[Bug c++/105467] New: Dependency file produced by C++ modules causes Ninja errors

["jpakkane at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105467

            Bug ID: 105467
           Summary: Dependency file produced by C++ modules causes Ninja
                    errors
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jpakkane at gmail dot com
  Target Milestone: ---

Currently if you try to build C++ modules using Ninja and a dependency file,
things fail with this fairly cryptic error message (that comes from Ninja):

inputs may not also have inputs

The commend line is this:

g++ -Igcc/modtest.p -Igcc '-I../test cases/unit/85 cpp modules/gcc'
-fdiagnostics-color=always -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch
-Wnon-virtual-dtor -std=c++20 -O0 -g -fmodules-ts -MD -MQ
gcc/modtest.p/src0.cxx.o -MF gcc/modtest.p/src0.cxx.o.d -o
gcc/modtest.p/src0.cxx.o -c '../test cases/unit/85 cpp modules/gcc/src0.cxx

and the generated dependency file looks like this:

gcc/modtest.p/src0.cxx.o gcm.cache/M0.gcm: \
 ../test\ cases/unit/85\ cpp\ modules/gcc/src0.cxx \
 /usr/include/stdc-predef.h
gcc/modtest.p/src0.cxx.o gcm.cache/M0.gcm: M1.c++m
M0.c++m: gcm.cache/M0.gcm
.PHONY: M0.c++m
gcm.cache/M0.gcm:| gcc/modtest.p/src0.cxx.o
CXX_IMPORTS += M1.c++m

This sets things up so that the object file depends on the sources and the
generated module file depends on the object file only. Ninja does not like this
and errors out because it thinks that outputs (the .o file in this case) may
not be used as a dependency to other outputs.

A possible fix would be to set the dependencies of the module output to be the
same source files as are used for the object file. Something like this:

gcc/modtest.p/src0.cxx.o gcm.cache/M0.gcm: \
 ../test\ cases/unit/85\ cpp\ modules/gcc/src0.cxx \
 /usr/include/stdc-predef.h
gcc/modtest.p/src0.cxx.o gcm.cache/M0.gcm: M1.c++m
M0.c++m: gcm.cache/M0.gcm
.PHONY: M0.c++m
gcm.cache/M0.gcm: \
 ../test\ cases/unit/85\ cpp\ modules/gcc/src0.cxx \
 /usr/include/stdc-predef.h
CXX_IMPORTS += M1.c++m

The corresponding Ninja bug is here:
https://github.com/ninja-build/ninja/issues/1962