[Bug preprocessor/127878] New: Module mapper file breaks makefile fragment generation.

[joergboe at snafu dot de via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127878

            Bug ID: 127878
           Summary: Module mapper file breaks makefile fragment
                    generation.
           Product: gcc
           Version: 16.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: preprocessor
          Assignee: unassigned at gcc dot gnu.org
          Reporter: joergboe at snafu dot de
  Target Milestone: ---

There is one header file:
> echo '' > header.h

Translating it into a header unit cmi file:
> g++ -x c++-user-header header.h -c -std=c++20 -fmodules 
> -flang-info-include-translate -flang-info-module-cmi
./header.h: note: writing CMI ‘gcm.cache/,/header.h.gcm’

One TU imports header.h and a named module:
> echo -e 'import "header.h";\nimport module;\n' > foo.cpp

Now I generate the makefile fragment:
> g++ foo.cpp -std=c++20 -fmodules -MM -flang-info-include-translate 
> -flang-info-module-cmi
In module imported at foo.cpp:1:1:
./header.h: note: reading CMI ‘gcm.cache/,/header.h.gcm’
foo.o: foo.cpp gcm.cache/,/header.h.gcm
foo.o: module.c++-module ./header.h.c++-module
CXX_IMPORTS += module.c++-module ./header.h.c++-module

So far it works as expected.

Now I add a module mapper file that specifies the location of the header cmi
file:
> echo -e '$root gcm.cache\n./header.h ,/header.h.gcm' > module-mapper.modm

Generating the makefile fragment again:
> g++ foo.cpp -std=c++20 -fmodules -MM -flang-info-include-translate 
> -flang-info-module-cmi -fmodule-mapper=module-mapper.modm
In module imported at foo.cpp:1:1:
./header.h: note: reading CMI ‘gcm.cache/,/header.h.gcm’
foo.cpp:2:1: fatal error: unknown compiled module interface: no such module
    2 | import module;
      | ^~~~~~
compilation terminated.

So it is impossible to generate the makefile fragments if the header unit cmi
files are not in the default place!