SUMMARY: -imacros switch breaks warning/error reporting

When the -imacros switch is used, and the source file has errors or warnings, the file name in the warning message gets lost and is replaced with <command-line>.

GCC VERSION:

$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.8/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.8.2-19ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-4.8/README.Bugs --enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.8 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.8 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object --disable-libmudflap --enable-plugin --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.8-amd64 --with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.8.2 (Ubuntu 4.8.2-19ubuntu1) 


COMMAND LINE WHICH TRIGGERS BUG:

g++ -imacros validinclude.h causeerror.cpp

COMPILER OUTPUT (the output itself is the bug):

<command-line>:0:1: error: ‘blah’ does not name a type

The source file causeerror.cpp has the one line:
blah

The valid file validinclude.h has the one line:
#define one 1

HERE IS THE .ii file:

# 1 "causeerror.cpp"
# 1 "<command-line>"
# 1 "./validinclude.h" 1
# 1 "<command-line>" 2
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 1 "causeerror.cpp"
# 1 "<command-line>" 2
blah