[Bug preprocessor/105412] New: [10/11/12 Regression] Missing phony target with -MP for first include when compiling from stdin

["jsm28 at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105412

            Bug ID: 105412
           Summary: [10/11/12 Regression] Missing phony target with -MP
                    for first include when compiling from stdin
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: preprocessor
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jsm28 at gcc dot gnu.org
  Target Milestone: ---

When generating dependencies with -M -MP, the phony target for the first
included header (typically stdc-predef.h) is missing if compiling from stdin,
with GCC 10 and later but not with GCC 9.  For example:

$ echo "#include <cstdlib>" |
./many9/install/compilers/x86_64-linux-gnu/bin/x86_64-glibc-linux-gnu-g++ -M
-MP -x c++ - | grep stdc-predef

/scratch/jmyers/glibc/many9/install/compilers/x86_64-linux-gnu/sysroot/usr/include/stdc-predef.h
\
/scratch/jmyers/glibc/many9/install/compilers/x86_64-linux-gnu/sysroot/usr/include/stdc-predef.h:

shows the expected phony target (the second line of output) with a GCC 9
compiler, but

$ echo "#include <cstdlib>" |
./many10/install/compilers/x86_64-linux-gnu/bin/x86_64-glibc-linux-gnu-g++ -M
-MP -x c++ - | grep stdc-predef

/scratch/jmyers/glibc/many10/install/compilers/x86_64-linux-gnu/sysroot/usr/include/stdc-predef.h
\

shows that line of output is missing with GCC 10 and later.

(This example is adapted from code in glibc's configure script, but the bug
only actually causes problems with glibc when the compiler is used with an old
sysroot lacking stdc-predef.h and so the missing phony target is that for
<cstdlib> itself.)