -Wstrict-prototypes -Wmissing-prototypes -pedantic -Wno-long-long  -fno-common   -DHAVE_CONFIG_H    -I. -Icp -I../../srcw/gcc -I../../srcw/gcc/cp -I../../srcw/gcc/../include  ../../srcw/gcc/cp/decl.c -o cp/decl.o
... lots of warnings...
../../srcw/gcc/cp/decl.c: In function `grokfndecl':
... more warnings...
../../srcw/gcc/cp/decl.c:5696: incompatible types in assignment
../../srcw/gcc/cp/decl.c:5686: warning: `ok' might be used uninitialized in this function
... more warnings...
make[1]: *** [cp/decl.o] Error 1
make[1]: Leaving directory `/swbuild/nightly/2003-11-12/sh-elf/gcc'

In this compiler, _Bool is implemented by a header file as an enum,
which is not assignment-compatible with a pointer.
If my reading of the c99 standard is correct, although _Bool
exists in c99, it is not compatible with any pointer type, and
thus assigning a union tree_node * to ok is a constraint violation
of 6.5.16.1 .

I've tried compiling decl.i with the freshly-build cc1, and after
removing a __thread parameter name from a declaration that came from
/usr/include/bits/sigthread.h, it indeed compiles without even warning
about the assignment to ok.

Further experimentation shows that cc1 doesn't warn even if c89 or c99
is specifically requested.

bash-2.05$ cat boolass.c
char *bar ();

int
f()
{
  _Bool i = bar ();
  return i;
}
bash-2.05$ ./cc1 boolass.c -Wall -pedantic -std=c89
 f

Execution times (seconds)
 expand                :   0.00 ( 0%) usr   0.00 ( 0%) sys   0.01 (50%) wall
 TOTAL                 :   0.01             0.01             0.02
bash-2.05$ ./cc1 boolass.c -Wall -pedantic -std=c99
 f

Execution times (seconds)
 parser                :   0.00 ( 0%) usr   0.01 (100%) sys   0.00 ( 0%) wall
 TOTAL                 :   0.01             0.01             0.02

So we have two problems:

- g++ can only be build using a recent gcc version with a non-standard
  extension.  IIRC we require c89 for GCC source that is to be compiled
  with the host compiler.
- gcc doesn't warn or error for using this extension, even when a standard
  is selected that requires a diagnostic.