[Bug driver/57951] New: -MG doesn't work with -MD

[Bug driver/57951] New: -MG doesn't work with -MD

[f.heckenbach@fh-soft.de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57951

            Bug ID: 57951
           Summary: -MG doesn't work with -MD
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: driver
          Assignee: unassigned at gcc dot gnu.org
          Reporter: f.heckenbach@fh-soft.de

This was reported as Debian bug #593086 several years ago ...

% gcc -MD -MG foo.c
cc1: error: -MG may only be used with -M or -MM

(Contents of source file don't matter.)

According to the manual

  `-MD' is equivalent to `-M -MF FILE', except that `-E' is not implied.

so I figure it should work. (It did in previous versions, e.g. 4.1.2)

There seems to be no work-around since "-M" implies "-E".

I guess the driver only needs to be told that "-MD" is also OK for "-MG" to
work since it implies "-M".

[Bug driver/57951] -MG doesn't work with -MD

[douglas at halo dot gen.nz]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57951

Douglas Bagnall <douglas at halo dot gen.nz> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |douglas at halo dot gen.nz

--- Comment #1 from Douglas Bagnall <douglas at halo dot gen.nz> ---
Also present in 4.8.1, and it also affects -MMD as you might expect.

[Bug driver/57951] -MG doesn't work with -MD

[f.heckenbach@fh-soft.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57951

--- Comment #2 from Frank Heckenbach <f.heckenbach@fh-soft.de> ---
Another bug that may be related to this one (and certainly depends
on it), originally reported as Debian bug #613551:

Tested with gcc-4.1 (apparently the last version that did allow "-MD -MG"):

When using "-MG", gcc doesn't give an error message, neither exits
with nonzero status when included files don't exist, even if not
only preprocessing ("-M"), but also when actually compiling ("-MD").

This can, in extreme cases, lead to miscompilation (if the included
file e.g. redefines existing macros), usually only to surprising
errors (the results of missing declarations from the include file,
without a leading hint to its absence).

% cat foo.c
#include "non-existing-file"
% gcc -MD -MG -c foo.c
% echo $?
0