GDB does not recognize cxx extension?

Hi Everyone,

I've got a C++ program built from a source called pem_test.cxx. I'm
having trouble getting GDB to accept it. Autocomplete does not work:

    (gdb) b pem_<TAB>
    pem_common.cpp  pem_common.h    pem_read.cpp    pem_write.cpp

And breakpoints don't work:

    (gdb) b pem_test.cxx : 85
    No source file named pem_test.cxx.
    Make breakpoint pending on future shared library load? (y or [n])

The file is in PWD:

    (gdb) shell ls *.cxx
    pem_test.cxx  test.cxx

I'm working on Ubuntu 18.04, using "GCC (Ubuntu
7.4.0-1ubuntu1~18.04.1)", "GDB (Ubuntu 8.1-0ubuntu3.1)", and
CXXFLAGS="-DDEBUG -g3 -O0 -Wall".

Searching the mailing list returns spurious hits:
https://sourceware.org/cgi-bin/search.cgi?wm=wrd&form=extended&m=all&s=D&ul=%2Fml%2Fgdb%2F%25&q=cxx%20extension

How do I have have GDB recognize the cxx extension as a C++ source file?

Thanks in advance.