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From: "jpakkane at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/105467] New: Dependency file produced by C++ modules causes
 Ninja errors
Date: Tue, 03 May 2022 15:04:37 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105467

            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=3Dalways -D_FILE_OFFSET_BITS=3D64 -Wall -Winvalid-pch
-Wnon-virtual-dtor -std=3Dc++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 +=3D 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 +=3D M1.c++m

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