[Bug c++/109718] New: Dependency generation for header-units and modules not possible

[Bug c++/109718] New: Dependency generation for header-units and modules not possible

[klaus.doldinger64 at googlemail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109718

            Bug ID: 109718
           Summary: Dependency generation for header-units and modules not
                    possible
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: klaus.doldinger64 at googlemail dot com
  Target Milestone: ---

To generate make-dependencies I use the following command line:

g++ -c -MD -MG -std=c++23 -fconcepts -fmodules-ts test.cc

where test.cc is:

-----------------
import <array>;

int main() {
}
-----------------


It gives the error:

cc1plus: error: '-MG' may only be used with '-M' or '-MM'

Removing -MG gives also an error, because the precompiled header does not yet
exist:

/usr/local/include/c++/14.0.0/iostream: error: failed to read compiled module:
No such file or directory
/usr/local/include/c++/14.0.0/iostream: note: compiled module file is
'gcm.cache/./usr/local/include/c++/14.0.0/iostream.gcm'
/usr/local/include/c++/14.0.0/iostream: note: imports must be built before
being imported
/usr/local/include/c++/14.0.0/iostream: fatal error: returning to the gate for
a mechanical issue

So, it looks that there is no possiblity to create a valid dependency file with
gcc.