From the unit test run [1]:
----------------------8<-----------------------
[ RUN ] FormatterTest.FormatLongDouble
/«BUILDDIR»/cppformat-1.1.0+ds/test/format-test.cc:1166: Failure
Value of: format("{0:+010.4g}", 392.65l)
Actual: "+0000392.7"
Expected: "+0000392.6"
[ FAILED ] FormatterTest.FormatLongDouble (0 ms)
---------------------->8-----------------------
I don't know whether this is an issue of flags or a compiler or the architecture or the test or the implementation. Please have a look.
[1] https://buildd.debian.org/status/fetch.php?pkg=cppformat&arch=powerpc&ver=1.1.0%2Bds-3&stamp=1434716610