Dependency file produced by C++ modules causes Ninja errors

Bug 105467 - Dependency file produced by C++ modules causes Ninja errors

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

-fdeps-format=p1689r5 will suppress this extra information. A new `-Mno-modules` flag may be more useful (and certainly discoverable).

It would be preferable to have the default work out of the box than having every end user having to add compiler flags to make things work.

Ninja is the most popular underlying build system for modules, having it work by default would make things easier for many people.