[bug #61171] Unexpected warning "Circular test.mm <- test.mm.o dependency dropped."

[anonymous]

URL:
  <https://savannah.gnu.org/bugs/?61171>

                 Summary: Unexpected warning "Circular test.mm <- test.mm.o
dependency dropped."
                 Project: make
            Submitted by: None
            Submitted on: Thu 16 Sep 2021 04:45:12 PM UTC
                Severity: 3 - Normal
              Item Group: Bug
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
       Component Version: 3.81
        Operating System: POSIX-Based
           Fixed Release: None
           Triage Status: None

    _______________________________________________________

Details:

Running `make` version 3.81 i386-apple-darwin11.3.0 on the `Makefile`:

```
test: test.mm.o
        c++ -framework Foundation -o $@ $^
test.mm.o: test.mm
        c++ -ObjC++ -c -o $@ $<
```

and having in place the following ObjectiveC++ source file `test.mm`:

```
#include <iostream>
#include "Foundation/Foundation.h"
int main() {
    NSLog(@"test");
    std::cout << "test\n";
}
```

results in the warning:

```
make: Circular test.mm <- test.mm.o dependency dropped.
c++ -ObjC++ -c -o test.mm.o test.mm
c++ -framework Foundation -o test test.mm.o
```

Invoking `make -p` reveals that there seems to be a build-in implicit rule for
`pattern stem: `test.mm'` which could be the root cause.

The same behavior can be reproduced with an even simpler `Makefile`:

```
test.mm.o: test.mm
        touch $@
```

Note that, e.g., the following `Makefile` executes without the warning:

```
test.mm.o: test.c
        touch $@
```

provide any file named `test.c` is in place. Also the same `Makefile` with
`test.c` replaced by `test.cc`, `test.cpp`, or `test.m` does not show a
warning.

I do not know if the build-in implicit rule for `%.mm` mentioned above is
intended like so. But even in case it is, it seems inconsistent with the ones
for `%.c`, `%.cc`, `%.cpp`, `%.m`, which do not cause a circular dependency.

An obvious workaround is adding another target:

```
test.mm:
        @echo "workaround"
```

or similar.

Thanks for looking into this if you find the time! And big thanks to all for
your awesome work!




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